Worker: postMessage() method

The postMessage() method of the Worker interface sends a message to the worker. The first parameter is the data to send to the worker. The data may be any JavaScript object that can be handled by the structured clone algorithm.

The Worker postMessage() method delegates to the MessagePort postMessage() method, which adds a task on the event loop corresponding to the receiving MessagePort.

The Worker can send back information to the thread that spawned it using the DedicatedWorkerGlobalScope.postMessage method.

Syntax

postMessage(message)
postMessage(message, transfer)

Parameters

message

The object to deliver to the worker; this will be in the data field in the event delivered to the DedicatedWorkerGlobalScope.message_event event. 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, a SyntaxError will be thrown by the parser. If the data to be passed to the worker is unimportant, null or undefined 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 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 or ImageBitmap objects that can be transferred. null is not an acceptable value for transfer.

Return value

None (undefined).

Examples

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.

const myWorker = new Worker("worker.js");

first.onchange = () => {
  myWorker.postMessage([first.value, second.value]);
  console.log("Message posted to worker");
};

second.onchange = () => {
  myWorker.postMessage([first.value, second.value]);
  console.log("Message posted to worker");
};

For a full example, see our simple worker example (run example).

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 minimum example has main create an ArrayBuffer and transfer it to myWorker, then has myWorker transfer it back to main, with the size logged at each step.

main.js code

// create worker
const myWorker = new Worker("myWorker.js");

// listen for myWorker to transfer the buffer back to main
myWorker.addEventListener("message", function handleMessageFromWorker(msg) {
  console.log("message from worker received in main:", msg);

  const bufTransferredBackFromWorker = msg.data;

  console.log(
    "buf.byteLength in main AFTER transfer back from worker:",
    bufTransferredBackFromWorker.byteLength
  );
});

// create the buffer
const myBuf = new ArrayBuffer(8);

console.log(
  "buf.byteLength in main BEFORE transfer to worker:",
  myBuf.byteLength
);

// send myBuf to myWorker and transfer the underlying ArrayBuffer
myWorker.postMessage(myBuf, [myBuf]);

console.log(
  "buf.byteLength in main AFTER transfer to worker:",
  myBuf.byteLength
);

myWorker.js code

// listen for main to transfer the buffer to myWorker
self.onmessage = function handleMessageFromMain(msg) {
  console.log("message from main received in worker:", msg);

  const bufTransferredFromMain = msg.data;

  console.log(
    "buf.byteLength in worker BEFORE transfer back to main:",
    bufTransferredFromMain.byteLength
  );

  // send buf back to main and transfer the underlying ArrayBuffer
  self.postMessage(bufTransferredFromMain, [bufTransferredFromMain]);

  console.log(
    "buf.byteLength in worker AFTER transfer back to main:",
    bufTransferredFromMain.byteLength
  );
};

Output logged

bash

buf.byteLength in main BEFORE transfer to worker:        8                     main.js:19
buf.byteLength in main AFTER transfer to worker:         0                     main.js:27

message from main received in worker:                    MessageEvent { ... }  myWorker.js:3
buf.byteLength in worker BEFORE transfer back to main:   8                     myWorker.js:7
buf.byteLength in worker AFTER transfer back to main:    0                     myWorker.js:15

message from worker received in main:                    MessageEvent { ... }  main.js:6
buf.byteLength in main AFTER transfer back from worker:  8                     main.js:10

byteLength goes to 0 after the ArrayBuffer is transferred. For a more sophisticated full working example of ArrayBuffer transfer, see this Firefox demo add-on: GitHub :: ChromeWorker - demo-transfer-arraybuffer

Specifications

Specification
HTML Standard # dom-worker-postmessage-dev

Browser compatibility

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