The postMessage()
method of the Worker
interface sends a message to the worker's inner scope. This accepts a single parameter, which is the data to send to the worker. The data may be any value or JavaScript object handled by the structured clone algorithm, which includes cyclical references.
The Worker
can send back information to the thread that spawned it using the DedicatedWorkerGlobalScope.postMessage
method.
Syntax
worker.postMessage(message, [transfer]);
Parameters
- message
- The object to deliver to the worker; this will be in the
data
field in the event delivered to theDedicatedWorkerGlobalScope.onmessage
handler. This may be any value or JavaScript object handled by the structured clone algorithm, which includes cyclical references. - If the
message
parameter is not provided, aTypeError
will be thrown. If the data to be passed to the worker is unimportant,null
orundefined
can be passed explicitly. - transfer Optional
- An optional array of
Transferable
objects to transfer ownership of. If the ownership of an object is transferred, it becomes unusable (neutered) in the context it was sent from and becomes available only to the worker it was sent to. - Transferable objects are instances of classes like
ArrayBuffer
,MessagePort
orImageBitmap
objects that can be transferred.null
is not an acceptable value fortransfer
.
Returns
Void.
Example
The following code snippet shows the creation of a Worker
object using the Worker()
constructor. When either of two form inputs (first
and second
) have their values changed, change
events invoke postMessage()
to send the value of both inputs to the current worker.
var myWorker = new Worker('worker.js'); first.onchange = function() { myWorker.postMessage([first.value,second.value]); console.log('Message posted to worker'); } second.onchange = function() { myWorker.postMessage([first.value,second.value]); console.log('Message posted to worker'); }
For a full example, see ourBasic dedicated worker example (run dedicated worker).
Note: postMessage()
can only send a single object at once. As seen above, if you want to pass multiple values you can send an array.
Transfer Example
This example shows a Firefox add-on that transfers an ArrayBuffer
from the main thread to the ChromeWorker
, and then the ChromeWorker
transfers it back to the main thread.
Main thread code:
var myWorker = new ChromeWorker(self.path + 'myWorker.js'); function handleMessageFromWorker(msg) { console.log('incoming message from worker, msg:', msg); switch (msg.data.aTopic) { case 'do_sendMainArrBuff': sendMainArrBuff(msg.data.aBuf) break; default: throw 'no aTopic on incoming message to ChromeWorker'; } } myWorker.addEventListener('message', handleMessageFromWorker); // Ok lets create the buffer and send it var arrBuf = new ArrayBuffer(8); console.info('arrBuf.byteLength pre transfer:', arrBuf.byteLength); myWorker.postMessage( { aTopic: 'do_sendWorkerArrBuff', aBuf: arrBuf // The array buffer that we passed to the transferrable section 3 lines below }, [ arrBuf // The array buffer we created 9 lines above ] ); console.info('arrBuf.byteLength post transfer:', arrBuf.byteLength);
Worker code
self.onmessage = function (msg) { switch (msg.data.aTopic) { case 'do_sendWorkerArrBuff': sendWorkerArrBuff(msg.data.aBuf) break; default: throw 'no aTopic on incoming message to ChromeWorker'; } } function sendWorkerArrBuff(aBuf) { console.info('from worker, PRE send back aBuf.byteLength:', aBuf.byteLength); self.postMessage({aTopic:'do_sendMainArrBuff', aBuf:aBuf}, [aBuf]); console.info('from worker, POST send back aBuf.byteLength:', aBuf.byteLength); }
Output logged
arrBuf.byteLength pre transfer: 8 bootstrap.js:40 arrBuf.byteLength post transfer: 0 bootstrap.js:42 from worker, PRE send back aBuf.byteLength: 8 myWorker.js:5:2 incoming message from worker, msg: message { ... } bootstrap.js:20 got back buf in main thread, aBuf.byteLength: 8 bootstrap.js:12 from worker, POST send back aBuf.byteLength: 0 myWorker.js:7:2
byteLength
goes to 0 as it is transferred. To see a full working example of this Firefox demo add-on see here: GitHub :: ChromeWorker - demo-transfer-arraybuffer
Specifications
Specification | Status | Comment |
---|---|---|
HTML Living Standard The definition of 'Worker.postMessage()' in that specification. |
Living Standard |
Browser compatibility
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
postMessage | Chrome Full support Yes | Edge Full support 12 | Firefox Full support Yes | IE
Full support
10
| Opera Full support 47 | Safari Full support Yes | WebView Android Full support Yes | Chrome Android Full support Yes | Firefox Android Full support Yes | Opera Android Full support 44 | Safari iOS Full support Yes | Samsung Internet Android Full support Yes |
Legend
- Full support
- Full support
- See implementation notes.
- See implementation notes.
[1] Internet Explorer does not support Transferable
objects.
See also
- The
Worker
interface it belongs to.