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## Workers
Deno supports
[`Web Worker API`](https://developer.mozilla.org/en-US/docs/Web/API/Worker/Worker).
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:
```ts
// Good
new Worker("./worker.js", { type: "module" });
// Bad
new Worker("./worker.js");
new Worker("./worker.js", { type: "classic" });
```
### Permissions
Creating a new `Worker` instance is similar to a dynamic import; therefore Deno
requires appropriate permission for this action.
For workers using local modules; `--allow-read` permission is required:
```ts
// main.ts
new Worker("./worker.ts", { type: "module" });
// worker.ts
console.log("hello world");
self.close();
```
```shell
$ deno run main.ts
error: Uncaught PermissionDenied: read access to "./worker.ts", run again with the --allow-read flag
$ deno run --allow-read main.ts
hello world
```
For workers using remote modules; `--allow-read` permission is required:
```ts
// main.ts
new Worker("https://example.com/worker.ts", { type: "module" });
// worker.ts
console.log("hello world");
self.close();
```
```shell
$ deno run main.ts
error: Uncaught PermissionDenied: net access to "https://example.com/worker.ts", run again with the --allow-net flag
$ deno run --allow-net main.ts
hello world
```
### Using Deno in worker
> This is an unstable Deno feature. Learn more about
> [unstable features](./stability.md).
By default `Deno` namespace is not available in worker scope.
To add `Deno` namespace pass `deno: true` option when creating new worker:
```ts
// 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
```
```shell
$ 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.
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