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Window: postMessage() method

{{ApiRef("HTML DOM")}} 

The window.postMessage() method safely enables cross-origin communication between {{domxref("Window")}}  objects; e.g., between a page and a pop-up that it spawned, or between a page and an iframe embedded within it.

Normally, scripts on different pages are allowed to access each other if and only if the pages they originate from share the same origin (also known as the “same-origin policy”). window.postMessage() provides a controlled mechanism to securely circumvent this restriction (if used properly).

Furthermore, an accessing script must have obtained the window object of the accessed document beforehand. This can occur through methods such as window.open() for popups or iframe.contentWindow for iframes.

Broadly, one window may obtain a reference to another (e.g., via targetWindow = window.opener), and then dispatch a {{domxref("MessageEvent")}}  on it with targetWindow.postMessage(). The receiving window is then free to handle this event as needed. The arguments passed to window.postMessage() (i.e., the “message”) are exposed to the receiving window through the event object.

Syntax

postMessage(message)
postMessage(message, targetOrigin)
postMessage(message, targetOrigin, transfer)

postMessage(message, options)

Parameters

Return value

None ({{jsxref("undefined")}} ).

The dispatched event

A window can listen for dispatched messages by executing the following JavaScript:

window.addEventListener(
  "message",
  (event) => {
    if (event.origin !== "http://example.org:8080") return;

    // …
  },
  false,
);

The properties of the dispatched message are:

Security concerns

If you do not expect to receive messages from other sites, do not add any event listeners for message events. This is a completely foolproof way to avoid security problems.

If you do expect to receive messages from other sites, always verify the sender’s identity using the origin and possibly source properties. Any window (including, for example, http://evil.example.com) can send a message to any other window within the iframe hierarchy from top to every iframe below of the current document. Having verified identity, however, you still should always verify the syntax of the received message. Otherwise, a security hole in the site you trusted to send only trusted messages could then open a cross-site scripting hole in your site.

Always specify an exact target origin, not *, when you use postMessage to dispatch data to other windows. A malicious site can change the location of the window without your knowledge, and therefore it can intercept the data sent using postMessage.

Secure shared memory messaging

If postMessage() throws when used with {{jsxref("SharedArrayBuffer")}}  objects, you might need to make sure you cross-site isolated your site properly. Shared memory is gated behind two HTTP headers:

Cross-Origin-Opener-Policy: same-origin
Cross-Origin-Embedder-Policy: require-corp

To check if cross origin isolation has been successful, you can test against the {{domxref("Window.crossOriginIsolated")}}  property available to window and worker contexts:

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

if (crossOriginIsolated) {
  const buffer = new SharedArrayBuffer(16);
  myWorker.postMessage(buffer);
} else {
  const buffer = new ArrayBuffer(16);
  myWorker.postMessage(buffer);
}

Examples

/*
 * In window A's scripts, with A being on http://example.com:8080:
 */

const popup = window.open(/* popup details */);

// When the popup has fully loaded, if not blocked by a popup blocker:

// This does nothing, assuming the window hasn't changed its location.
popup.postMessage(
  "The user is 'bob' and the password is 'secret'",
  "https://secure.example.net",
);

// This will successfully queue a message to be dispatched to the popup, assuming
// the window hasn't changed its location.
popup.postMessage("hello there!", "http://example.com");

window.addEventListener(
  "message",
  (event) => {
    // Do we trust the sender of this message? (might be
    // different from what we originally opened, for example).
    if (event.origin !== "http://example.com") return;

    // event.source is popup
    // event.data is "hi there yourself! the secret response is: rheeeeet!"
  },
  false,
);

/*
 * In the popup's scripts, running on http://example.com:
 */

// Called sometime after postMessage is called
window.addEventListener("message", (event) => {
  // Do we trust the sender of this message?
  if (event.origin !== "http://example.com:8080") return;

  // event.source is window.opener
  // event.data is "hello there!"

  // Assuming you've verified the origin of the received message (which
  // you must do in any case), a convenient idiom for replying to a
  // message is to call postMessage on event.source and provide
  // event.origin as the targetOrigin.
  event.source.postMessage(
    "hi there yourself! the secret response " + "is: rheeeeet!",
    event.origin,
  );
});

Notes

Any script in a document in a window can request to let a message get dispatched to a document in another window whose window object it has obtained, by calling .postMessage() on that window object. Consequently, any event listener used to receive messages must first check the identity of the sender of the message, using the origin and possibly source properties. This cannot be overstated: Failure to check the origin and possibly source properties enables cross-site scripting attacks.

As with any asynchronously-dispatched script (timeouts, user-generated events), it is not possible for the caller of postMessage to detect when an event handler listening for events sent by postMessage throws an exception.

After postMessage() is called, the {{domxref("MessageEvent")}}  will be dispatched only after all pending execution contexts have finished. For example, if postMessage() is invoked in an event handler, that event handler will run to completion, as will any remaining handlers for that same event, before the {{domxref("MessageEvent")}}  is dispatched.

The value of the origin property of the dispatched event is not affected by the current value of document.domain in the calling window.

For IDN host names only, the value of the origin property is not consistently Unicode or punycode; for greatest compatibility check for both the IDN and punycode values when using this property if you expect messages from IDN sites. This value will eventually be consistently IDN, but for now you should handle both IDN and punycode forms.

The value of the origin property when the sending window contains a javascript: or data: URL is the origin of the script that loaded the URL.

Using window.postMessage in extensions {{Non-standard_inline}} 

window.postMessage is available to JavaScript running in chrome code (e.g., in extensions and privileged code), but the source property of the dispatched event is always null as a security restriction. (The other properties have their expected values.)

It is not possible for content or web context scripts to specify a targetOrigin to communicate directly with an extension (either the background script or a content script). Web or content scripts can use window.postMessage with a targetOrigin of "*" to broadcast to every listener, but this is discouraged, since an extension cannot be certain the origin of such messages, and other listeners (including those you do not control) can listen in.

Content scripts should use {{WebExtAPIRef("runtime.sendMessage")}}  to communicate with the background script. Web context scripts can use custom events to communicate with content scripts (with randomly generated event names, if needed, to prevent snooping from the guest page).

Lastly, dispatching a message to a page at a file: URL currently requires that the targetOrigin argument be "*". file:// cannot be used as a security restriction; this restriction may be modified in the future.

Specifications

{{Specifications}} 

Browser compatibility

{{Compat}} 

See also

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