denoland-deno/docs/runtime/workers.md

Workers

Deno supports Web Worker API.

Workers can be used to run code on multiple threads. Each instance of Worker is run on a separate thread, dedicated only to that worker.

Currently Deno supports only module type workers; thus it's essential to pass type: "module" option when creating new worker:

// Good
new Worker("./worker.js", { type: "module" });

// Bad
new Worker("./worker.js");
new Worker("./worker.js", { type: "classic" });

Using Deno in worker

UNSTABLE: This feature is unstable and requires --unstable flag

By default Deno namespace is not available in worker scope.

To add Deno namespace pass deno: true option when creating new worker:

// main.js
const worker = new Worker("./worker.js", { type: "module", deno: true });
worker.postMessage({ filename: "./log.txt" });

// worker.js
self.onmessage = async (e) => {
  const { filename } = e.data;
  const text = await Deno.readTextFile(filename);
  console.log(text);
  self.close();
};

// log.txt
hello world

$ deno run --allow-read --unstable main.js
hello world

When Deno namespace is available in worker scope; the worker inherits parent process permissions (the ones specified using --allow-* flags).

We intend to make permissions configurable for workers.