MessagePort: postMessage() method

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.

None (undefined).

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).

• Using channel messaging