MessagePort: postMessage() method
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2015.
Note: This feature is available in Web Workers.
The postMessage()
method of the
MessagePort
interface sends a message from the port, and optionally,
transfers ownership of objects to other browsing contexts.
Syntax
postMessage(message)
postMessage(message, transfer)
postMessage(message, options)
Parameters
message
-
The message you want to send through the channel. This can be of any basic data type. Multiple data items can be sent as an array.
transfer
Optional-
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-
An optional object containing the following properties:
transfer
Optional-
Has the same meaning as the
transfer
parameter.
Return value
None (undefined
).
Examples
In the following code block, you can see a new channel being created using the
MessageChannel()
constructor. When the
IFrame has loaded, we pass MessageChannel.port2
to the IFrame using
window.postMessage
along with a message. The iframe receives the message,
and sends a message back on the MessageChannel
using postMessage()
.
The handleMessage
handler then responds to a message being sent back from the iframe using
onmessage
, putting it into a paragraph —
MessageChannel.port1
is listened to, to check when the message arrives.
const channel = new MessageChannel();
const para = document.querySelector("p");
const ifr = document.querySelector("iframe");
const otherWindow = ifr.contentWindow;
ifr.addEventListener("load", iframeLoaded, false);
function iframeLoaded() {
otherWindow.postMessage("Transferring message port", "*", [channel.port2]);
}
channel.port1.onmessage = handleMessage;
function handleMessage(e) {
para.innerHTML = e.data;
}
// in the iframe…
window.addEventListener("message", (event) => {
const messagePort = event.ports?.[0];
messagePort.postMessage("Hello from the iframe!");
});
For a full working example, see our channel messaging basic demo on GitHub (run it live too).
Specifications
Specification |
---|
HTML Standard # dom-messageport-postmessage-dev |
Browser compatibility
BCD tables only load in the browser