The MessageChannel interface of the Channel Messaging API allows us to create a new message channel and send data through it via its two MessagePort properties.

MessageChannel.port1 Read only

Returns port1 of the channel.

MessageChannel.port2 Read only

Returns port2 of the channel.

MessageChannel()

Returns a new MessageChannel object with two new MessagePort objects.

In the following example, you can see a new channel being created using the MessageChannel.MessageChannel constructor.

When the IFrame has loaded, we register an onmessage handler for MessageChannel.port1 and transfer MessageChannel.port2 to the IFrame using the window.postMessage method along with a message.

When a message is received back from the IFrame, the onMessage function outputs the message to a paragraph.

const channel = new MessageChannel();
const output = document.querySelector(".output");
const iframe = document.querySelector("iframe");

// Wait for the iframe to load
iframe.addEventListener("load", onLoad);

function onLoad() {
  // Listen for messages on port1
  channel.port1.onmessage = onMessage;

  // Transfer port2 to the iframe
  iframe.contentWindow.postMessage("Hello from the main page!", "*", [
    channel.port2,
  ]);
}

// Handle messages received on port1
function onMessage(e) {
  output.innerHTML = e.data;
}

For a full working example, see our channel messaging basic demo on GitHub (run it live too).

• Using channel messaging