{{APIRef("Web Workers API")}}
The postMessage() method of the {{domxref("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 {{domxref("DedicatedWorkerGlobalScope.postMessage")}} method.
Syntax
myWorker.postMessage(aMessage, transferList);
Parameters
- aMessage
- The object to deliver to the worker; this will be in the data field in the event delivered to the {{domxref("DedicatedWorkerGlobalScope.onmessage")}} handler. This may be any value or JavaScript object handled by the structured clone algorithm, which includes cyclical references.
- transferList {{optional_inline}}
- An optional array of {{domxref("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.
- Only {{domxref("MessagePort")}} and {{domxref("ArrayBuffer")}} objects can be transferred.
nullis not an acceptable value for thetransferList.
Returns
Void.
Example
The following code snippet shows the creation of a {{domxref("Worker")}} object using the {{domxref("Worker.Worker", "Worker()")}} constructor. When either of two form inputs (first and second) have their values changed, {{event("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 |
|---|---|---|
| {{SpecName('HTML WHATWG', "#dom-worker-postmessage", "Worker.postMessage()")}} | {{Spec2('HTML WHATWG')}} | No change from {{SpecName("Web Workers")}}. |
| {{SpecName('Web Workers', "#dom-worker-postmessage", "Worker.postMessage()")}} | {{Spec2('Web Workers')}} | Initial definition. |
Browser compatibility
| Feature | Chrome | Firefox (Gecko) | Internet Explorer | Opera | Safari (WebKit) |
|---|---|---|---|---|---|
| Basic support | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} | 10.0 [1] | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} |
| Feature | Android | Firefox Mobile (Gecko) | Firefox OS (Gecko) | IE Phone | Opera Mobile | Safari Mobile |
|---|---|---|---|---|---|---|
| Basic support | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} | 10.0 [1] | {{CompatVersionUnknown}} | {{CompatVersionUnknown}} |
[1] Internet Explorer does not support {{domxref("Transferable")}} objects.
See also
- The {{domxref("Worker")}} interface it belongs to.