Using Fetch - Web APIs
e resources it's retrieving. supplying request options the fetch() method can optionally accept a second parameter, an init object that allows you to control a number of different settings: see fetch() for the full options available, and more details. // example post method implementation: async function postdata(url = '', data = {}) { // default options are marked with * const response = await fetch(url, { method: 'post', // *get, post, put, delete, etc. mode: 'cors', // no-cors, *cors, same-origin cache: 'no-cache', // *default, no-cache, reload, force-cache, only-if-cached credentials: 'same-origin', // include, *same-origin, omit headers: { 'content-type': 'application/json' // 'content-type': 'application/x-www-form-urlencoded', }, redirect: ...
..., not strings. if you want to fetch a text file and process it line by line, it is up to you to handle these complications. the following example shows one way to do this by creating a line iterator (for simplicity, it assumes the text is utf-8, and doesn't handle fetch errors). ...
...complications. the following example shows one way to do this by creating a line iterator (for simplicity, it assumes the text is utf-8, and doesn't handle fetch errors). ...
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LockManager.request() - Web APIs
ropped if it was not already granted. callback … return value a promise that resolves with undefined when the request is granted. examples general example the following example shows the basic use of the request() method with an asynchronous function as the callback. once the callback is invoked, no other running code on this orign can hold `my_resource` until the callback returns. await navigator.locks.request('my_resource', async lock => { // the lock was granted. }); mode example the following example shows how to use the mode option for readers and writers. notice that both functions use a lock called my_resource. the do_read() requests a lock in 'shared' mode meaning that multiple calls may occur simultaneously across different event handlers, tabs, or workers. async ...
...t('my_resource', async lock => { // the lock was granted. }); mode example the following example shows how to use the mode option for readers and writers. notice that both functions use a lock called my_resource. the do_read() requests a lock in 'shared' mode meaning that multiple calls may occur simultaneously across different event handlers, tabs, or workers. ...
... workers. async function do_read() { await navigator.locks.request('my_resource', {mode: 'shared'}, async lock => { // read code here. }); } the do_write() function use the same lock but in 'exclusive' mode which will delay invocation of the request() call in do_read() until the write operation has completed. this applies across event handlers, tabs, or workers. function do_write() { await navigator.locks.request('my_resource', {mode: 'exclusive'}, async lock => { // write code here. }); } ifavailable example to grab a lock only if it isn't already being held, use the ifavailable option. in this function await means the method will not return until the callback is complete. since the lock is only granted if it was available, this call avoids needing to wait on the lock bein...
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SubtleCrypto.wrapKey() - Web APIs
ey(keymaterial, salt) { return window.crypto.subtle.derivekey( { "name": "pbkdf2", salt: salt, "iterations": 100000, "hash": "sha-256" }, keymaterial, { "name": "aes-kw", "length": 256}, true, [ "wrapkey", "unwrapkey" ] ); } /* wrap the given key. */ async function wrapcryptokey(keytowrap) { // get the key encryption key const keymaterial = await getkeymaterial(); salt = window.crypto.getrandomvalues(new uint8array(16)); const wrappingkey = await getkey(keymaterial, salt); return window.crypto.subtle.wrapkey( "raw", keytowrap, wrappingkey, "aes-kw" ); } /* generate an encrypt/decrypt secret key, then wrap it. */ window.crypto.subtle.generatekey( { name: "aes-gcm", length: 256, }, true, ["encrypt"...
...y(keymaterial, salt) { return window.crypto.subtle.derivekey( { "name": "pbkdf2", salt: salt, "iterations": 100000, "hash": "sha-256" }, keymaterial, { "name": "aes-gcm", "length": 256}, true, [ "wrapkey", "unwrapkey" ] ); } /* wrap the given key. */ async function wrapcryptokey(keytowrap) { // get the key encryption key const keymaterial = await getkeymaterial(); salt = window.crypto.getrandomvalues(new uint8array(16)); const wrappingkey = await getkey(keymaterial, salt); iv = window.crypto.getrandomvalues(new uint8array(12)); return window.crypto.subtle.wrapkey( "pkcs8", keytowrap, wrappingkey, { name: "aes-gcm", iv: iv } ); } /* generate a sign/verify key pair, then wrap the private key. */ ...
...y(keymaterial, salt) { return window.crypto.subtle.derivekey( { "name": "pbkdf2", salt: salt, "iterations": 100000, "hash": "sha-256" }, keymaterial, { "name": "aes-cbc", "length": 256}, true, [ "wrapkey", "unwrapkey" ] ); } /* wrap the given key. */ async function wrapcryptokey(keytowrap) { // get the key encryption key const keymaterial = await getkeymaterial(); salt = window.crypto.getrandomvalues(new uint8array(16)); const wrappingkey = await getkey(keymaterial, salt); iv = window.crypto.getrandomvalues(new uint8array(16)); return window.crypto.subtle.wrapkey( "spki", keytowrap, wrappingkey, { name: "aes-cbc", iv: iv } ); } /* generate an encrypt/decrypt key pair, then wrap it. */ window.cr...
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Introduction to the Real-time Transport Protocol (RTP) - Web APIs
ccess this gives you to the internals of streams to make adjustments. among the simplest things you can do is to implement a "hold" feature, wherein a participant in a call can click a button and turn off their microphone, begin sending music to the other peer instead, and stop accepting incoming audio. note: this example makes use of modern javascript features including async functions and the await expression. this enormously simplifies and makes far more readable the code dealing with the promises returned by webrtc methods. in the examples below, we'll refer to the peer which is turning "hold" mode on and off as the local peer and the user being placed on hold as the remote peer. activating hold mode local peer when the local user decides to enable hold mode, the enablehold() meth...
...fer to the peer which is turning "hold" mode on and off as the local peer and the user being placed on hold as the remote peer. activating hold mode local peer when the local user decides to enable hold mode, the enablehold() method below is called. it accepts as input a mediastream containing the audio to play while the call is on hold. async function enablehold(audiostream) { try { await audiotransceiver.sender.replacetrack(audiostream.getaudiotracks()[0]); audiotransceiver.receiver.track.enabled = false; audiotransceiver.direction = "sendonly"; } catch(err) { /* handle the error */ } } the three lines of code within the try block perform the following steps: replace their outgoing audio track with a mediastreamtrack containing hold music. disable the incomin...
...ne audio, can come from anywhere. one possibility is to have a hidden <audio> element and use htmlaudioelement.capturestream() to get its audio stream. remote peer on the remote peer, when we receive an sdp offer with the directionality set to "sendonly", we handle it using the holdrequested() method, which accepts as input an sdp offer string. async function holdrequested(offer) { try { await peerconnection.setremotedescription(offer); await audiotransceiver.sender.replacetrack(null); audiotransceiver.direction = "recvonly"; await sendanswer(); } catch(err) { /* handle the error */ } } the steps taken here are: set the remote description to the specified offer by calling rtcpeerconnection.setremotedescription(). replace the audio transceiver's rtcrtpsender's ...
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Web Locks API - Web APIs
d indexed db, each tab could try to acquire a "my_net_db_sync" lock, but only one tab will succeed (the leader election pattern.) the api is used as follows: the lock is requested. work is done while holding the lock in an asynchronous task. the lock is automatically released when the task completes. navigator.locks.request('my_resource', async lock => { // the lock has been acquired. await do_something(); await do_something_else(); // now the lock will be released. }); while a lock is held, requests for the same lock from this execution context, or from other tabs/workers, will be queued. the first queued request will be granted only when the lock is released. the api provides optional functionality that may be used as needed, including: returning values from the asynchro...
... and a callback. the callback is invoked when the lock is granted. the lock is automatically released when the callback returns, so usually the callback is an async function, which causes the lock to be released only when the async function has completely finished. ...
...ock is automatically released when the callback returns, so usually the callback is an async function, which causes the lock to be released only when the async function has completely finished. the request() method itself returns a promise which resolves once the lock has been released; within an async function, a script can await the call to make the asynchronous code flow linear. for example: await do_something_without_lock(); // request the lock. ...
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Index - Learn web development
s. every time a web page does more than just sit there and display static information for you to look at—displaying timely content updates, interactive maps, animated 2d/3d graphics, scrolling video jukeboxes, or more—you can bet that javascript is probably involved. ...
...we'll probably add to this resource as time goes on. ...
...pt, learn, promises, async, asynchronous, catch, finally, then promises are a comparatively new feature of the javascript language that allow you to defer further actions until after a previous action has completed, or respond to its failure. ...
...ingscripting, guide, introducing, javascript, learn, promises, async, asynchronous, await, callbacks in this article we briefly recap the problems associated with synchronous javascript, and take a first look at some of the different asynchronous techniques you'll encounter, showing how they can help us solve such problems. 55 making asynchronous programming easier with async and await beginner, codingscripting, guide, javascript, learn, promises, async, asynchronous, await more recent additions to the javascript language are async functions and the await keyword, part of the so-called ecmascript 2017 javascript edition (see ecmascript next support in mozilla). these features basically act as syntactic sugar on top of promises, making asynchronous code easier to write...

ReadableStreamDefaultReader.read() - Web APIs
return reader.read().then(processtext); }); } example 2 - handling text line by line this example shows how you might fetch a text file and handle it as a stream of text lines. it deals with stream chunks not ending on line boundaries and converting from uint8array to strings. ...
...ws how you might fetch a text file and handle it as a stream of text lines. it deals with stream chunks not ending on line boundaries and converting from uint8array to strings. ...
...se.body.getreader(); let {value: chunk, done: readerdone} = await reader.read(); chunk = chunk ? utf8decoder.decode(chunk) : ""; let re = /\r\n|\n|\r/gm; let startindex = 0; let result; for (;;) { let result = re.exec(chunk); if (!result) { if (readerdone) { break; } let remainder = chunk.substr(startindex); ({value: chunk, done: readerdone} = await reader.read()); chunk = remainder + (chunk ? ...
...{value: chunk, done: readerdone} = await reader.read()); chunk = remainder + (chunk ? utf8decoder.decode(chunk) : ""); startindex = re.lastindex = 0; continue; } yield chunk.substring(startindex, result.index); startindex = re.lastindex; } if (startindex < chunk.length) { // last line didn't end in a newline char yield chunk.substr(startindex); } } for await (let line of maketextfilelineiterator(urloffile)) { processline(line); } specifications specification status comment streamsthe definition of 'read()' in that specification. living standard initial definition. ...

Using the Screen Capture API - Web APIs
n sharing on browsers that support it but do not implement the current standard api. this works with at least chrome, edge, and firefox. capturing screen contents capturing screen contents as a live mediastream is initiated by calling navigator.mediadevices.getdisplaymedia(), which returns a promise that resolves to a stream containing the live screen contents. ...
...e, and firefox. capturing screen contents capturing screen contents as a live mediastream is initiated by calling navigator.mediadevices.getdisplaymedia(), which returns a promise that resolves to a stream containing the live screen contents. starting screen capture: async/await style async function startcapture(displaymediaoptions) { let capturestream = null; try { capturestream = await navigator.mediadevices.getdisplaymedia(displaymediaoptions); } catch(err) { console.error("error: " + err); } return capturestream; } you can write this code either using an asynchronous function and the await operator, as shown above, or using the promise directly, as seen below. starting screen capture: promise style function startcapture(displaymediaoptions) { let capturestream ...
...ole.error(), and so on to log information to the log box in the document. starting display capture the startcapture() method, below, starts the capture of a mediastream whose contents are taken from a user-selected area of the screen. startcapture() is called when the "start capture" button is clicked. ...
...upon resolution, the promise returns a mediastream, which will stream the contents of the screen, window, or other region selected by the user. the stream is connected to the <video> element by storing the returned mediastream into the element's srcobject. ...

SubtleCrypto.deriveKey() - Web APIs
name: "ecdh", public: publickey }, privatekey, { name: "aes-gcm", length: 256 }, false, ["encrypt", "decrypt"] ); } async function agreesharedsecretkey() { // generate 2 ecdh key pairs: one for alice and one for bob // in more normal usage, they would generate their key pairs // separately and exchange public keys securely let aliceskeypair = await window.crypto.subtle.generatekey( { name: "ecdh", namedcurve: "p-384" }, false, ["derivekey"] ); let bobskeypair = await window.crypto.subtle.generatekey( { name: "ecdh", namedcurve: "p-384" }, false, ["derivekey"] ); // alice then generates a secret key using her private key and bob's public key. let alicessecretkey = await deri...
...ubtle.generatekey( { name: "ecdh", namedcurve: "p-384" }, false, ["derivekey"] ); // alice then generates a secret key using her private key and bob's public key. let alicessecretkey = await derivesecretkey(aliceskeypair.privatekey, bobskeypair.publickey); // bob generates the same secret key using his private key and alice's public key. let bobssecretkey = await derivesecretkey(bobskeypair.privatekey, aliceskeypair.publickey); // alice can then use her copy of the secret key to encrypt a message to bob. let encryptbutton = document.queryselector(".ecdh .encrypt-button"); encryptbutton.addeventlistener("click", () => { encrypt(alicessecretkey); }); // bob can use his copy to decrypt the message. let decryptbutton = document.queryselector...
...he derivekey method. the key material is a password supplied by the user. ...
...l is a password supplied by the user. ...

SubtleCrypto.exportKey() - Web APIs
unknown format. typeerror raised when trying to use an invalid format. examples note: you can try the working examples out on github. raw export this example exports an aes key as an arraybuffer containing the bytes for the key. see the complete code on github. /* export the given key and write it into the "exported-key" space. */ async function exportcryptokey(key) { const exported = await window.crypto.subtle.exportkey( "raw", key ); const exportedkeybuffer = new uint8array(exported); const exportkeyoutput = document.queryselector(".exported-key"); exportkeyoutput.textcontent = `[${exportedkeybuffer}]`; } /* generate an encrypt/decrypt secret key, then set up an event listener on the "export" button. */ window.crypto.subtle.generatekey( { name: "aes-gcm", ...
...ed. see the complete code on github. /* convert an arraybuffer into a string from https://developers.google.com/web/updates/2012/06/how-to-convert-arraybuffer-to-and-from-string */ function ab2str(buf) { return string.fromcharcode.apply(null, new uint8array(buf)); } /* export the given key and write it into the "exported-key" space. ...
...ct. see the complete code on github. /* convert an arraybuffer into a string from https://developers.google.com/web/updates/2012/06/how-to-convert-arraybuffer-to-and-from-string */ function ab2str(buf) { return string.fromcharcode.apply(null, new uint8array(buf)); } /* export the given key and write it into the "exported-key" space. ...
...tbutton = document.queryselector(".spki"); exportbutton.addeventlistener("click", () => { exportcryptokey(keypair.publickey); }); }); json web key import this code exports an ecdsa private signing key as a json web key object. see the complete code on github. /* export the given key and write it into the "exported-key" space. */ async function exportcryptokey(key) { const exported = await window.crypto.subtle.exportkey( "jwk", key ); const exportkeyoutput = document.queryselector(".exported-key"); exportkeyoutput.textcontent = json.stringify(exported, null, " "); } /* generate a sign/verify key pair, then set up an event listener on the "export" button. */ window.crypto.subtle.generatekey( { name: "ecdsa", namedcurve: "p-384" }, true, ["sign", "veri...

SubtleCrypto.sign() - Web APIs
the complete source code on github. /* fetch the contents of the "message" textbox, and encode it in a form we can use for the sign operation. */ function getmessageencoding() { const messagebox = document.queryselector(".rsassa-pkcs1 #message"); let message = messagebox.value; let enc = new textencoder(); return enc.encode(message); } let encoded = getmessageencoding(); let signature = await window.crypto.subtle.sign( "rsassa-pkcs1-v1_5", privatekey, encoded ); rsa-pss this code fetches the contents of a text box, encodes it for signing, and signs it with a private key. see the complete source code on github. /* fetch the contents of the "message" textbox, and encode it in a form we can use for the sign operation. */ function getmessageencoding() { const messagebox = docu...
... see the complete source code on github. /* fetch the contents of the "message" textbox, and encode it in a form we can use for the sign operation. */ function getmessageencoding() { const messagebox = document.queryselector(".rsa-pss #message"); let message = messagebox.value; let enc = new textencoder(); return enc.encode(message); } let encoded = getmessageencoding(); let signature = await window.crypto.subtle.sign( { name: "rsa-pss", saltlength: 32, }, privatekey, encoded ); ecdsa this code fetches the contents of a text box, encodes it for signing, and signs it with a private key. see the complete source code on github. /* fetch the contents of the "message" textbox, and encode it in a form we can use for the sign operation. */ function getmessageencoding() { ...
...y. see the complete source code on github. /* fetch the contents of the "message" textbox, and encode it in a form we can use for the sign operation. */ function getmessageencoding() { const messagebox = document.queryselector(".ecdsa #message"); let message = messagebox.value; let enc = new textencoder(); return enc.encode(message); } let encoded = getmessageencoding(); let signature = await window.crypto.subtle.sign( { name: "ecdsa", hash: {name: "sha-384"}, }, privatekey, encoded ); hmac this code fetches the contents of a text box, encodes it for signing, and signs it with a secret key. see the complete source code on github. /* fetch the contents of the "message" textbox, and encode it in a form we can use for the sign operation. */ function getmessageencoding...
...ey. see the complete source code on github. /* fetch the contents of the "message" textbox, and encode it in a form we can use for the sign operation. */ function getmessageencoding() { const messagebox = document.queryselector(".hmac #message"); let message = messagebox.value; let enc = new textencoder(); return enc.encode(message); } let encoded = getmessageencoding(); let signature = await window.crypto.subtle.sign( "hmac", key, encoded ); specifications specification status comment web cryptography apithe definition of 'subtlecrypto.sign()' in that specification. recommendation initial definition. ...

SubtleCrypto.unwrapKey() - Web APIs
erial() { let password = window.prompt("enter your password"); let enc = new textencoder(); return window.crypto.subtle.importkey( "raw", enc.encode(password), {name: "pbkdf2"}, false, ["derivebits", "derivekey"] ); } /* derive an aes-kw key using pbkdf2. */ async function getunwrappingkey() { // 1. get the key material (user-supplied password) const keymaterial = await getkeymaterial(); // 2 initialize the salt parameter. // the salt must match the salt originally used to derive the key. // in this example it's supplied as a constant "saltbytes". const saltbuffer = bytestoarraybuffer(saltbytes); // 3 derive the key from key material and salt return window.crypto.subtle.derivekey( { "name": "pbkdf2", salt: saltbuffer, "iteration...
...keymaterial, { "name": "aes-kw", "length": 256}, true, [ "wrapkey", "unwrapkey" ] ); } /* unwrap an aes secret key from an arraybuffer containing the raw bytes. takes an array containing the bytes, and returns a promise that will resolve to a cryptokey representing the secret key. */ async function unwrapsecretkey(wrappedkey) { // 1. get the unwrapping key const unwrappingkey = await getunwrappingkey(); // 2. initialize the wrapped key const wrappedkeybuffer = bytestoarraybuffer(wrappedkey); // 3. unwrap the key return window.crypto.subtle.unwrapkey( "raw", // import format wrappedkeybuffer, // arraybuffer representing key to unwrap unwrappingkey, // cryptokey representing key encryption key "aes-kw", // algori...
...rial() { let password = window.prompt("enter your password"); let enc = new textencoder(); return window.crypto.subtle.importkey( "raw", enc.encode(password), {name: "pbkdf2"}, false, ["derivebits", "derivekey"] ); } /* derive an aes-gcm key using pbkdf2. */ async function getunwrappingkey() { // 1. get the key material (user-supplied password) const keymaterial = await getkeymaterial(); // 2 initialize the salt parameter. // the salt must match the salt originally used to derive the key. // in this example it's supplied as a constant "saltbytes". const saltbuffer = bytestoarraybuffer(saltbytes); // 3 derive the key from key material and salt return window.crypto.subtle.derivekey( { "name": "pbkdf2", salt: saltbuffer, "iteration...
... { "name": "aes-gcm", "length": 256}, true, [ "wrapkey", "unwrapkey" ] ); } /* unwrap an rsa-pss private signing key from an arraybuffer containing the raw bytes. takes an array containing the bytes, and returns a promise that will resolve to a cryptokey representing the private key. */ async function unwrapprivatekey(wrappedkey) { // 1. get the unwrapping key const unwrappingkey = await getunwrappingkey(); // 2. initialize the wrapped key const wrappedkeybuffer = bytestoarraybuffer(wrappedkey); // 3. initialize the iv const ivbuffer = bytestoarraybuffer(ivbytes); // 4. unwrap the key return window.crypto.subtle.unwrapkey( "pkcs8", // import format wrappedkeybuffer, // arraybuffer representing key to unwrap unwrappingkey, // cryp...

SubtleCrypto.verify() - Web APIs
coded message-to-sign and verify it against the stored signature. * if it checks out, set the "valid" class on the signature. * otherwise set the "invalid" class. */ async function verifymessage(publickey) { const signaturevalue = document.queryselector(".rsassa-pkcs1 .signature-value"); signaturevalue.classlist.remove("valid", "invalid"); let encoded = getmessageencoding(); let result = await window.crypto.subtle.verify( "rsassa-pkcs1-v1_5", publickey, signature, encoded ); signaturevalue.classlist.add(result ? "valid" : "invalid"); } rsa-pss this code uses a public key to verify a signature. see the complete code on github. /* fetch the contents of the "message" textbox, and encode it in a form we can use for sign operation. */ function getmessageencoding() { ...
...he encoded message-to-sign and verify it against the stored signature. * if it checks out, set the "valid" class on the signature. * otherwise set the "invalid" class. */ async function verifymessage(publickey) { const signaturevalue = document.queryselector(".rsa-pss .signature-value"); signaturevalue.classlist.remove("valid", "invalid"); let encoded = getmessageencoding(); let result = await window.crypto.subtle.verify( { name: "rsa-pss", saltlength: 32, }, publickey, signature, encoded ); signaturevalue.classlist.add(result ? "valid" : "invalid"); } ecdsa this code uses a public key to verify a signature. see the complete code on github. /* fetch the contents of the "message" textbox, and encode it in a form we can use for sign operation. */ f...
... the encoded message-to-sign and verify it against the stored signature. * if it checks out, set the "valid" class on the signature. * otherwise set the "invalid" class. */ async function verifymessage(publickey) { const signaturevalue = document.queryselector(".ecdsa .signature-value"); signaturevalue.classlist.remove("valid", "invalid"); let encoded = getmessageencoding(); let result = await window.crypto.subtle.verify( { name: "ecdsa", hash: {name: "sha-384"}, }, publickey, signature, encoded ); signaturevalue.classlist.add(result ? "valid" : "invalid"); } hmac this code uses a secret key to verify a signature. see the complete code on github. /* fetch the contents of the "message" textbox, and encode it in a form we can use for sign operation...
...tch the encoded message-to-sign and verify it against the stored signature. * if it checks out, set the "valid" class on the signature. * otherwise set the "invalid" class. */ async function verifymessage(key) { const signaturevalue = document.queryselector(".hmac .signature-value"); signaturevalue.classlist.remove("valid", "invalid"); let encoded = getmessageencoding(); let result = await window.crypto.subtle.verify( "hmac", key, signature, encoded ); signaturevalue.classlist.add(result ? "valid" : "invalid"); } specifications specification status comment web cryptography apithe definition of 'subtlecrypto.verify()' in that specification. recommendation initial definition. ...

WebGLRenderingContext.vertexAttribPointer() - Web APIs
will be stored in the array buffer as follows: 00 00 80 3f 00 00 00 40 00 00 0c 3f 7f 00 00 00 7f ff 3f ff creating the array buffer first, we dynamically create the array buffer from json data using a dataview. note the use of true because webgl expects our data to be in little-endian. ...
... 00 00 80 3f 00 00 00 40 00 00 0c 3f 7f 00 00 00 7f ff 3f ff creating the array buffer first, we dynamically create the array buffer from json data using a dataview. note the use of true because webgl expects our data to be in little-endian. ...
...al[2] * 0x7f); dv.setint8(20 * i + 15, 0); dv.setuint16(20 * i + 16, vertices[i].texcoord[0] * 0xffff, true); dv.setuint16(20 * i + 18, vertices[i].texcoord[1] * 0xffff, true); } for higher performance, we could also do the previous json to arraybuffer conversion on the server-side, e.g. with node.js. ...
...etuint16(20 * i + 16, vertices[i].texcoord[0] * 0xffff, true); dv.setuint16(20 * i + 18, vertices[i].texcoord[1] * 0xffff, true); } for higher performance, we could also do the previous json to arraybuffer conversion on the server-side, e.g. with node.js. ...

Using Promises - JavaScript
.catch() handlers or onrejected. this is very much modeled after how synchronous code works: try { const result = syncdosomething(); const newresult = syncdosomethingelse(result); const finalresult = syncdothirdthing(newresult); console.log(`got the final result: ${finalresult}`); } catch(error) { failurecallback(error); } this symmetry with asynchronous code culminates in the async/await syntactic sugar in ecmascript 2017: async function foo() { try { const result = await dosomething(); const newresult = await dosomethingelse(result); const finalresult = await dothirdthing(newresult); console.log(`got the final result: ${finalresult}`); } catch(error) { failurecallback(error); } } it builds on promises, e.g. dosomething() is the same function as befor...
...result3) */ timing to avoid surprises, functions passed to then() will never be called synchronously, even with an already-resolved promise: promise.resolve().then(() => console.log(2)); console.log(1); // 1, 2 instead of running immediately, the passed-in function is put on a micro...
...accept any number of functions as arguments, and will return a new function that accepts an initial value to be passed through the composition pipeline: const transformdata = composeasync(func1, func2, func3); const result3 = transformdata(data); in ecmascript 2017, sequential composition can be done more simply with async/await: let result; for (const f of [func1, func2, func3]) { result = await f(result); } /* use last result (i.e. result3) */ timing to avoid surprises, functions passed to then() will never be called synchronously, even with an already-resolved promise: promise.resolve().then(() => console.log(2)); console.log(1); // 1, 2 instead of running immediately, the passed-in function is put on a microtask queue, which means it runs later when the queue is emptied at the e...
... soon as you get a new promise, return it immediately, to flatten things: dosomething() .then(function(result) { return dosomethingelse(result); }) .then(newresult => dothirdthing(newresult)) .then(() => dofourththing()) .catch(error => console.error(error)); note that () => x is short for () => { return x; }. now we have a single deterministic chain with proper error handling. using async/await addresses most, if not all of these problems—the tradeoff being that the most common mistake with that syntax is forgetting the await keyword. when promises and tasks collide if you run into situations in which you have promises and tasks (such as events or callbacks) which are firing in unpredictable orders, it's possible you may benefit from using a microtask to check status or balance out ...

import - JavaScript
name as short } from '/modules/my-module.js'; import a module for its side effects only import an entire module for side effects only, without importing anything. this runs the module's global code, but doesn't actually import any values. import '/modules/my-module.js'; this works with dynamic imports as well: (async () => { if (somethingistrue) { // import module for side effects await import('/modules/my-module.js'); } })(); if your project uses packages that export esm, you can also import them for side effects only. this will run the code in the package entry point file (and any files it imports) only. importing defaults it is possible to have a default export (whether it is an object, a function, a class, etc.). the import statement may then be used to import such def...
...om '/modules/my-module.js'; // mymodule used as a namespace or import mydefault, {foo, bar} from '/modules/my-module.js'; // specific, named imports when importing a default export with dynamic imports, it works a bit differently. you need to destructure and rename the "default" key from the returned object. (async () => { if (somethingistrue) { const { default: mydefault, foo, bar } = await import('/modules/my-module.js'); } })(); dynamic imports the standard import syntax is static and will always result in all code in the imported module being evaluated at load time. in situations where you wish to load a module conditionally or on demand, you can use a dynamic import instead. the following are some reasons why you might need to use dynamic import: when importing statically...
...necessary. the static form is preferable for loading initial dependencies, and can benefit more readily from static analysis tools and tree shaking. to dynamically import a module, the import keyword may be called as a function. when used this way, it returns a promise. import('/modules/my-module.js') .then((module) => { // do something with the module. }); this form also supports the await keyword. let module = await import('/modules/my-module.js'); examples standard import the code below shows how to import from a secondary module to assist in processing an ajax json request. the module: file.js function getjson(url, callback) { let xhr = new xmlhttprequest(); xhr.onload = function () { callback(this.responsetext) }; xhr.open('get', url, true); xhr.send(); } ...
...getusefulcontents } from '/modules/file.js'; getusefulcontents('http://www.example.com', data => { dosomethinguseful(data); }); dynamic import this example shows how to load functionality on to a page based on a user action, in this case a button click, and then call a function within that module. this is not the only way to implement this functionality. the import() function also supports await. const main = document.queryselector("main"); for (const link of document.queryselectorall("nav > a")) { link.addeventlistener("click", e => { e.preventdefault(); import('/modules/my-module.js') .then(module => { module.loadpageinto(main); }) .catch(err => { main.textcontent = err.message; }); }); } specifications specification ...

Audio for Web games - Game development
lavi-play-label">play</span> </button> </li> </ul> <p class="sourced">all tracks sourced from <a href="http://jplayer.org/">jplayer.org</a></p> </section> all of these tracks are the same tempo and are designed to be synchronized with each other, so we need to make sure they are loaded and available to the api before we are able to play them. we can do this with javascript's async/await functionality. once they are available to play, we need to make sure they start at the correct point that other tracks might be playing at, so they sync up. let's create our audio context: // for cross browser compatibility const audiocontext = window.audiocontext || window.webkitaudiocontext; const audioctx = new audiocontext(); now let's select all the <li> elements; later we can harness t...
...'s select all the <li> elements; later we can harness these elements to give us access to the track file path and each individual play button. const trackels = document.queryselectorall('li'); we want to make sure each file has loaded and been decoded into a buffer before we use it, so let's create an async function to allow us to do this: async function getfile(filepath) { const response = await fetch(filepath); const arraybuffer = await response.arraybuffer(); const audiobuffer = await audioctx.decodeaudiodata(arraybuffer); return audiobuffer; } we can then use the await operator when calling this function, which ensures that we can run subsequent code when it has finished executing. let's create another async function to set up the sample — we can combine the two async funct...
...we need an offset here, so if we have started one file playing, we have a record of how far through to start another file. start() takes two optional parameters. ...

ClipboardItem - Web APIs
a blob of the requested mime type, or an error if the mime type is not found. examples writing to clipboard here we're writing a new clipboarditem.clipboarditem() to the clipboard by requesting a png image using the fetch api, and in turn, the responses' blob() method, to create the new clipboarditem. async function writeclipimg() { try { const imgurl = '/myimage.png'; const data = await fetch(imgurl); const blob = await data.blob(); await navigator.clipboard.write([ new clipboarditem({ [blob.type]: blob }) ]); console.log('fetched image copied.'); } catch(err) { console.error(err.name, err.message); } } reading from the clipboard here we're returning all items on the clipboard via the clipboard.read() method. then utilizing the cli...
...g('fetched image copied.'); } catch(err) { console.error(err.name, err.message); } } reading from the clipboard here we're returning all items on the clipboard via the clipboard.read() method. then utilizing the clipboarditem.types property to set the gettype() argument and return the corresponding blob object. ...
...n the clipboard via the clipboard.read() method. then utilizing the clipboarditem.types property to set the gettype() argument and return the corresponding blob object. async function getclipboardcontents() { try { const clipboarditems = await navigator.clipboard.read(); for (const clipboarditem of clipboarditems) { for (const type of clipboarditem.types) { const blob = await clipboarditem.gettype(type); // we can now use blob here } } } catch (err) { console.error(err.name, err.message); } } specifications specification status comment clipboard api and eventsthe definition of 'clipboarditem' in that specification. working draft initial definition. ...

ConvolverNode - Web APIs
the following example shows basic usage of an audiocontext to create a convolver node. note: you will need to find an impulse response to complete the example below. see this codepen for an applied example. ...
...e a convolver node. note: you will need to find an impulse response to complete the example below. see this codepen for an applied example. ...
...impulse response to complete the example below. see this codepen for an applied example. let audioctx = new window.audiocontext(); async function createreverb() { let convolver = audioctx.createconvolver(); // load impulse response from file let response = await fetch("path/to/impulse-response.wav"); let arraybuffer = await response.arraybuffer(); convolver.buffer = await audioctx.decodeaudiodata(arraybuffer); return convolver; } ... let reverb = await createreverb(); // someotheraudionode -> reverb -> destination someotheraudionode.connect(reverb); reverb.connect(audioctx.destination); specifications specification status comment web audio apithe definition of 'convolvernode' in that specification. working draft ...

PaymentAddress - Web APIs
: "standard", label: "standard shipping", amount: { currency: "usd", value: "0.00" }, selected: true, }, ], }; const options = { requestshipping: true }; async function dopaymentrequest() { const request = new paymentrequest(supportedinstruments, details, options); // add event listeners here. // call show() to trigger the browser's payment flow. ...
... selected: true, }, ], }; const options = { requestshipping: true }; async function dopaymentrequest() { const request = new paymentrequest(supportedinstruments, details, options); // add event listeners here. // call show() to trigger the browser's payment flow. const response = await request.show(); // process payment. ...
... const request = new paymentrequest(supportedinstruments, details, options); // add event listeners here. // call show() to trigger the browser's payment flow. const response = await request.show(); // process payment. const json = response.tojson(); const httpresponse = await fetch("/pay/", { method: "post", body: json }); const result = httpresponse.ok ? "success" : "failure"; await response.complete(result); } dopaymentrequest(); once the payment flow has been triggered using paymentrequest.show() and the promise resolves successfully, the paymentresponse object available inside the fulfilled promise (instrumentresponse above) will have a paymentresponse.details property that will contain response details. this has to conform to the structure defined by the basiccardrespo...

PaymentRequest.canMakePayment() - Web APIs
nmakepayment() to determine if the browser will let the user pay using payment request api, and if it won't, you could fall back to another payment method, or offer a list of methods that aren't handled by payment request api (or even provide instructions for paying by mail or by phone). syntax paymentrequest.canmakepayment() .then( canpay => { ... }) .catch( error => { ... }) canpay = await paymentrequest.canmakepayment(); returns a promise to a boolean that resolves to true if the user agent supports any of the payment methods supplied when instantiating the request using the paymentrequest constructor. if the payment can't be processed, the promise receives a value of false. note: if you call this too often, the browser may reject the returned promise with a domexception. p...
...ack depending on the resolution of the promise. ...
... currency: "usd", value: "0.00", }, }, }; const supportsapplepay = new paymentrequest( [{ supportedmethods: "https://apple.com/apple-pay" }], details ).canmakepayment(); // supports apple pay? if (await supportsapplepay) { // show apple pay logo, for instance return; } // otherwise... let's see if we can use basic card const supportsbasiccard = await new paymentrequest( [{ supportedmethods: "basic-card" }], details ).canmakepayment(); if (supportsbasiccard) { // show basic card support return; } // otherwise, make payments using html form element } specifications specification status comment payment request apithe definition of 'canmakepayment()' in that specification. candidate recommen...

SubtleCrypto.deriveBits() - Web APIs
see the supported algorithms section of the derivekey() documentation. examples note: you can try the working examples on github. ecdh in this example alice and bob each generate an ecdh key pair. we then use alice's private key and bob's public key to derive a shared secret. see the complete code on github. ...
... the user. */ function getkeymaterial() { const password = window.prompt("enter your password"); const enc = new textencoder(); return window.crypto.subtle.importkey( "raw", enc.encode(password), {name: "pbkdf2"}, false, ["derivebits", "derivekey"] ); } /* derive some bits from a password supplied by the user. ...
... enc = new textencoder(); return window.crypto.subtle.importkey( "raw", enc.encode(password), {name: "pbkdf2"}, false, ["derivebits", "derivekey"] ); } /* derive some bits from a password supplied by the user. ...

SubtleCrypto.digest() - Web APIs
subtlecrypto.sign() instead. examples basic example this example encodes a message, then calculates its sha-256 digest and logs the digest length: const text = 'an obscure body in the s-k system, your majesty. the inhabitants refer to it as the planet earth.'; async function digestmessage(message) { const encoder = new textencoder(); const data = encoder.encode(message); const hash = await crypto.subtle.digest('sha-256', data); return hash; } const digestbuffer = await digestmessage(text); console.log(digestbuffer.bytelength); converting a digest to a hex string the digest is returned as an arraybuffer, but for comparison and display digests are often represented as hex strings. this example calculates a digest, then converts the arraybuffer to a hex string: const text = 'an...
...n represented as hex strings. this example calculates a digest, then converts the arraybuffer to a hex string: const text = 'an obscure body in the s-k system, your majesty. ...
... recommendation initial definition. ...

WebGL best practices - Web APIs
ct(); return; } if (res == gl.timeout_expired) { settimeout(test, interval_ms); return; } resolve(); } test()); }); } async function getbuffersubdataasync( gl, target, buffer, srcbyteoffset, dstbuffer, /* optional */ dstoffset, /* optional */ length) { const sync = gl.fencesync(gl.sync_gpu_commands_complete, 0); gl.flush(); await clientwaitasync(gl, sync, 0, 10); gl.deletesync(sync); gl.bindbuffer(target, buffer); gl.getbuffersubdata(target, srcbyteoffset, dstbuffer, dstoffset, length); gl.bindbuffer(target, null); return dest; } async function readpixelsasync(gl, x, y, w, h, format, type, dest) { const buf = gl.createbuffer(); gl.bindbuffer(gl.pixel_pack_buffer, buf); gl.bufferdata(gl.pixel_pack_buffer...
... use requestpostanimationframe not requestanimationframe while it's well-known that apps should use requestanimationframe ("raf") instead of settimeout (et al) to redraw on-demand, what's less well-known is...
...misc/webgl/device-pixel-presnap.html resizeobserver and 'device-pixel-content-box' on supporting browsers (chromium?), resizeobserver can be used with 'device-pixel-content-box' to request a callback that includes the true device pixel size of an element. ...

Compiling an Existing C Module to WebAssembly - WebAssembly
the first question you need to answer is: how do i get the image into wasm? looking at the encoding api of libwebp, you'll find that it expects an array of bytes in rgb, rgba, bgr or bgra. ...
...he image into wasm? looking at the encoding api of libwebp, you'll find that it expects an array of bytes in rgb, rgba, bgr or bgra. luckily, the canvas api has canvasrenderingcontext2d.getimagedata — that gives you an uint8clampedarray containing the image data in rgba: async function loadimage(src) { // load image const imgblob = await fetch(src).then(resp => resp.blob()); const img = await createimagebitmap(imgblob); // make canvas same size as image const canvas = document.createelement('canvas'); canvas.width = img.width; canvas.height = img.height; // draw image onto canvas const ctx = canvas.getcontext('2d'); ctx.drawimage(img, 0, 0); return ctx.getimagedata(0, 0, img.width, img.height); } now it's "only" a matter of copying the data from javascript into wasm. ...
...d, it is treated as just a number. after exposing the function to javascript using cwrap, you can use that number to find the start of our buffer and copy the image data: const api = { version: module.cwrap('version', 'number', []), create_buffer: module.cwrap('create_buffer', 'number', ['number', 'number']), destroy_buffer: module.cwrap('destroy_buffer', '', ['number']), }; const image = await loadimage('./image.jpg'); const p = api.create_buffer(image.width, image.height); module.heap8.set(image.data, p); // ... call encoder ... api.destroy_buffer(p); encode the image the image is now available in wasm. it is time to call the webp encoder to do its job. looking at the webp documentation, you'll find that webpencodergba seems like a perfect fit. the function takes a pointer to the in...

Introducing asynchronous JavaScript - Learn web development
ations can be made to run in order, one after another. the catch() block at the end runs if any of the .then() blocks fail — in a similar way to synchronous try...catch blocks, an error object is made available inside the catch(), which can be used to report the kind of error that has occurred. note however that synchronous try...catch won't work with promises, although it will work with async/await, as you'll learn later on. note: you'll learn a lot more about promises later on in the module, so don't worry if you don't understand them fully yet. the event queue async operations like promises are put into an event queue, which runs after the main thread has finished processing so that they do not block subsequent javascript code from running. the queued operations will complete as soo...
...rn when it makes more sense to choose an asynchronous technique over a synchronous one. previous overview: asynchronous next in this module general asynchronous programming concepts introducing asynchronous javascript cooperative asynchronous javascript: timeouts and intervals graceful asynchronous programming with promises making asynchronous programming easier with async and await choosing the right approach ...

Performance best practices for Firefox front-end engineers
s - however, bug 1434376 has recently landed some chromeonly helpers to the window binding to make this simpler. if you want to queue up some javascript to run after the next "natural" style and layout flush, try: // suppose we want to get the computed "display" style of some node without // causing a style flush. we could do it this way: async function nodeisdisplaynone(node) { let display = await window.promisedocumentflushed(() => { // do _not_ under any circumstances write to the dom in one of these // callbacks! return window.getcomputedstyle(node).display; }); return display == "none"; } see detecting and avoiding synchronous reflow for a more advanced example of getting layout information, and then setting it safely, without causing flushes. promisedocumentflush...
...that do so. if you require size or position information, you have a few options. bug 1434376 has recently landed a helper in the window binding to make it easier for priviledged code to queue up javascript to run when we know that the dom is not dirty, and size, position, and style information is cheap to query for. here's an example: async function matchwidth(elem, otherelem) { let width = await window.promisedocumentflushed(() => { // do _not_ under any circumstances write to the dom in one of these // callbacks! return elem.clientwidth; }); requestanimationframe(() => { otherelem.style.width = `${width}px`; }); } please see the section on promisedocumentflushed in detecting and avoiding synchronous style flushes for more information on how to use the api. note ...

Clients - Web APIs
rl and returns a promise for the new windowclient. clients.claim() allows an active service worker to set itself as the controller for all clients within its scope. examples the following example shows an existing chat window or creates a new one when the user clicks a notification. ...
... working draft initial definition ...

ClipboardItem() - Web APIs
itetext() methods of the clipboard interface. examples the below example requests a png image using the fetch api, and in turn, the responses' blob() method, to create a new clipboarditem and write it to the clipboard, using the clipboard api. note: you can only pass in one clipboard item at a time. ...
...erface. examples the below example requests a png image using the fetch api, and in turn, the responses' blob() method, to create a new clipboarditem and write it to the clipboard, using the clipboard api. note: you can only pass in one clipboard item at a time. async function writeclipimg() { try { const imgurl = '/myimage.png'; const data = await fetch(imgurl); const blob = await data.blob(); await navigator.clipboard.write([ new clipboarditem({ [blob.type]: blob }) ]); console.log('fetched image copied.'); } catch(err) { console.error(err.name, err.message); } } specifications specification status comment clipboard api and eventsthe definition of 'clipboarditem' in that specification. working draft ...

ClipboardItem.getType() - Web APIs
type. typeerror no parameter is specified or the type is not that of the clipboarditem. examples in the following example, we're returning all items on the clipboard via the clipboard.read() method. then utilizing the clipboarditem.types property to set the gettype() argument and return the corresponding blob object. ...
...n the clipboard via the clipboard.read() method. then utilizing the clipboarditem.types property to set the gettype() argument and return the corresponding blob object. async function getclipboardcontents() { try { const clipboarditems = await navigator.clipboard.read(); for (const clipboarditem of clipboarditems) { for (const type of clipboarditem.types) { const blob = await clipboarditem.gettype(type); // we can now use blob here } } } catch (err) { console.error(err.name, err.message); } } specifications specification status comment clipboard api and eventsthe definition of 'clipboarditem' in that specification. working draft initial definition. ...

ClipboardItem.types - Web APIs
. examples in the below example, we're returning all items on the clipboard via the clipboard.read() method. then checking the types property for available types before utilizing the clipboarditem.gettype() method to return the blob object. if no clipboards contents is found for the specified type, an error is returned. ...
...le types before utilizing the clipboarditem.gettype() method to return the blob object. if no clipboards contents is found for the specified type, an error is returned. async function getclipboardcontents() { try { const clipboarditems = await navigator.clipboard.read(); for (const clipboarditem of clipboarditems) { for (const type of clipboarditem.types) { const blob = await clipboarditem.gettype(type); // we can now use blob here } } } catch (err) { console.error(err.name, err.message); } } specifications specification status comment clipboard api and eventsthe definition of 'clipboarditem' in that specification. working draft initial definition. ...

ContentIndex.add() - Web APIs
// our content const item = { id: 'post-1', url: '/posts/amet.html', ...
... // our content const item = { id: 'post-1', url: '/posts/amet.html', title: 'amet consectetur adipisicing', description: 'repellat et quia iste possimus ducimus aliquid a aut eaque nostrum.', icons: [{ src...

ContentIndex.getAll() - Web APIs
exceptions no exceptions are thrown. if there are no items in the content index, an empty array is returned. examples the below example shows an asynchronous function that retrieves items within the content index and iterates over each entry, building a list for the interface. ...
...ntent index, an empty array is returned. examples the below example shows an asynchronous function that retrieves items within the content index and iterates over each entry, building a list for the interface. ...

HTMLImageElement.complete - Web APIs
's worth noting that due to the image potentially being received asynchronously, the value of complete may change while your script is running. examples consider a photo library app that provides the ability to open images into a lightbox mode for improved viewing as well as editing of the image. these photos may be very large, so you don't want to wait for them to load, so your code uses async/await to load the images in the background. but imagine that you have other code that needs to only run when the image has completed loading, such as a command that performs red-eye removal on the image in the lightbox. while ideally this command wouldn't even be executed if the image hasn't fully loaded, for improved reliability you want to check to ensure this is the case. so the fixredeyecommand()...
...*/ function fixredeyecommand() { if (lightboxelem.style.display === "block" && lightboximgelem.complete) { fixredeye(lightboximgelem); } else { /* can't start doing this until the image is fully loaded */ } } specifications specification status com...

HTMLMediaElement.setSinkId() - Web APIs
ication is authorized to use the specified device. syntax htmlmediaelement.setsinkid(sinkid).then(function() { ... }) returns a promise that resolves to undefined. parameters sinkid the mediadeviceinfo.deviceid of the audio output device. ...
...eters sinkid the mediadeviceinfo.deviceid of the audio output device. exceptions exception explanation domexception no permission to use the requested device examples const devices = await navigator.mediadevices.enumeratedevices(); const audiodevices = devices.filter(device => device.kind === 'audiooutput'); const audio = document.createelement('audio'); await audio.setsinkid(audiodevices[0].deviceid); console.log('audio is being played on ' + audio.sinkid); specifications specification status comment audio output devices apithe definition of 'sinkid' in that specification. candidate recommendation initial definition. older versions of this spec were called "media capture output". ...

HTMLMediaElement.srcObject - Web APIs
patibility table for what is actually supported). usage notes older versions of the media source specification required using createobjecturl() to create an object url then setting src to that url. now you can just set srcobject to the mediastream directly. examples basic example in this example, a mediastream from a camera is assigned to a newly-created <video> element. const mediastream = await navigator.mediadevices.getusermedia({video: true}); const video = document.createelement('video'); video.srcobject = mediastream; in this example, a new mediasource is assigned to a newly-created <video> element. const mediasource = new mediasource(); const video = document.createelement('video'); video.srcobject = mediasource; supporting fallback to the src property the examples below suppor...
...onst video = document.createelement('video'); video.srcobject = mediasource; supporting fallback to the src property the examples below support older browser versions that require you to create an object url and assign it to src if srcobject isn't supported. first, a mediastream from a camera is assigned to a newly-created <video> element, with fallback for older browsers. const mediastream = await navigator.mediadevices.getusermedia({video: true}); const video = document.createelement('video'); if ('srcobject' in video) { video.srcobject = mediastream; } else { // avoid using this in new browsers, as it is going away. video.src = url.createobjecturl(mediastream); } second, a new mediasource is assigned to a newly-created <video> element, with fallback for older browsers and browser...

MediaRecorder: error event - Web APIs
s attempted using an unsupported codec. bubbles no cancelable no interface mediarecordererrorevent event handler property onerror for details of the all the possible errors see the documentation for the event handler property: onerror. ...
...ord() { const stream = await navigator.mediadevices.getusermedia({audio: true}); const recorder = new mediarecorder(stream); recorder.addeventlistener('error', (event) => { console.error(`error recording stream: ${event.error.name}`) }); recorder.start(); } record(); the same, but using the onerror event handler property: async function record() { const stream = await navigator.mediadevices.getusermedia({audio: true}); const recorder = new mediarecorder(stream); recorder.onerror = (event) => { console.error(`error recording stream: ${event.error.name}`) }; recorder.start(); } record(); specifications specification status mediastream recording working draft ...

NavigationPreloadManager - Web APIs
tate() returns a promise that resolves to an object with properties indicating whether preload is enabled and the contents of the service-worker-navigation-preload. ...
...ationpreload) { // enable navigation preloads! await self.registration.navigationpreload.enable(); } }()); }); using a preloaded response the following example shows the implementation of a fetch event that uses a preloaded response. addeventlistener('fetch', event => { event.respondwith(async function() { // respond from the cache if we can const cachedresponse = await caches.match(event.request); if (cachedresponse) return cachedresponse; // else, use the preloaded response, if it's there const response = await event.preloadresponse; if (response) return response; // else try the network. return fetch(event.request); }()); }); specifications specification status comment service workersthe definition of 'nav...

PaymentRequest: merchantvalidation event - Web APIs
alidation url in accordance with the payment method documention. typically, a client should not access the validation url. request.addeventlistener("merchantvalidation", event => { event.complete(async () => { const merchantserverurl = window.location.origin + '/validate?url=' + encodeuricomponent(event.validationurl); // get validation data, and complete validation; return await fetch(merchantserverurl).then(response => response.text()); }, false); }; const response = await request.show(); how merchant server handles the validation depends on the server implementation and payment method documentation. the content delivered by the validation server is forwarded to the merchant server and is then returned from the fetch() call's fulfillment handler to the complete() me...
...know if the merchant is validated. you can also use the onmerchantvalidation event handler property to set up the handler for this event: request.onmerchantvalidation = event => { event.complete(async () => { const merchantserverurl = window.location.origin + '/validate?url=' + encodeuricomponent(event.validationurl); // get validation data, and complete validation; return await fetch(merchantserverurl).then(response => response.text()); }); }; const response = await request.show(); for more information, see merchant validation in payment processing concepts. related events payerdetailchange, paymentmethodchange, shippingaddresschange, and shippingoptionchange specifications specification status comment payment request apithe definition...

PaymentRequest.shippingOption - Web APIs
cand...
... candidate recommendation initial definition. browser compatibi...

PaymentResponse.onpayerdetailchange - Web APIs
ame = payername; } if (oldpayeremail !== payeremail) { promisestovalidate.push(validateemail(payeremail)); oldpayeremail = payeremail; } if (oldpayerphone !== payerphone) { promisestovalidate.push(validatephone(payerphone)); oldpayerphone = payerphone; } // as each validation promise resolves, add the results of the // validation to the errors list const errors = await promise.all(promisestovalidate).then(results => results.reduce((errors, result), object.assign(errors, result)) ); // if we found any errors, wait for them to be corrected if (object.getownpropertynames(errors).length) { await response.retry(errors); } else { // we have a good payment; send the data to the server await fetch("/pay-for-things/", { method: "post", body: re...
... candidate recommendation initial definition. ...

PaymentResponse: payerdetailchange event - Web APIs
ame = payername; } if (oldpayeremail !== payeremail) { promisestovalidate.push(validateemail(payeremail)); oldpayeremail = payeremail; } if (oldpayerphone !== payerphone) { promisestovalidate.push(validatephone(payerphone)); oldpayerphone = payerphone; } // as each validation promise resolves, add the results of the // validation to the errors list const errors = await promise.all(promisestovalidate).then(results => results.reduce((errors, result), object.assign(errors, result)) ); // if we found any errors, wait for them to be corrected if (object.getownpropertynames(errors).length) { await response.retry(errors); } else { // we have a good payment; send the data to the server await fetch("/pay-for-things/", { method: "post", body: re...
...} specifications specification status comment payment request apithe definition of 'payerdetailchang...

RTCPeerConnection.addTrack() - Web APIs
, you don't need to deal with managing what track is in what stream, so you might as well just let the transceiver handle it for you. here's an example showing a function that uses getusermedia() to obtain a stream from a user's camera and microphone, then adds each track from the stream to the peer connection, without specifying a stream for each track: async opencall(pc) { const gumstream = await navigator.mediadevices.getusermedia( {video: true, audio: true}); for (const track of gumstream.gettracks()) { pc.addtrack(track); } } the result is a set of tracks being sent to the remote peer, with no stream associations. the handler for the track event on the remote peer will be responsible for determining what stream to add each track to, even if that means...
...k associations are automatically managed for you by the webrtc infrastructure. this includes things like changes to the transceiver's direction and tracks being halted using removetrack(). ...

RTCPeerConnection.setLocalDescription() - Web APIs
scriptions one of the advantages of the parameter-free form of setlocaldescription() is that it lets you simplify your negotiation code substantially. this is all your negotiationneeded event handler needs to look like, for the most part. just add the signaling server code, which here is represented by the call to signalremotepeer(). pc.addeventlistener("negotiationneeded", async (event) => { await pc.setlocaldescription(); signalremotepeer({ description: pc.localdescription }); }); other than error handling, that's about it! providing your own offer or answer the example below shows the implementation of a handler for the negotiationneeded event that explicitly creates an offer, rather than letting setlocaldescription() do it. async function handlenegotiationneededevent() { try { ...
...setlocaldescription(); signalremotepeer({ description: pc.localdescription }); }); other than error handling, that's about it! providing your own offer or answer the example below shows the implementation of a handler for the negotiationneeded event that explicitly creates an offer, rather than letting setlocaldescription() do it. async function handlenegotiationneededevent() { try { await offer = pc.createoffer(); pc.setlocaldescription(offer); signalremotepeer({ description: pc.localdescription }); } catch(err) { reporterror(err); } } this begins by creating an offer by calling createoffer(); when that succeeds, we call setlocaldescription(). we can then send the newly-created offer along to the other peer using the signaling server, which here is done by calling...

RTCRemoteOutboundRtpStreamStats.localId - Web APIs
and the second, networkteststop(), captures a second report, then uses the two reports to output some information about the network conditions... xxx ... networkteststart() this function simply calls the rtcpeerconnection method getstats() to request an rtcstatsreport and store it in the variable startreport. let startreport; async function networkteststart(pc) { if (pc) { startreport = await pc.getstats(); } } given an rtcpeerconnection, pc, this calls its getstats() method to obtain a statistics report object, which it stores in startreport for use once the end-of-test data has been collected by networkteststop(). networkteststop() the networkteststop() function obtains a second report, endreport, then uses the two reports together to determine several... xxx ... finding paire...
...being tested, calls getstats() to get a new rtcstatsreport with current statistics, then computes the results it's looking for, outputting those results as appropriate to the user by appending appropriate html to the contents of the <div> element whose class is stats-box. ...

Screen Wake Lock API - Web APIs
wing example demonstrates how to request a wakelocksentinel object. the wakelock.request method is promise based and so we can create an asynchronous function, which in turn updates the ui to reflect the wake lock is active. ...
...ase', () => { // the wake lock has been released statuselem.textcontent = 'wake lock has been released'; }); reacquiring a wake lock the following code reacquires the wake lock should the visibility of the document change and the wake lock is released. document.addeventlistener('visibilitychange', () => { if (wakelock !== null && document.visibilitystate === 'visible') { wakelock = await navigator.wakelock.request('screen'); } }); putting it all together you can find the complete code on github here. the demo uses a button to acquire a wake lock and also release it, which in turn updates the ui. the ui also updates if the wake lock is released automatically for any reason. there's a checkbox which when checked, will automatically reacquire the wake lock if the document's visi...

USBDevice.clearHalt() - Web APIs
tnumber indicates the number of the endpoint to clear. the promise will reject if an invalid endpoint is supplied. return value a promise. example the following example shows how to test for and clear a 'stall' condition in the result of a data transfer. what data can be passed to a usb device and how it is passed is particular and unique to each device. ...
... each device. while (true) { let result = await data.transferin(1, 6); if (result.data && result.data.bytelength === 6) { console.log('channel 1: ' + result.data.getuint16(0)); console.log('channel 2: ' + result.data.getuint16(2)); console.log('channel 5: ' + result.data.getuint16(4)); } if (result.status === 'stall') { console.warn('endpoint stalled. clearing.'); await device.clearhalt('in', 1); } } specifications specification status comment webusbthe definition of 'clearhalt()' in that specification. draft initial definition. ...

WebGLRenderingContext.makeXRCompatible() - Web APIs
putcanvas.addeventlistener("webglcontextlost", (event) => { /* the context has been lost but can be restored */ event.canceled = true; }); /* when the gl context is reconnected, reload the resources for the current scene. ...
...these both u...

Writing WebSocket client applications - Web APIs
request for the protocol upgrade is implicit when we address the web server as ws://www.example.com or wss://www.example.com. sending data to the server once you've opened your connection, you can begin transmitting data to the server. to do this, simply call the websocket object's send() method for each message you want to send: examplesocket.send("here's some text that the server is urgently awaiting!"); you can send data as a string, blob, or arraybuffer. as establishing a connection is asynchronous and prone to failure there is no guarantee that calling the send() method immediately after creating a websocket object will be successful. we can at least be sure that attempting to send data only takes place once a connection is established by defining an onopen event handler to do the wo...
...rone to failure there is no guarantee that calling the send() method immediately after creating a websocket object will be successful. we can at least be sure that attempting to send data only takes place once a connection is established by defining an onopen event handler to do the work: examplesocket.onopen = function (event) { examplesocket.send("here's some text that the server is urgently awaiting!"); }; using json to transmit objects one handy thing you can do is use json to send reasonably complex data to the server. for example, a chat program can interact with a server using a protocol implemented using packets of json-encapsulated data: // send text to all users through the server function sendtext() { // construct a msg object containing the data the server needs to process ...

Advanced techniques: Creating and sequencing audio - Web APIs
ators together using the methods we've already looked at, however, in this section, we'll load in a sample file instead so we can take a look at what's involved. loading the sample we want to make sure our file has loaded and been decoded into a buffer before we use it, so let's create an async function to allow us to do this: async function getfile(audiocontext, filepath) { const response = await fetch(filepath); const arraybuffer = await response.arraybuffer(); const audiobuffer = await audiocontext.decodeaudiodata(arraybuffer); return audiobuffer; } we can then use the await operator when calling this function, which ensures that we can only run subsequent code when it has finished executing. let's create another async function to set up the sample — we can combine the two asy...
...it operator when calling this function, which ensures that we can only run subsequent code when it has finished executing. let's create another async function to set up the sample — we can combine the two async functions in a nice promise pattern to perform further actions when this file is loaded and buffered: async function setupsample() { const filepath = 'dtmf.mp3'; const sample = await getfile(audioctx, filepath); return sample; } note: you can easily modify the above function to take an array of files and loop over them to load more than one sample. this would be very handy for more complex instruments, or gaming. we can now use setupsample() like so: setupsample() .then((sample) => { // sample is our buffered file // ... }); when the sample is re...

Symbol.asyncIterator - JavaScript
the source for this interactive example is stored in a github repository. if you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request. description the symbol.asynciterator symbol is a builtin symbol that is used to access an object's @@asynciterator method. ...
...no enumerable no configurable no examples user-defined async iterables you can define your own async iterable by setting the [symbol.asynciterator] property on an object. const myasynciterable = { async* [symbol.asynciterator]() { yield "hello"; yield "async"; yield "iteration!"; } }; (async () => { for await (const x of myasynciterable) { console.log(x); // expected output: // "hello" // "async" // "iteration!" } })(); when creating an api, remember that async iterables are designed to represent something iterable — like a stream of data or a list —, not to completely replace callbacks and events in most situations. built-in async iterables ...

Operator precedence - JavaScript
18 postfix increment n/a … ++ postfix decrement … -- 17 logical not right-to-left ! ...
... 18 postfix increment n/a … ++ postfix decrement … -- 17 logical not right-to-left ! ...

async function expression - JavaScript
iife (immediately invoked function expression) which runs as soon as it is defined. see also the chapter about functions for more information. ...
...sion) which runs as soon as it is defined. see also the chapter about functions for more information. examples simple example function resolveafter2seconds(x) { return new promise(resolve => { settimeout(() => { resolve(x); }, 2000); }); }; const add = async function(x) { // async function expression assigned to a variable let a = await resolveafter2seconds(20); let b = await resolveafter2seconds(30); return x + a + b; }; add(10).then(v => { console.log(v); // prints 60 after 4 seconds. }); (async function(x) { // async function expression used as an iife let p_a = resolveafter2seconds(20); let p_b = resolveafter2seconds(30); return x + await p_a + await p_b; })(10).then(v => { console.log(v); // prints 60 after 2 seconds. }); specifications ...

Enhanced Extension Installation - Archive of obsolete content
remote compatibility information that supercedes the compatibility information held by the item. if there is, and this remote information makes it so that the item is now compatible with the running version of the application, the item is configured in the manner described above (metadata written, added to container). finalization for themes, the item is immediately installed fully, rather than awaiting the next restart, since themes do not supply xpcom components, preferences defaults etc. for extensions, on the next startup, the startup system notices the presence of the .autoreg file and starts the extension manager with a "dirty" flag, telling it to finish any install operations that may be pending. the extension manager loads the xml/rdf datasources (this is allowable and necessary bec...

CSS3 - Archive of obsolete content
experimental parts are vendor-prefixed and should either be avoided in production environments, or used with extreme caution as both their syntax and semantics can change in the future. modules and the standardization process css level 2 needed 9 years, from august 2002 to june 2011 to reach the recommendation status. ...

Index - Archive of obsolete content
with xml data. 3243 xul user interfaces advanced, css, css:getting_started, example, guide, needsupdate, web, xul this page illustrates mozilla's specialized language for creating user interfaces. 3244 css3 css, needsupdate, reference css3 is the latest evolution of the cascading style sheets language and aims at extending css2.1. it brings a lot of long-awaited novelties, like rounded corners, shadows, gradients, transitions or animations, as well as new layouts like multi-columns, flexible box or grid layouts. 3245 case sensitivity in class and id names css, html no summary! 3246 creating a dynamic status bar extension add-ons, extensions, needsupdate concepts covered in the previous sample won't be reiterate...

The new nsString class implementation (1999) - Archive of obsolete content
: class nsimemoryagent : nsisupports { void* new(nsint32 asize)=0; //used for both alloc and realloc void* delete(void* aptr)=0; }; internationalization the new nsstrimpl/nsstring implementation addresses at least two of the primary concerns of our i18n team. first, nsstrimpl offers charset conversion hooks for use during construction, comparison and assignment. (these are stubbed out today awaiting their review and implementation). second, they are concerned that programmers be prevented from abusing the string classes in a number of ways. to wit: they want to ensure that the underlying buffers cannot be corrupted or altered erroneously they want to ensure that the appropriate set of conversion functions get applied they want some control over the usage pattern of strings, such that ...

ECMAScript 2016 to ES.Next support in Mozilla - Archive of obsolete content
includes() (firefox 43) exponentiation operator (firefox 52) ecmascript 2017 object.values() (firefox 47) object.entries() (firefox 47) string.prototype.padstart() (firefox 48) string.prototype.padend() (firefox 48) object.getownpropertydescriptors() (firefox 50) async functions async function (firefox 52) async function expression (firefox 52) asyncfunction (firefox 52) await (firefox 52) trailing commas in function parameter lists (firefox 52) ecmascript 2018 spread in object literals and rest parameters (firefox 55) for await...of (firefox 57) global_objects/sharedarraybuffer (firefox 57, with flags) global_objects/promise/finally (firefox 58) global_objects/regexp/dotall (not yet implemented; in other browsers) regexp lookbehind assertions (not yet ...

Archive of obsolete content
mple for further details. creating a status bar extension many of the concepts introduced here apply to any xul-based application; however, to keep from getting completely overwhelmed, we're going to focus specifically on firefox. css3 css3 is the latest evolution of the cascading style sheets language and aims at extending css2.1. it brings a lot of long-awaited novelties, like rounded corners, shadows, gradients, transitions or animations, as well as new layouts like multi-columns, flexible box or grid layouts. element events archived event pages firefox developer tools these are articles related to the firefox developer tools, which are no longer current. gecko compat...

General asynchronous programming concepts - Learn web development
his module, we'll explore further why asynchronous code matters and how to design code that avoids some of the problems described above. in this module general asynchronous programming concepts introducing asynchronous javascript cooperative asynchronous javascript: timeouts and intervals graceful asynchronous programming with promises making asynchronous programming easier with async and await choosing the right approach ...

Graceful asynchronous programming with Promises - Learn web development
nce the functions just pass their arguments directly, so there isn't any need for that extra layer of functions: choosetoppings().then(placeorder).then(collectorder).then(eatpizza).catch(failurecallback); this is not quite as easy to read, however, and this syntax might not be usable if your blocks are more complex than what we've shown here. note: you can make further improvements with async/await syntax, which we'll dig into in the next article. at their most basic, promises are similar to event listeners, but with a few differences: a promise can only succeed or fail once. it cannot succeed or fail twice and it cannot switch from success to failure or vice versa once the operation has completed. if a promise has succeeded or failed and you later add a success/failure callback, the ...

Cooperative asynchronous JavaScript: Timeouts and intervals - Learn web development
pecially those that manipulate the dom) can really slow down a page if you're not careful. previous overview: asynchronous next in this module general asynchronous programming concepts introducing asynchronous javascript cooperative asynchronous javascript: timeouts and intervals graceful asynchronous programming with promises making asynchronous programming easier with async and await choosing the right approach ...

Asynchronous JavaScript - Learn web development
new feature of the javascript language that allow you to defer further actions until after the previous action has completed, or respond to its failure. this is really useful for setting up a sequence of operations to work correctly. this article shows you how promises work, where you'll see them in use in webapis, and how to write your own. making asynchronous programming easier with async and await promises can be somewhat complex to set up and understand, and so modern browsers have implemented async functions and the await operator. the former allows standard functions to implicitly behave asynchronously with promises, whereas the latter can be used inside async functions to wait for promises before the function continues. this makes chaining promises simpler and easier to read. choosin...

Server-side web frameworks - Learn web development
bly, which enables the compilation of binary code for use on the client-side. deno aims to fill in some of the loop-holes in node.js by providing a mechanism that naturally maintains better security. deno's features include: security by default. deno modules restrict permissions to file, network, or environment access unless explicitly allowed. typescript support out-of-the-box. first-class await mechanism. built-in testing facility and code formatter (deno fmt) (javascript) browser compatibility: deno programs that are written completely in javascript excluding the deno namespace (or feature test for it), should work directly in any modern browser. script bundling into a single javascript file. deno provides an easy yet powerful way to use javascript for both client- and server-side...

Ember resources and troubleshooting - Learn web development
data will run in to, it is possible to roll your own front-end data client. a common alternative is to any fully-featured front-end data client is the fetch api. using the design patterns provided by the framework, a route using fetch() would look something like this: import route from '@ember/routing/route'; export default class myroute extends route { async model() { let response = await fetch('some/url/to/json/data'); let json = await response.json(); return { data: json }; } } see more information on specifying the route's model here. why can't i just use javascript? this is the most common question ember folks hear from people who have previous experience with react. while it is technically possible to use jsx, or any other form of dom creation, there...

Getting started with Svelte - Learn web development
u like, and in addition you can insert valid javascript expression inside single curly brackets ({}). in this case we are embedding the value of the name prop right after the hello text. <main> <h1>hello {name}!</h1> <p>visit the <a href="https://svelte.dev/tutorial">svelte tutorial</a> to learn how to build svelte apps.</p> </main> svelte also supports tags like {#if...}, {#each...}, and {#await...} — these examples allow you to conditionally render a portion of the markup, iterate through a list of elements, and work with async values, respectively. the <style> section if you have experience working with css, the following snippet should make sense: <style> main { text-align: center; padding: 1em; max-width: 240px; margin: 0 auto; } h1 { color: #ff3e00; ...

Advanced Svelte: Reactivity, lifecycle, accessibility - Learn web development
provides a better way to handle these cases. the tick() function returns a promise that resolves as soon as any pending state changes have been applied to the dom (or immediately, if there are no pending state changes). let's try it now. first of all, import tick at the top of the <script> section alongside your existing import: import { tick } from 'svelte' next, call tick() with await from an async function; update onedit() like so: async function onedit() { editing = true // enter editing mode await tick() nameel.focus() } if you try it now you'll see that everything works as expected. note: to see another example using tick(), visit the svelte tutorial. adding functionality to html elements with the use:action directive next up, ...

AddonManager
ge types these constants represent the lists of types of changes that can occur to add-ons during startup; they're used with the getstartupchanges(), addstartupchange(), and removestartupchange() methods. constant description startup_change_installed a list of add-ons that were detected as newly-installed during application startup. this doesn't include add-ons that were awaiting installation the last time the application was running. startup_change_changed a list of add-ons that were detected as having changed during startup. this includes an add-on being moved to a new location, changing version, or having been detected as possibly altered. startup_change_uninstalled a list of add-ons that were detected as having been uninstalled during startu...

Mozilla Development Strategies
e patch doesn't take twice as long to review as a ten line patch, it usually takes much more time. if you can divide and conquer what you are working on into smaller chunks, you will find you'll get faster reviews. of course, not everything can be divided up. and, shorter fixes aren't necessarily better than longer ones. having parallel trees should allow you to work on other items while you are awaiting reviews. get advice before working on a patch or feature, instead of after if you are blocked, or stumped as to what to work on, talk to a lead sooner rather than later. most likely they will have a bug they can hand off to you, or they can help get you unblocked. since they are going to be reviewing your code later, run your plan of attack by them first. it's better to have an idea rejected...

TPS Tests
t val1 is equal to val2, otherwise an exception is thrown and the test fails. custom functions you can also write your own functions to be called as actions. for example, consider the first action in the phase above: [bookmarks.add, bookmarks_initial] you could rewrite this as a custom function so as to add some custom logging: [async () => { logger.loginfo("adding bookmarks_initial"); await bookmarks.add(bookmarks_initial); }] note that this is probably best used for debugging, and new tests that want custom behavior should add it to the tps addon so that other tests can use it. example test here's an example tps test to tie it all together. // phase declarations var phases = { "phase1": "profile1", "phase2": "profile2", "phase3": "profile1" }; //...

AudioContext.close() - Web APIs
exts from being created and used, suspend the progression of audio time in the audio context, and stop processing audio data. the returned promise resolves when all audiocontext-creation-blocking resources have been released. this method throws an invalid_state_err exception if called on an offlineaudiocontext. syntax var audioctx = new audiocontext(); audioctx.close().then(function() { ... }); await audioctx.close(); returns a promise that resolves with void. example the following snippet is taken from our audiocontext states demo (see it running live.) when the stop button is clicked, close() is called. when the promise resolves, the example is reset to its beginning state. stopbtn.onclick = function() { audioctx.close().then(function() { startbtn.removeattribute('disabled'); ...

AudioParamDescriptor - Web APIs
sed for all 128 samples // if no automation is scheduled for the moment. } }) return true } } registerprocessor('white-noise-processor', whitenoiseprocessor) next, in our main scripts file we'll load the processor, create an instance of audioworkletnode passing it the name of the processor, and connect the node to an audio graph. ...

AudioWorkletGlobalScope.registerProcessor - Web APIs
working draft ...

AudioWorkletGlobalScope - Web APIs
able const usefulvariable = 42 console.log(usefulvariable) registerprocessor('test-processor', testprocessor) next, in our main scripts file we'll load the processor, create an instance of audioworkletnode — passing the name of the processor to it — and connect the node to an audio graph. we should see the output of console.log calls in the console: const audiocontext = new audiocontext() await audiocontext.audioworklet.addmodule('test-processor.js') const testnode = new audioworkletnode(audiocontext, 'test-processor') testnode.connect(audiocontext.destination) specifications specification status comment web audio apithe definition of 'audioworkletglobalscope' in that specification. working draft ...

AudioWorkletNode.parameters - Web APIs
sed for all 128 samples // if no automation is scheduled for the moment. } }) return true } } registerprocessor('white-noise-processor', whitenoiseprocessor) next, in our main scripts file we'll load the processor, create an instance of audioworkletnode passing it the name of the processor, and connect the node to an audio graph. ...

AudioWorkletNode.port - Web APIs
sole.log(e.data) this.port.postmessage('pong') } } process (inputs, outputs, parameters) { return true } } registerprocessor('ping-pong-processor', pingpongprocessor) now in our main scripts file we'll load the processor, create an instance of audioworkletnode passing the name of the processor, and connect the node to an audio graph. ...

AudioWorkletNode - Web APIs
(let i = 0; i < channel.length; i++) { channel[i] = math.random() * 2 - 1 } }) return true } } registerprocessor('white-noise-processor', whitenoiseprocessor) next, in our main script file we'll load the processor, create an instance of audioworkletnode passing it the name of the processor, and connect the node to an audio graph. const audiocontext = new audiocontext() await audiocontext.audioworklet.addmodule('white-noise-processor.js') const whitenoisenode = new audioworkletnode(audiocontext, 'white-noise-processor') whitenoisenode.connect(audiocontext.destination) specifications specification status comment web audio apithe definition of 'audioworkletnode' in that specification. working draft ...

AudioWorkletProcessor() - Web APIs
essor('test-processor', testprocessor) next, in our main script file we'll load the processor, create an instance of audioworkletnode passing it the name of the processor and options object. in the options object we pass processoroptions with a map instance under someusefulvariable key. we don't pass numberofinputs and see how it gets its default value. ...

AudioWorkletProcessor.parameterDescriptors (static getter) - Web APIs
sed for all 128 samples // if no automation is scheduled for the moment. } }) return true } } registerprocessor('white-noise-processor', whitenoiseprocessor) next, in our main scripts file we'll load the processor, create an instance of audioworkletnode passing it the name of the processor, and connect the node to an audio graph. ...

AudioWorkletProcessor.port - Web APIs
sole.log(e.data) this.port.postmessage('pong') } } process (inputs, outputs, parameters) { return true } } registerprocessor('ping-pong-processor', pingpongprocessor) now in our main scripts file we'll load the processor, create an instance of audioworkletnode passing the name of the processor, and connect the node to an audio graph. ...

AudioWorkletProcessor - Web APIs
et i = 0; i < channel.length; i++) { channel[i] = math.random() * 2 - 1 } }) return true } } registerprocessor('white-noise-processor', whitenoiseprocessor) next, in our main script file we'll load the processor, create an instance of audioworkletnode, passing it the name of the processor, then connect the node to an audio graph. const audiocontext = new audiocontext() await audiocontext.audioworklet.addmodule('white-noise-processor.js') const whitenoisenode = new audioworkletnode(audiocontext, 'white-noise-processor') whitenoisenode.connect(audiocontext.destination) specifications specification status comment web audio apithe definition of 'audioworkletprocessor' in that specification. working draft ...

Blob.arrayBuffer() - Web APIs
returns a promise that resolves with an arraybuffer that contains the blob's data in binary form. exceptions while this method doesn't throw exceptions, it may reject the promise. ...

Blob.text() - Web APIs
returns a promise that resolves with a usvstring which contains the blob's data as a text string. the data is always presumed to be in utf-8 format. ...

Blob - Web APIs
to use a filereader. the following code reads the content of a blob as a typed array: const reader = new filereader(); reader.addeventlistener('loadend', () => { // reader.result contains the contents of blob as a typed array }); reader.readasarraybuffer(blob); another way to read content from a blob is to use a response. the following code reads the content of a blob as text: const text = await (new response(blob)).text(); or by using blob.prototype.text(): const text = await blob.text(); by using other methods of filereader, it is possible to read the contents of a blob as a string or a data url. specifications specification status comment file apithe definition of 'the blob interface' in that specification. working draft initial definition. ...

Client.postMessage() - Web APIs
destination side and they are no longer usable on the sending side. return value undefined. examples sending a message from a service worker to a client: addeventlistener('fetch', event => { event.waituntil(async function() { // exit early if we don't have access to the client. // eg, if it's cross-origin. if (!event.clientid) return; // get the client. const client = await clients.get(event.clientid); // exit early if we don't get the client. // eg, if it closed. if (!client) return; // send a message to the client. client.postmessage({ msg: "hey i just got a fetch from you!", url: event.request.url }); }()); }); receiving that message: navigator.serviceworker.addeventlistener('message', event => { console.log(event.da...

Clients.claim() - Web APIs
gator.serviceworker in any clients that become controlled by this service worker. when a service worker is initially registered, pages won't use it until they next load. the claim() method causes those pages to be controlled immediately. be aware that this results in your service worker controlling pages that loaded regularly over the network, or possibly via a different service worker. syntax await clients.claim(); parameters none. return value a promise that resolves to undefined. example the following example uses claim() inside service worker's "activate" event listener so that clients loaded in the same scope do not need to be reloaded before their fetches will go through this service worker. self.addeventlistener('activate', event => { event.waituntil(clients.claim()); }); s...

ContentIndex.delete() - Web APIs
hat resolves with undefined exceptions no exceptions are thrown. examples below is an asynchronous function, that removes an item from the content index. we receive a reference to the current serviceworkerregistration, which allows us to access the index property and thus access the delete method. async function unregistercontent(article) { // reference registration const registration = await navigator.serviceworker.ready; // feature detect content index if (!registration.index) return; // unregister content from index await registration.index.delete(article.id); } the delete method can also be used within the service worker scope. self.registration.index.delete('my-id'); specifications specification status comment unknownthe definition of '...

CustomElementRegistry.whenDefined() - Web APIs
menu-container'); const placeholder = container.queryselector('.menu-placeholder'); // fetch all the children of menu that are not yet defined. const undefinedelements = container.queryselectorall(':not(:defined)'); const promises = [...undefinedelements].map( button => customelements.whendefined(button.localname) ); // wait for all the children to be upgraded, // then remove the placeholder. await promise.all(promises); container.removechild(placeholder); specifications specification status comment html living standardthe definition of 'customelements.whendefined()' in that specification. living standard initial definition. ...

Document.hasFocus() - Web APIs
syntax var focused = document.hasfocus(); return value false if the active element in the document has no focus; true if the active element in the document has focus. example this example checks whether the document has focus every 300 milliseconds. to test the functionality of hasfocus(), click on the button to open a new window, and try switching between the two pages. ...

EffectTiming.fill - Web APIs
set: 0 }, 200); for some complex effects where animations layer on top of one another, it may be necessary to use a fill mode temporarily to capture the final value of an animation before canceling it. elem.addeventlistener('click', async evt => { const animation = elem.animate( { transform: `translate(${evt.clientx}px, ${evt.clienty}px)` }, { duration: 800, fill: 'forwards' } ); await animation.finished; // commitstyles will record the style up to and including `animation` and // update elem’s specified style with the result. animation.commitstyles(); animation.cancel(); }); specifications specification status comment web animationsthe definition of 'fill' in that specification. working draft editor's draft. ...

ExtendableEvent.waitUntil() - Web APIs
, until all the promises passed to it settle. note: the behaviour described in the above paragraph was fixed in firefox 43 (see bug 1180274). syntax extendableevent.waituntil(promise); parameters a promise. return value undefined. example using waituntil() within a service worker's install event: addeventlistener('install', event => { const precache = async () => { const cache = await caches.open('static-v1'); return cache.addall([ '/', '/about/', '/static/styles.css' ]); }; event.waituntil(precache()); }); specifications specification status comment service workersthe definition of 'waituntil()' in that specification. working draft initial definition ...

FetchEvent.preloadResponse - Web APIs
promise that back to the controlled page to fetchevent.respondwith(). this promise resolves to the first matching url request in the cache object. if no match is found, the code checks for a preloaded response. else it fetches a response from the network. addeventlistener('fetch', event => { event.respondwith(async function() { // respond from the cache if we can const cachedresponse = await caches.match(event.request); if (cachedresponse) return cachedresponse; // else, use the preloaded response, if it's there const response = await event.preloadresponse; if (response) return response; // else try the network. return fetch(event.request); }()); }); specifications specification status comment service workersthe definition of 'pre...

FetchEvent.navigationPreload - Web APIs
orker registration. syntax var promise = fetchevent.navigationpreload value a promise that resolves to the instance of navigationpreloadmanager. example the following example shows the implementation of a fetch event that uses a preloaded response. addeventlistener('fetch', event => { event.respondwith(async function() { // respond from the cache if we can const cachedresponse = await caches.match(event.request); if (cachedresponse) return cachedresponse; // else, use the preloaded response, if it's there const response = await event.preloadresponse; if (response) return response; // else try the network. return fetch(event.request); }()); }); specifications specification status comment service workersthe definition of 'na...

FetchEvent.respondWith() - Web APIs
has not been dispatched or respondwithwith() has already been invoked. examples this fetch event tries to return a response from the cache api, falling back to the network otherwise. addeventlistener('fetch', event => { // prevent the default, and handle the request ourselves. event.respondwith(async function() { // try to get the response from a cache. const cachedresponse = await caches.match(event.request); // return it if we found one. if (cachedresponse) return cachedresponse; // if we didn't find a match in the cache, use the network. return fetch(event.request); }()); }); note: caches.match() is a convenience method. equivalent functionality is to call cache.match() on each cache (in the order returned by caches.keys()) until a response is returne...

FetchEvent - Web APIs
finds a match in the cache, it asynchronously updates the cache for next time. self.addeventlistener('fetch', event => { // let the browser do its default thing // for non-get requests. if (event.request.method != 'get') return; // prevent the default, and handle the request ourselves. event.respondwith(async function() { // try to get the response from a cache. const cache = await caches.open('dynamic-v1'); const cachedresponse = await cache.match(event.request); if (cachedresponse) { // if we found a match in the cache, return it, but also // update the entry in the cache in the background. event.waituntil(cache.add(event.request)); return cachedresponse; } // if we didn't find a match in the cache, use the network. return fet...

FontFace.FontFace() - Web APIs
criptors optional a set of optional descriptors passed as an object. it can have the following keys: family: family style: style weight: weight stretch: stretch unicoderange: unicode range variant: variant featuresettings: feature settings example async function loadfonts() { const font = new fontface('myfont', 'url(myfont.woff)'); // wait for font to be loaded await font.load(); // add font to document document.fonts.add(font); // enable font with css class document.body.classlist.add('fonts-loaded'); } specifications specification status comment css font loading module level 3the definition of 'fontface constructor' in that specification. working draft initial definition ...

HTMLMediaElement.play() - Web APIs
oplay guide for media and web audio apis. example this example demonstrates how to confirm that playback has begun and how to gracefully handle blocked automatic playback: let videoelem = document.getelementbyid("video"); let playbutton = document.getelementbyid("playbutton"); playbutton.addeventlistener("click", handleplaybutton, false); playvideo(); async function playvideo() { try { await videoelem.play(); playbutton.classlist.add("playing"); } catch(err) { playbutton.classlist.remove("playing"); } } function handleplaybutton() { if (videoelem.paused) { playvideo(); } else { videoelem.pause(); playbutton.classlist.remove("playing"); } } in this example, playback of video is toggled off and on by the async playvideo() function. it tries to play the v...

LockManager.query() - Web APIs
esolves with an object containing information about held and pending locks. syntax var promise<lockmanagersnapshot> = lockmanager.query() parameters none. return value a promise that resolves with a lockmanagersnapshot containing the following properties. held: an array of lock objects for held locks. pending: an array of lock objects for pending lock requests. example const state = await navigator.locks.query(); for (const lock of state.held) { console.log(`held lock: name ${lock.name}, mode ${lock.mode}`); } for (const request of state.pending) { console.log(`requested lock: name ${request.name}, mode ${request.mode}`); } specifications specification status comment web locks apithe definition of 'query()' in that specification. draft initial ...

MediaDevices.getDisplayMedia() - Web APIs
ated which initiates screen capture given a set of options specified by the displaymediaoptions parameter. the options are specified in the form of a mediastreamconstraints object which specifies the preferred stream configuration and the display surface from which video is to be captured. async function startcapture(displaymediaoptions) { let capturestream = null; try { capturestream = await navigator.mediadevices.getdisplaymedia(displaymediaoptions); } catch(err) { console.error("error: " + err); } return capturestream; } this uses await to asynchronously wait for getdisplaymedia() to resolve with a mediastream which contains the display contents as requested by the specified options. the stream is then returned to the caller for use, perhaps for adding to a webrtc call u...

MediaDevices.getUserMedia() - Web APIs
notallowederror or notfounderror respectively. note: it's possible for the returned promise to neither resolve nor reject, as the user is not required to make a choice at all and may simply ignore the request. ...

createMediaKeys() - Web APIs
the mediakeysystemaccess.createmediakeys() method returns a promise that resolves to a new mediakeys object. syntax var mediakeys = await mediakeysystemaccess.createmediakeys(); specifications specification status comment encrypted media extensionsthe definition of 'createmediakeys()' in that specification. recommendation initial definition ...

MediaTrackSupportedConstraints.cursor - Web APIs
video element referenced by the variable videoelem. async function capturewithcursor() { let supportedconstraints = navigator.mediadevices.getsupportedconstraints(); let displaymediaoptions = { video: { displaysurface: "browser" }, audio: false; }; if (supportedconstraints.cursor) { displaymediaoptions.video.cursor = "always"; } try { videoelem.srcobject = await navigator.mediadevices.getdisplaymedia(displaymediaoptions); } catch(err) { /* handle the error */ } } specifications specification status comment screen capturethe definition of 'mediatracksupportedconstraints.cursor' in that specification. unknown initial definition ...

MediaTrackSupportedConstraints.displaySurface - Web APIs
e returned stream to the video element referenced by the variable videoelem. async function capture() { let supportedconstraints = navigator.mediadevices.getsupportedconstraints(); let displaymediaoptions = { video: { }, audio: false; }; if (supportedconstraints.displaysurface) { displaymediaoptions.video.displaysurface = "monitor"; } try { videoelem.srcobject = await navigator.mediadevices.getdisplaymedia(displaymediaoptions); } catch(err) { /* handle the error */ } } specifications specification status comment screen capturethe definition of 'mediatracksupportedconstraints.displaysurface' in that specification. working draft initial definition ...

MediaTrackSupportedConstraints.logicalSurface - Web APIs
e returned stream to the video element referenced by the variable videoelem. async function capture() { let supportedconstraints = navigator.mediadevices.getsupportedconstraints(); let displaymediaoptions = { video: { }, audio: false; }; if (supportedconstraints.logicalsurface) { displaymediaoptions.video.logicalsurface = "monitor"; } try { videoelem.srcobject = await navigator.mediadevices.getdisplaymedia(displaymediaoptions); } catch(err) { /* handle the error */ } } specifications specification status comment screen capturethe definition of 'mediatracksupportedconstraints.logicalsurface' in that specification. working draft initial definition ...

Navigator.share() - Web APIs
the web share api to share mdn's url. the javascript looks like this: const sharedata = { title: 'mdn', text: 'learn web development on mdn!', url: 'https://developer.mozilla.org', } const btn = document.queryselector('button'); const resultpara = document.queryselector('.result'); // must be triggered some kind of "user activation" btn.addeventlistener('click', async () => { try { await navigator.share(sharedata) resultpara.textcontent = 'mdn shared successfully' } catch(err) { resultpara.textcontent = 'error: ' + err } }); sharing files to share files, first test for and call navigator.canshare(). then include an array of files in the call to navigator.share(): notice: that the sample handles feature detection by testing for navigator.canshare() rather than for ...

PaymentMethodChangeEvent.methodDetails - Web APIs
f (ev.methodname === "https://apple.com/apple-pay") { switch (cardtype) { case "visa": // do apple pay specific handling for visa card... // methoddetails contains the card information const result = calculatediscount(ev.methoddetails); object.assign(newstuff, result); break; } } // finally... ev.updatewith(newstuff); }; const response = await request.show(); note that the methoddetails property is being used by the calculatediscount() function to compute any payment discount, then updatewith() is called to update the event with the computed update. specifications specification status comment payment request apithe definition of 'paymentmethodchangeevent.methoddetails' in that specification. candidate reco...

PaymentMethodChangeEvent.methodName - Web APIs
f (ev.methodname === "https://apple.com/apple-pay") { switch (cardtype) { case "visa": // do apple pay specific handling for visa card... // methoddetails contains the card information const result = calculatediscount(ev.methoddetails); object.assign(newstuff, result); break; } } // finally... ev.updatewith(newstuff); }; const response = await request.show(); specifications specification status comment payment request apithe definition of 'paymentmethodchangeevent.methodname' in that specification. candidate recommendation initial definition. ...

PaymentRequest.prototype.id - Web APIs
onst details = { id: "super-store-order-123-12312", total: { label: "total due", amount: { currency: "usd", value: "65.00" }, }, }; const request = new paymentrequest(methoddata, details); console.log(request.id); // super-store-order-123-12312 the id is then also available in the paymentresponse returned from the show() method, but under the requestid attribute. const response = await request.show(); console.log(response.requestid === request.id); // and in serialized form too const json = response.tojson(); console.log(json.requestid,response.requestid, request.id); syntax var id = paymentrequest.id value a domstring. specifications specification status comment payment request apithe definition of 'id' in that specification. candidate recomm...

PaymentRequest.onmerchantvalidation - Web APIs
as allowed to use this payment handler. examples an example merchant validation handler for the paymentrequest object request looks like this: request.onmerchantvalidation = ev => { ev.complete(async () => { const merchantserverurl = window.location.origin + '/validation?url=' + encodeuricomponent(ev.validationurl); // get validation data, and complete validation; return await fetch(merchantserverurl).then(r => r.text()); }) }; const response = await request.show(); for more information, see merchant validation in payment processing concepts. specifications specification status comment payment request apithe definition of 'onmerchantvalidation' in that specification. candidate recommendation initial definition. ...

PaymentRequest.onpaymentmethodchange - Web APIs
thodname === "https://apple.com/apple-pay") { switch (cardtype) { case "store": // do apple pay specific handling for store card... // methoddetails contains the store card information const result = calculatediscount(ev.methoddetails); object.assign(newstuff, result); break; } } // finally... ev.updatewith(newstuff); }; const response = await request.show(); specifications specification status comment payment request apithe definition of 'onpaymentmethodchange' in that specification. candidate recommendation initial definition. ...

Web Push API Notifications best practices - Web APIs
ush notification should be useful and time-sensitive, and the user should always be asked for permission before sending the first one, and be offered an easy way to opt out of getting more in the future. there are some basic questions you can answer to determine if a push notification is needed: is there someone waiting in real-time for a response? in our above example, the program manager is awaiting your response and so a push notification is appropriate. is up-to-the minute updating necessary? i use a service that aggregates different social media news sources. when a story i’m interested in is trending, i’d like to get a notification! is there breaking news that is timely? this is where it gets a little tricky. sometimes news sites request push notifications so they can essentiall...

RTCPeerConnection.peerIdentity - Web APIs
is example, a function, getidentityassertion(), is created which asynchronously waits for the peer's identity to be verified, then returns the identity to the caller. if an error occurs and the promise is rejected, this logs the error to the console and returns null to the caller. let pc = new rtcpeerconnection(); /* ... */ async function getidentityassertion(pc) { try { const identity = await pc.peeridentity; return identity; } catch(err) { console.log("error identifying remote peer: ", err); return null; } } specifications specification status comment identity for webrtc candidate recommendation initial specification. ...

RTCPeerConnection.setRemoteDescription() - Web APIs
guishable from rtcsessiondescriptioninit, you can also pass in an rtcsessiondescription. this lets you simplify code such as the following: mypeerconnection.setremotedescription(new rtcsessiondescription(description)) .then(function () { return createmystream(); }) to simply be: mypeerconnection.setremotedescription(description) .then(function () { return createmystream(); }) using async/await syntax, you can further simplify this to: await mypeerconnection.setremotedescription(description); createmystream(); since it's unnecessary, the rtcsessiondescription() constructor is deprecated. return value a promise which is fulfilled once the value of the connection's remotedescription is successfully changed or rejected if the change cannot be applied (for example, if the specified desc...

Screen Capture API - Web APIs
network. screen capture api concepts and usage the screen capture api is relatively simple to use. its sole method is mediadevices.getdisplaymedia(), whose job is to ask the user to select a screen or portion of a screen to capture in the form of a mediastream. to start capturing video from the screen, you call getdisplaymedia() on the instance of media navigator.mediadevices: capturestream = await navigator.mediadevices.getdisplaymedia(displaymediaoptions); the promise returned by getdisplaymedia() resolves to a mediastream which streams the captured media. see the article using the screen capture api for a more in-depth look at how to use the api to capture screen contents as a stream. additions to existing interfaces the screen capture api doesn't have any interfaces of its own; inst...

ServiceWorkerContainer: message event - Web APIs
eive messages from the service worker. bubbles no cancelable no interface messageevent event handler property onmessage examples in this example the service worker get the client's id from a fetch event and then sends it a message using client.postmessage: // in the service worker async function messageclient(clientid) { const client = await clients.get(clientid); client.postmessage('hi client!'); } addeventlistener('fetch', (event) => { messageclient(event.clientid); event.respondwith(() => { // ... }); }); the client can receive the message by listening to the message event: // in the page being controlled navigator.serviceworker.addeventlistener('message', (message) => { console.log(message); }); spec...

ServiceWorkerRegistration.index - Web APIs
interface returns a reference to the contentindex interface, which allows for indexing of offline content. syntax var a contentindex object = serviceworkerregistration.index; value a contentindex object examples you can access the property from either your main script or the registered service worker. here is an example from the main script: // reference registration const registration = await navigator.serviceworker.ready; // feature detection if ('index' in registration) { // content index api functionality const contentindex = registration.index; } from the service worker: // service worker script const contentindex = self.registration.index; specifications specification status comment unknownthe definition of 'index' in that specification. ...

SourceBuffer.appendBufferAsync() - Web APIs
is fulfilled when the buffer has been added successfully to the sourcebuffer, or null if the request could not be initiated. example this simplified example async function, fillsourcebuffer(), takes as input parameters buffersource, buffer, and a sourcebuffer to which to append the source media from the buffer. async function fillsourcebuffer(buffer, msbuffer) { try { while(true) { await msbuffer.appendbufferasync(buffer); } } catch(e) { handleexception(e); } } specifications not currently part of any specification. this is being experimented with under the auspices of the web platform incubator community group (wicg). specification status comment media source extensions recommendation initial definition; does not include this method.

SourceBuffer.removeAsync() - Web APIs
ble representing the end of the time range, in seconds. return value a promise whose fulfillment handler is executed once the buffers in the specified time range have been removed from the sourcebuffer. example this example establishes an asynchronous function, emptysourcebuffer(), which simply clears the contents of the specified sourcebuffer. async function emptysourcebuffer(msbuffer) { await msbuffer.removeasync(0, infinity).catch(function(e) { handleexception(e); } } specifications not currently part of the mse specification. specification status comment media source extensions recommendation initial definition. ...

SubtleCrypto.encrypt() - Web APIs
); // counter will be needed for decryption counter = window.crypto.getrandomvalues(new uint8array(16)); return window.crypto.subtle.encrypt( { name: "aes-ctr", counter, length: 64 }, key, encoded ); } let iv = new uint8array(16); let key = new uint8array(16); let data = new uint8array(12345); //crypto functions are wrapped in promises so we have to use await and make sure the function that //contains this code is an async function //encrypt function wants a cryptokey object const key_encoded = await crypto.subtle.importkey( "raw", key.buffer, 'aes-ctr' , false, ["encrypt", "decrypt"]); const encrypted_content = await window.crypto.subtle.encrypt( { name: "aes-ctr", counter: iv, length: 128 }, key_encoded, da...

TransformStream - Web APIs
le read only the readable end of a transformstream. transformstream.writable read only the writable end of a transformstream. methods none examples anything-to-uint8array stream in the following example, a transform stream passes through all chunks it receives as uint8array values. const transformcontent = { start() {}, // required. ...

URL - Web APIs
ate the url query string. to get the search params from the current window's url, you can do this: // https://some.site/?id=123 const parsedurl = new url(window.location.href); console.log(parsedurl.searchparams.get("id")); // "123" the tostring() method of url just returns the value of the href property, so the constructor can be used to normalize and encode a url directly. const response = await fetch(new url('http://www.example.com/démonstration.html')); specifications specification status comment file apithe definition of 'url' in that specification. working draft added the static methods url.createobjecturl() and url.revokeobjecturl(). urlthe definition of 'api' in that specification. living standard initial definition (implements ur...

USBDevice.claimInterface() - Web APIs
face is claimed for exclusive access. syntax var promise = usbdevice.claiminterface(interfacenumber) parameters interfacenumber the index of one of the interfaces supported by the device. interfaces are device-specific. return value a promise. example the following example shows claiminterface() in the context of connecting to a usb device. async function connectdevice(usbdevice) { await usbdevice.open(); if (usbdevice.configuration === null) await usbdevice.selectconfiguration(1); await usbdevice.claiminterface(0); } specifications specification status comment webusbthe definition of 'claiminterface()' in that specification. draft initial definition. ...

USBDevice.configuration - Web APIs
ace returns a usbconfiguration object for the currently selected interface for a paired usb device. syntax var usbconfiguration = usbdevice.configuration value a usbconfiguration object. example the following example uses this property to test for the existence of a usbconfiguration property to select a configuration before claiming an interface. async function connectdevice(usbdevice) { await usbdevice.open(); if (usbdevice.configuration === null) await usbdevice.selectconfiguration(1); await usbdevice.claiminterface(0); } specifications specification status comment webusbthe definition of 'configuration' in that specification. draft initial definition. ...

USBDevice.opened - Web APIs
efore calling usbdevice.controltransferout to set a specified led color. what data can be passed to a usb device and how it is passed is particular and unique to each device. async setdevicecolor(usbdevice, r, g, b) { if (device.opened) { // this hypothetical usb device requires that the data passed to // it be in a uint8array. let payload = new uint8array([r, g, b]); await usbdevice.controltransferout({ requesttype: 'vendor', recipient: 'device', request: 1, value: 0, index: 0, }, payload); } } specifications specification status comment webusbthe definition of 'opened' in that specification. draft initial definition. ...

WakeLock.request() - Web APIs
ck policy. document is not fully active. document is hidden. user agent could not acquire platform's wake lock. examples the following asynchronous function requests a wakelocksentinel object. the request() method is wrapped in a try...catch statement to account for if the browser refuses the request for any reason. const requestwakelock = async () => { try { const wakelock = await navigator.wakelock.request('screen'); } catch (err) { // the wake lock request fails - usually system related, such low as battery console.log(`${err.name}, ${err.message}`); } } requestwakelock(); specifications specification status comment screen wake lock apithe definition of 'request()' in that specification. unknown initial definition. ...

WakeLock - Web APIs
request requests a wakelocksentinel object, which returns a promise that resolves with a wakelocksentinel object. examples the following asynchronous function requests a wakelocksentinel object. the wakelock.request method is wrapped in a try...catch statement to account for if the browser refuses the request for any reason. const requestwakelock = async () => { try { const wakelock = await navigator.wakelock.request('screen'); } catch (err) { // the wake lock request fails - usually system related, such low as battery console.log(`${err.name}, ${err.message}`); } } requestwakelock(); specifications specification status comment screen wake lock apithe definition of 'wakelock' in that specification. unknown initial definition. ...

WakeLockSentinel.type - Web APIs
entinel.type; value a string representation of the currently acquired wake lock type. type read only return values are: 'screen': a screen wake lock. prevents devices from dimming or locking the screen. examples this example shows an asynchronous function that acquires a wakelocksentinel, then logs the type to the console. const requestwakelock = async () => { wakelock = await navigator.wakelock.request('screen'); console.log(wakelock.type); // logs 'screen' }; requestwakelock(); specifications specification status comment screen wake lock apithe definition of 'wakelocktype' in that specification. unknown initial definition. ...

WakeLockSentinel - Web APIs
ynchronous function which requests a wakelocksentinel. once acquired we listen for the onrelease event which can be used to give appropriate ui feedback. the sentinel can be acquired or released via appropriate interactions. ...

Compressed texture formats - Web APIs
of 4. ...

Movement, orientation, and motion: A WebXR example - Web APIs
ebxr session; we'll see that below. the webxr session is toggled on and off by the handler for click events on the button, whose label is appropriately set to either "enter webxr" or "exit webxr". this is done by the onxrbuttonclick() event handler. async function onxrbuttonclick(event) { if (!xrsession) { navigator.xr.requestsession(session_type) .then(sessionstarted); } else { await xrsession.end(); if (xrsession) { sessionended(); } } } this begins by looking at the value of xrsession to see if we already have a xrsession object representing an ongoing webxr session. if we don't, the click represents a request to enable webxr mode, so we check to see if the desired webxr session type is supported. if it is, we call sessionstarted() to begin running the sce...

Rendering and the WebXR frame animation callback - Web APIs
ientation set at the viewer's starting position and viewing direction. once that's in hand, you request that the browser call your rendering function the next time it needs a framebuffer to render your scene. this is done by calling the xrsession method requestanimationframe(). ...

Spaces and reference spaces: Spatial tracking in WebXR - Web APIs
n while performing this transition, you can use the xrframe method getviewerpose() to obtain the current xrviewerpose, switch the session, then use the saved viewer pose to restore the viewer's position and facing. let viewerpose = frame.getviewerpose(worldreferencespace); let newsession = navigator.xr.requestsession("immersive-vr", { requiredfeatures: "unbounded" }); worldreferencespace = await newsession.requestreferencespace("unbounded"); viewerpose = worldreferencespace.getoffsetreferencespace(viewerpose.transform); here, the viewer pose is obtained, with its transform defined relative to worldreferencespace, the current session's global reference space. then a new session is established and a reference space created to become the new world reference space. finally, the saved view...

Background audio processing using AudioWorklet - Web APIs
by its name connect inputs to the audioworkletnode and its outputs to appropriate destinations (either other nodes or to the audiocontext object's destination property. to use an audio worklet processor, you can use code similar to the following: let audiocontext = null; async function createmyaudioprocessor() { if (!audiocontext) { try { audiocontext = new audiocontext(); await audiocontext.resume(); await audiocontext.audioworklet.addmodule("module-url/module.js"); } catch(e) { return null; } } return new audioworkletnode(audiocontext, "processor-name"); } this createmyaudioprocessor() function creates and returns a new instance of audioworkletnode configured to use your audio processor. it also handles creating the audio context if it hasn't...

Worklet.addModule() - Web APIs
. the promise doesn't return any value. exceptions if addmodule() fails, it rejects the promise, delivering one of the following errors to the rejection handler. aborterror the specified script is invalid or could not be loaded. syntaxerror the specified moduleurl is invalid. ...

Overview of events and handlers - Developer guides
e page, the objects in the dom (document object model) tree including user interactions or modifications, the xmlhttprequest objects used for network requests, and the media objects such as audio and video, when the media stream players change state. although this list is currently incomplete. some notable events are: note: this list of events will need work to make relevant; that work is awaiting some global reorganization work on the documents. this will also need finding a good explanation of the events involved during page loading, such as discussed partially in this web page or in this stack overflow question. the global object window emits an event called 'load' when the page has finished rendering, meaning that all resources have been downloaded and acted upon, so that the s...

A re-introduction to JavaScript (JS tutorial) - JavaScript
javascript is now used by an incredible number of high-profile applications, showing that deeper knowledge of this technology is an important skill for any web or mobile developer. it's useful to start with an overview of the language's history. javascript was created in 1995 by brendan eich while he was an engineer at netscape. ...

Method definitions - JavaScript
index++ } } }; // the same object using shorthand syntax const obj2 = { * g() { let index = 0 while (true) { yield index++ } } }; const it = obj2.g() console.log(it.next().value) // 0 console.log(it.next().value) // 1 async methods async methods can also be defined using the shorthand syntax. // using a named property const obj3 = { f: async function () { await some_promise } } // the same object using shorthand syntax const obj3 = { async f() { await some_promise } } async generator methods generator methods can also be async. const obj4 = { f: async function* () { yield 1 yield 2 yield 3 } }; // the same object using shorthand syntax const obj4 = { async* f() { yield 1 yield 2 yield 3 } } method definition...

Array.prototype.forEach() - JavaScript
o determine if further iteration is required. foreach expects a synchronous function foreach does not wait for promises. kindly make sure you are aware of the implications while using promises(or async functions) as foreach callback. ...

AsyncFunction - JavaScript
it as a constructor. examples creating an async function from an asyncfunction() constructor function resolveafter2seconds(x) { return new promise(resolve => { settimeout(() => { resolve(x); }, 2000); }); } let asyncfunction = object.getprototypeof(async function(){}).constructor let a = new asyncfunction('a', 'b', 'return await resolveafter2seconds(a) + await resolveafter2seconds(b);'); a(10, 20).then(v => { console.log(v); // prints 30 after 4 seconds }); specifications specification ecmascript (ecma-262)the definition of 'asyncfunction object' in that specification. ...

Atomics.wait() - JavaScript
it returns a string which is either "ok", "not-equal", or "timed-out". note: this operation only works with a shared int32array and may not be allowed on the main thread. syntax atomics.wait(typedarray, index, value[, timeout]) parameters typedarray a shared int32array. index the position in the typedarray to wait on. value the expected value to test. timeout optional time to wait in milliseconds. infinity, if no time is provided. ...

Atomics - JavaScript
value at the specified index of the array with the provided value. returns the old value at that index. atomics.store() stores a value at the specified index of the array. returns the value. atomics.sub() subtracts a value at the specified index of the array. returns the old value at that index. atomics.wait() verifies that the specified index of the array still contains a value and sleeps awaiting or times out. returns either "ok", "not-equal", or "timed-out". if waiting is not allowed in the calling agent then it throws an error exception. (most browsers will not allow wait() on the browser's main thread.) atomics.xor() computes a bitwise xor on the value at the specified index of the array with the provided value. returns the old value at that index. examples using atomics const...

Symbol - JavaScript
bject is initialized with no own symbol properties, so that this array will be empty unless you've set symbol properties on the object. constructor symbol() creates a new symbol object. it is incomplete as a constructor because it does not support the syntax "new symbol()". static properties symbol.asynciterator a method that returns the default asynciterator for an object. used by for await...of. symbol.hasinstance a method determining if a constructor object recognizes an object as its instance. used by instanceof. symbol.isconcatspreadable a boolean value indicating if an object should be flattened to its array elements. used by array.prototype.concat(). symbol.iterator a method returning the default iterator for an object. used by for...of. symbol.match a method that matc...

Lexical grammar - JavaScript
ipt specification. they have no special functionality at present, but they might at some future time, so they cannot be used as identifiers. these are always reserved: enum the following are only reserved when they are found in strict mode code: implements interface let package private protected public static the following are only reserved when they are found in module code: await future reserved keywords in older standards the following are reserved as future keywords by older ecmascript specifications (ecmascript 1 till 3). abstract boolean byte char double final float goto int long native short synchronized throws transient volatile additionally, the literals null, true, and false cannot be used as identifiers in ecmascript. reserved word usage ...

Expressions and operators - JavaScript
xecution context. function the function keyword defines a function expression. class the class keyword defines a class expression. function* the function* keyword defines a generator function expression. yield pause and resume a generator function. yield* delegate to another generator function or iterable object. async function the async function defines an async function expression. await pause and resume an async function and wait for the promise's resolution/rejection. [] array initializer/literal syntax. {} object initializer/literal syntax. /ab+c/i regular expression literal syntax. ( ) grouping operator. left-hand-side expressions left values are the destination of an assignment. property accessors member operators provide access to a property or method of an ...

Statements and declarations - JavaScript
erty, a specified statement is executed. for...in iterates over the enumerable properties of an object, in arbitrary order. for each distinct property, statements can be executed. for...of iterates over iterable objects (including arrays, array-like objects, iterators and generators), invoking a custom iteration hook with statements to be executed for the value of each distinct property. for await...of iterates over async iterable objects, array-like objects, iterators and generators, invoking a custom iteration hook with statements to be executed for the value of each distinct property. while creates a loop that executes a specified statement as long as the test condition evaluates to true. the condition is evaluated before executing the statement. others debugger invokes any ava...

JavaScript reference - JavaScript
webassembly.linkerror webassembly.runtimeerror statements javascript statements and declarations control flowblock break continue empty if...else switch throw try...catch declarations var let const functions and classes function function* async function return class iterations do...while for for each...in for...in for...of for await...of while other debugger import label with expressions and operators javascript expressions and operators. primary expressionsthis function class function* yield yield* async function await [] {} /ab+c/i ( ) null left-hand-side expressions property accessors new new.target super ...obj increment & decrement a++ a-- ...

How to make PWAs re-engageable using Notifications and Push - Progressive web apps (PWAs)
ring, we use the registration object to subscribe, and then use the resulting subscription object to complete the whole process. in the registration part, the code looks like this: if(subscription) { return subscription; } if the user has already subscribed, we then return the subscription object and move to the subscription part. if not, we initialize a new subscription: const response = await fetch('./vapidpublickey'); const vapidpublickey = await response.text(); const convertedvapidkey = urlbase64touint8array(vapidpublickey); the app fetches the server's public key and converts the response to text; then it needs to be converted to a uint8array (to support chrome). to learn more about vapid keys you can read the sending vapid identified webpush notifications via mozilla’s push se...

Resources - XSLT: Extensible Stylesheet Language Transformations
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