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The window.postMessage() method safely enables cross-origin communication between 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 protocol, port number, and host (also known as the "same-origin policy"). window.postMessage() provides a controlled mechanism to securely circumvent this restriction (if used properly).

Broadly, one window may obtain a reference to another (e.g., via targetWindow = window.opener), and then dispatch a 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.


targetWindow.postMessage(message, targetOrigin, [transfer]);
A reference to the window that will receive the message. Methods for obtaining such a reference include:
Data to be sent to the other window. The data is serialized using the structured clone algorithm. This means you can pass a broad variety of data objects safely to the destination window without having to serialize them yourself. [1]
Specifies what the origin of targetWindow must be for the event to be dispatched, either as the literal string "*" (indicating no preference) or as a URI. If at the time the event is scheduled to be dispatched the scheme, hostname, or port of targetWindow's document does not match that provided in targetOrigin, the event will not be dispatched; only if all three match will the event be dispatched. This mechanism provides control over where messages are sent; for example, if postMessage() was used to transmit a password, it would be absolutely critical that this argument be a URI whose origin is the same as the intended receiver of the message containing the password, to prevent interception of the password by a malicious third party. Always provide a specific targetOrigin, not *, if you know where the other window's document should be located. Failing to provide a specific target discloses the data you send to any interested malicious site.
transfer Optional
Is a sequence of Transferable objects that are transferred with the message. The ownership of these objects is given to the destination side and they are no longer usable on the sending side.

The dispatched event

otherWindow can listen for dispatched messages by executing the following JavaScript:

window.addEventListener("message", receiveMessage, false);

function receiveMessage(event)
  if (event.origin !== "http://example.org:8080")

  // ...

The properties of the dispatched message are:

The object passed from the other window.
The origin of the window that sent the message at the time postMessage was called. This string is the concatenation of the protocol and "://", the host name if one exists, and ":" followed by a port number if a port is present and differs from the default port for the given protocol. Examples of typical origins are https://example.org (implying port 443), http://example.net (implying port 80), and http://example.com:8080. Note that this origin is not guaranteed to be the current or future origin of that window, which might have been navigated to a different location since postMessage was called.
A reference to the window object that sent the message; you can use this to establish two-way communication between two windows with different origins.

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, and you have no guarantees that an unknown sender will not send malicious messages. 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 send 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.


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

var 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'",

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

function receiveMessage(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")

  // event.source is popup
  // event.data is "hi there yourself!  the secret response is: rheeeeet!"
window.addEventListener("message", receiveMessage, false);
 * In the popup's scripts, running on <http://example.com>:

// Called sometime after postMessage is called
function receiveMessage(event)
  // Do we trust the sender of this message?
  if (event.origin !== "http://example.com:8080")

  // 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!",

window.addEventListener("message", receiveMessage, false);


Any window may access this method on any other window, at any time, regardless of the location of the document in the window, to send it a message. 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.

postMessage() schedules the MessageEvent to 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 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

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 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, posting 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.


Specification Status Comment
HTML Living Standard
The definition of 'postMessage()' in that specification.
Living Standard  

Browser compatibility


FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support1 Yes



Yes — 63


8 — 105

transfer argument ? Yes20 Yes ? ?
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes Yes Yes



Yes — 63

Yes Yes6 ?
transfer argument ? ? ?20 ? ? ?

1. Supports sending File and FileList objects between windows. This is only allowed if the recipient's principal is contained within the sender's principal for security reasons.

2. The message parameter is serialized using the structured clone algorithm. This means you can pass a broad variety of data objects safely to the destination window without having to serialize them yourself.

3. The message parameter must be a string.

4. IE10 had an important limitation: see this article for details.

5. Support only for <frame> and <iframe>.

6. For security reasons, to work properly on Safari, construct using document.getElementById('your-frame').contentWindow.


See also