Worker: postMessage() method

{{APIRef("Web Workers API")}} {{AvailableInWorkers("window_and_worker_except_service")}} 

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

The Worker can send back information to the thread that spawned it using the {{domxref("DedicatedWorkerGlobalScope.postMessage")}}  method.

Syntax

postMessage(message)
postMessage(message, transfer)
postMessage(message, options)

Parameters

• message

  • : The object to deliver to the worker; this will be in the data field in the event delivered to the {{domxref("DedicatedWorkerGlobalScope.message_event", "message")}}  event. This may be any value or JavaScript object handled by the structured clone algorithm, which includes cyclical references.

    The message parameter is mandatory. If the data to be passed to the worker is unimportant, null or undefined must be passed explicitly.

• transfer {{optional_inline}} 

  • : An optional array of transferable objects to transfer ownership of. The ownership of these objects is given to the destination side and they are no longer usable on the sending side. These transferable objects should be attached to the message; otherwise they would be moved but not actually accessible on the receiving end.

• options {{optional_inline}} 

  • : An optional object containing the following properties:
    • transfer {{optional_inline}} 
      • : Has the same meaning as the transfer parameter.

Return value

None ({{jsxref("undefined")}} ).

Examples

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, {{domxref("HTMLElement/change_event", "change")}}  events invoke postMessage() to send the value of both inputs to the current worker.

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

[first, second].forEach((input) => {
  input.onchange = () => {
    myWorker.port.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

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

{{Specifications}} 

Browser compatibility

{{Compat}} 

See also

• The {{domxref("Worker")}}  interface it belongs to.