Web Workers

Workers are the only way of running javascript off of the main thread. This can be useful for a wide variety of programs, especially those where there is a lot of computation that needs to be done without blocking a thread.

First we will define a web worker, we can receive messages from the main thread and do some processing as a result of them.

./worker.ts

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

Currently, Deno only supports module-type workers. To instantiate one we can use similar syntax to what is found on the web.

./main.ts

const worker = new Worker(
  new URL("./worker.ts", import.meta.url).href,
  {
    type: "module",
  },
);

We can send a message to the worker by using the `postMessage` method

./main.ts

worker.postMessage({ filename: "./log.txt" });

By default the worker inherits the permissions of the thread it was instantiated from. We can give workers certain permissions by using the deno.permissions option.

./main.ts

const worker2 = new Worker(
  new URL("./worker.ts", import.meta.url).href,
  {
    type: "module",
    deno: {
      permissions: {
        read: [
          new URL("./file_1.txt", import.meta.url),
          new URL("./file_2.txt", import.meta.url),
        ],
      },
    },
  },
);

Because we instantiated this worker with specific permissions, this will cause the worker to try to read a file it doesn't have access to and thus will throw a permission error.

./main.ts

worker2.postMessage({ filename: "./log.txt" });

Additional resources:

Checking for file existence Web Assembly