MessagePort: postMessage() method
The postMessage()
method of the
MessagePort
interface sends a message from the port, and optionally,
transfers ownership of objects to other browsing contexts.
Note: This feature is available in Web Workers
Syntax
js
postMessage(message, transferList)
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.
transferList
Optional-
Transferable objects to be transferred — these objects have their ownership transferred to the receiving browsing context, so are no longer usable by the sending browsing context.
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.
js
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