Introduction to Channel Messaging API

Given two scripts running in the same document, but in a different context, the Channel Messaging API allows them to communicate by passing messages through a channel.

This use case involves communication between

  • the document and an iframe
  • two iframes
  • two documents

How it works

Calling new MessageChannel() a message channel is initialized.

The channel has 2 properties, called

  • port1
  • port2

Those properties are a MessagePort object. port1 is the port used by the part that created the channel, and port2 is the port used by the channel receiver (by the way, the channel is bidirectional, so the receiver can send back messages as well).

Sending the message is done through the

otherWindow.postMessage()

method, where otherWindow is the other browsing context.

It accepts a message, an origin and the port.

A message can be a JavaScript basic value like strings, numbers, and some data structures are supported, namely

  • File
  • Blob
  • FileList
  • ArrayBuffer

“Origin” is a URI (e.g. https://example.org). You can use '*' to allow less strict checking, or specify a domain, or specify '/' to set a same-domain target, without needing to specify which domain is it.

The other browsing context listens for the message using MessagePort.onmessage, and it can respond back by using MessagePort.postMessage.

A channel can be closed by invoking MessagePort.close.

Let’s see a practical example in the next lesson.


An example with an iframe

Here’s an example of a communication happening between a document and an iframe embedded into it.

The main document defines an iframe and a span where we’ll print a message that’s sent from the iframe document. As soon as the iframe document is loaded, we send it a message on the channel we created.

<!DOCTYPE html>
<html>
  <body>
    <iframe src="iframe.html" width="500" height="500"></iframe>
    <span></span>
  </body>
  <script>
    const channel = new MessageChannel()
    const display = document.querySelector('span')
    const iframe = document.querySelector('iframe')

    iframe.addEventListener('load', () => {
      iframe.contentWindow.postMessage('Hey', '*', [channel.port2])
    }, false)

    channel.port1.onmessage = (e) => {
      para.innerHTML = e.data
    }
  </script>
</html>

The iframe page source is even simpler:

<!DOCTYPE html>
<html>
  <script>
  window.addEventListener("message", (event) => {
    if (event.origin !== "http://example.org:8080") {
      return
    }

    // process

    // send a message back
    event.ports[0].postMessage('Message back from the iframe')
  }, false)
  </script>
</html>

As you can see we don’t even need to initialize a channel, because the window.onmessage handler is automatically run when the message is received from the container page.

e is the event that’s sent, and is composed by the following properties:

  • data: the object that’s been sent from the other window
  • origin: the origin URI of the window that sent the message
  • source: the window object that sent the message

Always verify the origin of the message sender.

e.ports[0] is the way we reference port2 in the iframe, because ports is an array, and the port was added as the first element.


An example with a Service Worker

A Service Worker is an event-driven worker, a JavaScript file associated with web page. Check out the Service Workers guide to know more about them.

What’s important to know is that Service Workers are isolated from the main thread, and we must communicate with them using messages.

This is how a script attached to the main document will handle sending messages to the Service Worker:

// `worker` is the service worker already instantiated

const messageChannel = new MessageChannel()
messageChannel.port1.addEventListener('message', (event) => {
  console.log(event.data)
})
worker.postMessage(data, [messageChannel.port2])

In the Service Worker code, we add an event listener for the message event:

self.addEventListener('message', (event) => {
  console.log(event.data)
})

And it can send messages back by posting a message to messageChannel.port2, with

self.addEventListener('message', (event) => {
  event.ports[0].postMessage(data)
})

More on the inner workings of Service Workers in the Service Workers guide.