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BroadcastChannel

In Deno Deploy, code is run in different data centers around the world in order to reduce latency by servicing requests at the data center nearest to the client. In the browser, the BroadcastChannel API allows different tabs with the same origin to exchange messages. In Deno Deploy, the BroadcastChannel API provides a communication mechanism between the various instances; a simple message bus that connects the various Deploy instances world wide.

Constructor

The BroadcastChannel() constructor creates a new BroadcastChannel instance and connects to (or creates) the provided channel.

let channel = new BroadcastChannel(channelName);

Parameters

name type description
channelName string The name for the underlying broadcast channel connection.

The return type of the constructor is a BroadcastChannel instance.

Properties

name type description
name string The name of the underlying broadcast channel.
onmessage function (or null) The function that's executed when the channel recieves a new message ([MessageEvent][messageevent]).
onmessageerror function (or null) The function that's executed when the arrived message cannot be deserialized to a JavaScript data structure.

Methods

name description
close() Close the connection to the underlying channel. After closing, you can no longer post messages to the channel.
postMessage(message) Post a message to the underlying channel. The message can be a string, object literal, a number or any kind of [Object][object].

BroadcastChannel extends EventTarget, which allows you to use methods of EventTarget like addEventListener and removeEventListener on an instance of BroadcastChannel.

Example

A small example that has an endpoint to send a new message to all other actively running instances in different regions and another to fetch all messages from an instance.

import { serve } from "https://deno.land/std@0.114.0/http/server.ts";

const messages = [];
// Create a new broadcast channel named earth.
const channel = new BroadcastChannel("earth");
// Set onmessage event handler.
channel.onmessage = (event: MessageEvent) => {
  // Update the local state when other instances
  // send us a new message.
  messages.push(event.data);
};

function handler(req: Request): Response {
  const { pathname, searchParams } = new URL(req.url);

  // Handle /send?message=<message> endpoint.
  if (pathname.startsWith("/send")) {
    const message = searchParams.get("message");
    if (!message) {
      return new Response("?message not provided", { status: 400 });
    }

    // Update local state.
    messages.push(message);
    // Inform all other active instances of the deployment
    // about the new message.
    channel.postMessage(message);
    return new Response("message sent");
  }

  // Handle /messages request.
  if (pathname.startsWith("/messages")) {
    return new Response(JSON.stringify(messages), {
      "content-type": "application/json",
    });
  }

  return new Response("not found", { status: 404 });
}

console.log("Listening on http://localhost:8000");
serve(handler);

You can test this example by making an HTTP request to https://broadcast.deno.dev/send?message=Hello_from_<region> and then making another request to https://broadcast.deno.dev/messages from a different region (by using a VPN or some other way) to check if the first request's message is present in the second region.

We built a small chat application that you can play with at https://denochat.deno.dev/