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Communicate between workers with BroadcastChannel

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BroadcastChannel is a named bus. Every channel opened with the same name receives what the others post, with no direct references between them. That makes it a simple way for the main thread and workers to talk. On Deno Deploy the same API even spans separate instances of an application. This example runs a worker from an inline module and exchanges messages with it over a shared channel name.

The worker is defined inline as source code, so this example stays in one file. In a real program this would live in its own module, created with new Worker(new URL("./worker.ts", import.meta.url), { type: "module" }).
const workerCode = `
const channel = new BroadcastChannel("jobs");
A channel never receives its own posts, only posts from the others.
channel.onmessage = (event) => {
  if (event.data.type === "job") {
    channel.postMessage({ type: "result", note: "finished " + event.data.name });
    channel.close();
    self.close();
  }
};
Announce readiness, because messages posted before this channel existed would have been lost.
channel.postMessage({ type: "ready" });
`;
Turn the source into a Blob URL and start a module worker from it.
const blobUrl = URL.createObjectURL(
  new Blob([workerCode], { type: "application/javascript" }),
);
new Worker(blobUrl, { type: "module" });
Open a channel with the same name on the main thread. Wait for the worker to say it is listening, hand it a job, and print the result.
const channel = new BroadcastChannel("jobs");

channel.onmessage = (event) => {
  if (event.data.type === "ready") {
    console.log("worker is listening"); // worker is listening
    channel.postMessage({ type: "job", name: "resize avatar.png" });
  } else if (event.data.type === "result") {
    console.log(event.data.note); // finished resize avatar.png
Close the channel so the process can exit. The worker already closed its own side and shut itself down with self.close.
    channel.close();
    URL.revokeObjectURL(blobUrl);
  }
};

Run this example locally using the Deno CLI:

deno run https://docs.deno.com/examples/scripts/broadcast_channel.ts

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