MessagePort.postMessage()
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
port.postMessage(message, transferList);
Returns
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.
Example
In the following code block, you can see a new channel being created using the
MessageChannel.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 MessageChannel.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.
var channel = new MessageChannel();
var para = document.querySelector('p');
var ifr = document.querySelector('iframe');
var 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', function (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