To finish this module off, we'll provide a brief discussion of the different coding techniques and features we've discussed throughout, looking at which one you should use when, with recommendations and reminders of common pitfalls where appropriate. We'll probably add to this resource as time goes on.
Prerequisites: | Basic computer literacy, a reasonable understanding of JavaScript fundamentals. |
---|---|
Objective: | To be able to make a sound choice of when to use different asynchronous programming techniques. |
Asynchronous callbacks
Generally found in old-style APIs, involves a function being passed into another function as a parameter, which is then invoked when an asynchronous operation has been completed, so that the callback can in turn do something with the result. This is the precursor to promises; it's not as efficient or flexible. Use only when necessary.
Single delayed operation | Repeating operation | Multiple sequential operations | Multiple simultaneous operations |
---|---|---|---|
No | Yes (recursive callbacks) | Yes (nested callbacks) | No |
Code example
An example that loads a resource via the XMLHttpRequest
API (run it live, and see the source):
function loadAsset(url, type, callback) { let xhr = new XMLHttpRequest(); xhr.open('GET', url); xhr.responseType = type; xhr.onload = function() { callback(xhr.response); }; xhr.send(); } function displayImage(blob) { let objectURL = URL.createObjectURL(blob); let image = document.createElement('img'); image.src = objectURL; document.body.appendChild(image); } loadAsset('coffee.jpg', 'blob', displayImage);
Pitfalls
- Nested callbacks can be cumbersome and hard to read (i.e. "callback hell").
- Failure callbacks need to be called once for each level of nesting, whereas with promises you can just use a single
.catch()
block to handle the errors for the entire chain. - Async callbacks just aren't very graceful.
- Promise callbacks are always called in the strict order they are placed in the event queue; async callbacks aren't.
- Async callbacks lose full control of how the function will be executed when passed to a third-party library.
Browser compatibility
Really good general support, although the exact support for callbacks in APIs depends on the particular API. Refer to the reference documentation for the API you're using for more specific support info.
Further information
- Introducing asynchronous JavaScript, in particular Async callbacks
setTimeout()
setTimeout()
is a method that allows you to run a function after an arbitrary amount of time has passed.
Single delayed operation | Repeating operation | Multiple sequential operations | Multiple simultaneous operations |
---|---|---|---|
Yes | Yes (recursive timeouts) | Yes (nested timeouts) | No |
Code example
Here the browser will wait two seconds before executing the anonymous function, then will display the alert message (see it running live, and see the source code):
let myGreeting = setTimeout(function() { alert('Hello, Mr. Universe!'); }, 2000)
Pitfalls
You can use recursive setTimeout()
calls to run a function repeatedly in a similar fashion to setInterval()
, using code like this:
let i = 1; setTimeout(function run() { console.log(i); i++; setTimeout(run, 100); }, 100);
There is a difference between recursive setTimeout()
and setInterval()
:
- Recursive
setTimeout()
guarantees at least the specified amount of time (100ms in this example) will elapse between the executions; the code will run and then wait 100 milliseconds before it runs again. The interval will be the same regardless of how long the code takes to run. - With
setInterval()
, the interval we choose includes the time taken to execute the code we want to run in. Let's say that the code takes 40 milliseconds to run — the interval then ends up being only 60 milliseconds.
When your code has the potential to take longer to run than the time interval you’ve assigned, it’s better to use recursive setTimeout()
— this will keep the time interval constant between executions regardless of how long the code takes to execute, and you won't get errors.
Browser compatibility
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
setTimeout | Chrome Full support 30 | Edge Full support 12 | Firefox
Full support
1
| IE Full support 4 | Opera Full support 4 | Safari Full support 1 | WebView Android Full support 4.4 | Chrome Android Full support 30 | Firefox Android
Full support
4
| Opera Android Full support 10.1 | Safari iOS Full support 1 | Samsung Internet Android Full support 3.0 |
Supports parameters for callback | Chrome Full support Yes | Edge Full support 12 | Firefox Full support Yes | IE Full support 10 | Opera Full support Yes | Safari ? | WebView Android Full support Yes | Chrome Android Full support Yes | Firefox Android ? | Opera Android ? | Safari iOS ? | Samsung Internet Android Full support Yes |
Throttling of tracking timeout scripts | Chrome ? | Edge ? | Firefox Full support 55 | IE ? | Opera ? | Safari ? | WebView Android ? | Chrome Android ? | Firefox Android Full support 55 | Opera Android ? | Safari iOS ? | Samsung Internet Android ? |
Legend
- Full support
- Full support
- Compatibility unknown
- Compatibility unknown
- See implementation notes.
- See implementation notes.
Further information
- Cooperative asynchronous JavaScript: Timeouts and intervals, in particular setTimeout()
- setTimeout() reference
setInterval()
setInterval()
is a method that allows you to run a function repeatedly with a set interval of time between each execution. Not as efficient as requestAnimationFrame()
, but allows you to choose a running rate/frame rate.
Single delayed operation | Repeating operation | Multiple sequential operations | Multiple simultaneous operations |
---|---|---|---|
No | Yes | No (unless it is the same one) | No |
Code example
The following function creates a new Date()
object, extracts a time string out of it using toLocaleTimeString()
, and then displays it in the UI. We then run it once per second using setInterval()
, creating the effect of a digital clock that updates once per second (see this live, and also see the source):
function displayTime() { let date = new Date(); let time = date.toLocaleTimeString(); document.getElementById('demo').textContent = time; } const createClock = setInterval(displayTime, 1000);
Pitfalls
- The frame rate isn't optimized for the system the animation is running on, and can be somewhat inefficient. Unless you need to choose a specific (slower) framerate, it is generally better to use
requestAnimationFrame()
.
Browser compatibility
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
setInterval | Chrome Full support 30 | Edge Full support 12 | Firefox
Full support
1
| IE Full support 4 | Opera Full support 4 | Safari Full support 1 | WebView Android Full support 4.4 | Chrome Android Full support 30 | Firefox Android
Full support
4
| Opera Android Full support 10.1 | Safari iOS Full support 1 | Samsung Internet Android Full support 3.0 |
Supports parameters for callback | Chrome Full support Yes | Edge Full support 12 | Firefox Full support Yes | IE Full support 10 | Opera Full support Yes | Safari ? | WebView Android Full support Yes | Chrome Android Full support Yes | Firefox Android ? | Opera Android ? | Safari iOS ? | Samsung Internet Android Full support Yes |
Legend
- Full support
- Full support
- Compatibility unknown
- Compatibility unknown
- See implementation notes.
- See implementation notes.
Further information
- Cooperative asynchronous JavaScript: Timeouts and intervals, in particular setInterval()
- setInterval() reference
requestAnimationFrame()
requestAnimationFrame()
is a method that allows you to run a function repeatedly, and efficiently, at the best framerate available given the current browser/system. You should, if at all possible, use this instead of setInterval()
/recursive setTimeout()
, unless you need a specific framerate.
Single delayed operation | Repeating operation | Multiple sequential operations | Multiple simultaneous operations |
---|---|---|---|
No | Yes | No (unless it is the same one) | No |
Code example
A simple animated spinner; you can find this example live on GitHub (see the source code also):
const spinner = document.querySelector('div'); let rotateCount = 0; let startTime = null; let rAF; function draw(timestamp) { if(!startTime) { startTime = timestamp; } rotateCount = (timestamp - startTime) / 3; if(rotateCount > 359) { rotateCount %= 360; } spinner.style.transform = 'rotate(' + rotateCount + 'deg)'; rAF = requestAnimationFrame(draw); } draw();
Pitfalls
- You can't choose a specific framerate with
requestAnimationFrame()
. If you need to run your animation at a slower framerate, you'll need to usesetInterval()
or recursivesetTimeout()
.
Browser compatibility
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
requestAnimationFrame | Chrome
Full support
24
| Edge Full support 12 | Firefox
Full support
23
| IE Full support 10 | Opera
Full support
15
| Safari
Full support
6.1
| WebView Android
Full support
≤37
| Chrome Android
Full support
25
| Firefox Android
Full support
23
| Opera Android
Full support
14
| Safari iOS
Full support
7
| Samsung Internet Android
Full support
1.5
|
Return value | Chrome Full support 23 | Edge Full support 12 | Firefox Full support 11 | IE Full support 10 | Opera Full support 15 | Safari Full support 6.1 | WebView Android Full support Yes | Chrome Android Full support 25 | Firefox Android Full support 14 | Opera Android Full support 14 | Safari iOS Full support 6.1 | Samsung Internet Android Full support 1.5 |
Legend
- Full support
- Full support
- See implementation notes.
- See implementation notes.
- Requires a vendor prefix or different name for use.
- Requires a vendor prefix or different name for use.
Further information
- Cooperative asynchronous JavaScript: Timeouts and intervals, in particular requestAnimationFrame()
- requestAnimationFrame() reference
Promises
Promises are a JavaScript feature that allows you to run asynchronous operations and wait until it is definitely complete before running another operation based on its result. Promises are the backbone of modern asynchronous JavaScript.
Single delayed operation | Repeating operation | Multiple sequential operations | Multiple simultaneous operations |
---|---|---|---|
No | No | Yes | See Promise.all() , below |
Code example
The following code fetches an image from the server and displays it inside an <img>
element; see it live also, and see also the source code:
fetch('coffee.jpg') .then(response => response.blob()) .then(myBlob => { let objectURL = URL.createObjectURL(myBlob); let image = document.createElement('img'); image.src = objectURL; document.body.appendChild(image); }) .catch(e => { console.log('There has been a problem with your fetch operation: ' + e.message); });
Pitfalls
Promise chains can be complex and hard to parse. If you nest a number of promises, you can end up with similar troubles to callback hell. For example:
remotedb.allDocs({ include_docs: true, attachments: true }).then(function (result) { let docs = result.rows; docs.forEach(function(element) { localdb.put(element.doc).then(function(response) { alert("Pulled doc with id " + element.doc._id + " and added to local db."); }).catch(function (err) { if (err.name == 'conflict') { localdb.get(element.doc._id).then(function (resp) { localdb.remove(resp._id, resp._rev).then(function (resp) { // et cetera...
It is better to use the chaining power of promises to go with a flatter, easier to parse structure:
remotedb.allDocs(...).then(function (resultOfAllDocs) { return localdb.put(...); }).then(function (resultOfPut) { return localdb.get(...); }).then(function (resultOfGet) { return localdb.put(...); }).catch(function (err) { console.log(err); });
or even:
remotedb.allDocs(...) .then(resultOfAllDocs => { return localdb.put(...); }) .then(resultOfPut => { return localdb.get(...); }) .then(resultOfGet => { return localdb.put(...); }) .catch(err => console.log(err));
That covers a lot of the basics. For a much more complete treatment, see the excellent We have a problem with promises, by Nolan Lawson.
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Promise | Chrome Full support 32 | Edge Full support 12 | Firefox Full support 29 | IE No support No | Opera Full support 19 | Safari Full support 8 | WebView Android Full support 4.4.3 | Chrome Android Full support 32 | Firefox Android Full support 29 | Opera Android Full support 19 | Safari iOS Full support 8 | Samsung Internet Android Full support 2.0 | nodejs Full support 0.12 |
Promise() constructor | Chrome Full support 32 | Edge Full support 12 | Firefox
Full support
29
| IE No support No | Opera Full support 19 | Safari
Full support
8
| WebView Android Full support 4.4.3 | Chrome Android Full support 32 | Firefox Android
Full support
29
| Opera Android Full support 19 | Safari iOS
Full support
8
| Samsung Internet Android Full support 2.0 | nodejs
Full support
0.12
|
all() | Chrome Full support 32 | Edge Full support 12 | Firefox Full support 29 | IE No support No | Opera Full support 19 | Safari Full support 8 | WebView Android Full support 4.4.3 | Chrome Android Full support 32 | Firefox Android Full support 29 | Opera Android Full support 19 | Safari iOS Full support 8 | Samsung Internet Android Full support 2.0 | nodejs Full support 0.12 |
allSettled() | Chrome Full support 76 | Edge Full support 79 | Firefox Full support 71 | IE No support No | Opera Full support 63 | Safari Full support 13 | WebView Android Full support 76 | Chrome Android Full support 76 | Firefox Android No support No | Opera Android Full support 54 | Safari iOS Full support 13 | Samsung Internet Android No support No | nodejs Full support 12.9.0 |
any | Chrome Full support 85 | Edge No support No | Firefox Full support 79 | IE No support No | Opera No support No | Safari Full support 14 | WebView Android Full support 85 | Chrome Android Full support 85 | Firefox Android No support No | Opera Android No support No | Safari iOS Full support 14 | Samsung Internet Android No support No | nodejs No support No |
catch() | Chrome Full support 32 | Edge Full support 12 | Firefox Full support 29 | IE No support No | Opera Full support 19 | Safari Full support 8 | WebView Android Full support 4.4.3 | Chrome Android Full support 32 | Firefox Android Full support 29 | Opera Android Full support 19 | Safari iOS Full support 8 | Samsung Internet Android Full support 2.0 | nodejs Full support 0.12 |
finally() | Chrome Full support 63 | Edge Full support 18 | Firefox Full support 58 | IE No support No | Opera Full support 50 | Safari Full support 11.1 | WebView Android Full support 63 | Chrome Android Full support 63 | Firefox Android Full support 58 | Opera Android Full support 46 | Safari iOS Full support 11.3 | Samsung Internet Android Full support 8.0 | nodejs Full support 10.0.0 |
race() | Chrome Full support 32 | Edge Full support 12 | Firefox Full support 29 | IE No support No | Opera Full support 19 | Safari Full support 8 | WebView Android Full support 4.4.3 | Chrome Android Full support 32 | Firefox Android Full support 29 | Opera Android Full support 19 | Safari iOS Full support 8 | Samsung Internet Android Full support 2.0 | nodejs Full support 0.12 |
reject() | Chrome Full support 32 | Edge Full support 12 | Firefox Full support 29 | IE No support No | Opera Full support 19 | Safari Full support 8 | WebView Android Full support 4.4.3 | Chrome Android Full support 32 | Firefox Android Full support 29 | Opera Android Full support 19 | Safari iOS Full support 8 | Samsung Internet Android Full support 2.0 | nodejs Full support 0.12 |
resolve() | Chrome Full support 32 | Edge Full support 12 | Firefox Full support 29 | IE No support No | Opera Full support 19 | Safari Full support 8 | WebView Android Full support 4.4.3 | Chrome Android Full support 32 | Firefox Android Full support 29 | Opera Android Full support 19 | Safari iOS Full support 8 | Samsung Internet Android Full support 2.0 | nodejs Full support 0.12 |
then() | Chrome Full support 32 | Edge Full support 12 | Firefox Full support 29 | IE No support No | Opera Full support 19 | Safari Full support 8 | WebView Android Full support 4.4.3 | Chrome Android Full support 32 | Firefox Android Full support 29 | Opera Android Full support 19 | Safari iOS Full support 8 | Samsung Internet Android Full support 2.0 | nodejs Full support 0.12 |
Legend
- Full support
- Full support
- No support
- No support
- See implementation notes.
- See implementation notes.
Further information
Promise.all()
A JavaScript feature that allows you to wait for multiple promises to complete before then running a further operation based on the results of all the other promises.
Single delayed operation | Repeating operation | Multiple sequential operations | Multiple simultaneous operations |
---|---|---|---|
No | No | No | Yes |
Code example
The following example fetches several resources from the server, and uses Promise.all()
to wait for all of them to be available before then displaying all of them — see it live, and see the source code:
function fetchAndDecode(url, type) { // Returning the top level promise, so the result of the entire chain is returned out of the function return fetch(url).then(response => { // Depending on what type of file is being fetched, use the relevant function to decode its contents if(type === 'blob') { return response.blob(); } else if(type === 'text') { return response.text(); } }) .catch(e => { console.log(`There has been a problem with your fetch operation for resource "${url}": ` + e.message); }); } // Call the fetchAndDecode() method to fetch the images and the text, and store their promises in variables let coffee = fetchAndDecode('coffee.jpg', 'blob'); let tea = fetchAndDecode('tea.jpg', 'blob'); let description = fetchAndDecode('description.txt', 'text'); // Use Promise.all() to run code only when all three function calls have resolved Promise.all([coffee, tea, description]).then(values => { console.log(values); // Store each value returned from the promises in separate variables; create object URLs from the blobs let objectURL1 = URL.createObjectURL(values[0]); let objectURL2 = URL.createObjectURL(values[1]); let descText = values[2]; // Display the images in <img> elements let image1 = document.createElement('img'); let image2 = document.createElement('img'); image1.src = objectURL1; image2.src = objectURL2; document.body.appendChild(image1); document.body.appendChild(image2); // Display the text in a paragraph let para = document.createElement('p'); para.textContent = descText; document.body.appendChild(para); });
Pitfalls
- If a
Promise.all()
rejects, then one or more of the promises you are feeding into it inside its array parameter must be rejecting, or might not be returning promises at all. You need to check each one to see what they returned.
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
all() | Chrome Full support 32 | Edge Full support 12 | Firefox Full support 29 | IE No support No | Opera Full support 19 | Safari Full support 8 | WebView Android Full support 4.4.3 | Chrome Android Full support 32 | Firefox Android Full support 29 | Opera Android Full support 19 | Safari iOS Full support 8 | Samsung Internet Android Full support 2.0 | nodejs Full support 0.12 |
Legend
- Full support
- Full support
- No support
- No support
Further information
Async/await
Syntactic sugar built on top of promises that allows you to run asynchronous operations using syntax that's more like writing synchronous callback code.
Single delayed operation | Repeating operation | Multiple sequential operations | Multiple simultaneous operations |
---|---|---|---|
No | No | Yes | Yes (in combination with Promise.all() ) |
Code example
The following example is a refactor of the simple promise example we saw earlier that fetches and displays an image, written using async/await (see it live, and see the source code):
async function myFetch() { let response = await fetch('coffee.jpg'); let myBlob = await response.blob(); let objectURL = URL.createObjectURL(myBlob); let image = document.createElement('img'); image.src = objectURL; document.body.appendChild(image); } myFetch();
Pitfalls
- You can't use the
await
operator inside a non-async
function, or in the top level context of your code. This can sometimes result in an extra function wrapper needing to be created, which can be slightly frustrating in some circumstances. But it is worth it most of the time. - Browser support for async/await is not as good as that for promises. If you want to use async/await but are concerned about older browser support, you could consider using the BabelJS library — this allows you to write your applications using the latest JavaScript and let Babel figure out what changes if any are needed for your user’s browsers.
Browser compatibility
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
async function | Chrome Full support 55 | Edge Full support 15 | Firefox Full support 52 | IE No support No | Opera Full support 42 | Safari Full support 10.1 | WebView Android Full support 55 | Chrome Android Full support 55 | Firefox Android Full support 52 | Opera Android Full support 42 | Safari iOS Full support 10.3 | Samsung Internet Android Full support 6.0 | nodejs
Full support
7.6.0
|
Legend
- Full support
- Full support
- No support
- No support
- User must explicitly enable this feature.
- User must explicitly enable this feature.
Further information
- Making asynchronous programming easier with async and await
- Async function reference
- Await operator reference