Worker eval as ES Module

[targos]

Unless I missed it, there doesn't seem to be a way of creating a Worker from an ESM source string (with eval: true).

Example that should be possible somehow:

import { Worker } from 'worker_threads';

new Worker(
  `
  import { join } from 'path';
  console.log(join('hello', 'world'));
`,
  { eval: true },
);

Related: #21502

[devsnek]

Also this compat isn't theoretical, i've been able to run wasm threading demos in node just by creating globals of MessagePort and Worker from worker_threads.

[guybedford]

Here is my best shot at a single worker.mjs file that works in both browsers and Node.js:

const isNode = typeof process === 'object' && process.versions && process.versions.node;

// Give me Top-Level Await!
Promise.resolve()
.then(async () => {
  const workerThreads = isNode && await import('worker_threads');
  const fileURLToPath = isNode && (await import('url')).fileURLToPath;

  const isMainThread = isNode ? workerThreads.isMainThread : typeof WorkerGlobalScope === 'undefined' || self instanceof WorkerGlobalScope === false;

  if (isMainThread) {
    const { Worker } = isNode ? workerThreads : window;

    const worker = new Worker(isNode ? fileURLToPath(import.meta.url) : import.meta.url, { type: 'module' });

    worker.postMessage('hello');

    if (isNode)
      worker.on('message', onMessage);
    else
      worker.addEventListener('message', onMessage);

    function onMessage (e) {
      console.log(isNode ? e : e.data);
    }
  }

  // Worker
  else {
    if (isNode)
      workerThreads.parentPort.on('message', onMessage);
    else
      addEventListener('message', onMessage);

    function onMessage(e) {
      (isNode ? workerThreads.parentPort : self).postMessage((isNode ? e : e.data) + ' world');
    }
  }
});

The story works, but it took me at least half an hour to get all the details right. All in all it needs 9 branches to handle platform checks. Supporting "type": "module" was not even an afterthought in that process.

This is what I mean when I say that arguing for "type": "module" for workers as if it somehow is a compatibility issue at this point, is to ignore those 9 other conditional branches which truly do affect this workflow, while that one does not.

[addaleax]

@guybedford I wouldn’t call it a compatibility issue so much as a familiarity issue – the Worker API was written to match Node’s needs and differences from the Web platform, but it was also intended to be familiar to those who use Workers on the Web.

[guybedford]

@addaleax I am only arguing against the points made by @devsnek and @GeoffreyBooth that we have to use new Worker(..., { "type": "module" }) here in the name of browser compatibility, by clearly trying to show the API differences. I am fully behind the approach of worker_threads in that the browserify implementation of worker_threads would effectively be acting as an adapter for the browser though, exactly based on web workers API being an overall guide.

The reason I'm making this argument is because Node.js modules have a way to know the module type of every single module, which we've carefully worked out. We specially don't allow inline type information for the loader, so I don't think we should make workers an exception here. If we were to support "type": "module" for worker instantiation this introduces a new out-of-band mechanism that further complicates module interpretation, whereas the current techniques of module detection currently work perfectly fine for worker threads.

This is why my suggestion is to rely on the content-type for data URIs like we do in the module loader, or to have something like "eval": "module" specifically not to create the assumption for users that "type": "module" for workers is supported.

[devsnek]

using onmessage and a separate file for the worker (which is the common case on the web) you end up code that i would feel comfortable describing as portable. obviously one can write code that is not compatible (using isMainThread, addEventListener, etc) but it isn't required.

[aduh95]

Related #34584

[dmail]

I ran into this issue today and there is a "workaround" to be able to execute ESM in worker as explained in #36985 (comment)

import { Worker } from "node:worker_threads"

const worker = new Worker(new URL('data:text/javascript,import fs from "fs"'));

However the code inside the data url throws when it contains relative urls.

import { Worker } from 'node:worker_threads'

const worker = new Worker(new URL('data:text/javascript,import "./file.mjs"'));

Ideally I could do something as below:

new Worker(`
  import value from "./file.js";
  console.log(value);
`,
  { eval: true, type: 'module', url: import.meta.url },
);

I understand it's not possible today but is there something I can do with the API offered by Node.js?

It does not seems doable with vm.Module nor Loaders

[aduh95]

I don't know if that fits your use-case, but you should be able to do the following:

const toDataUrl = js => new URL(`data:text/javascript,${encodeURIComponent(js)}`);
const toAbsoluteUrl = relativeUrl => JSON.stringify(new URL(relativeUrl, import.meta.url));

new Worker(toDataUrl(`
  import value from ${toAbsoluteUrl('./file.js')};
  console.log(value);
`)
);

[dmail]

Converting to absolute urls would work indeed!
In my use case the string passed to the worker is dynamic. But I can use babel to transform and resolve all relative import specifiers found in the code 🤔 .

Good enough for me.

Edit: Actually no 😬 . There is several other cases where node throw ERR_INVALID_URL errors

new Worker(toDataUrl(`
  import value from "leftpad";
  console.log(value);
`)
);

☝️ In the code above "leftpad" would be a package declared in my package.json. But as code is executed in the context of a data url, Node.js won't find my package.json.

[dmail]

I guess I could resolve all specifiers with import.meta.resolve but it's experimental so I would be forced to put --experimental-import-meta-resolve in all my node commands using this technic 😬

[ljharb]

It also doesn’t work properly, even with the flag.

[foosmithco]

If ESM is unavoidable, it may be sensible to write the eval'd code to file first before calling the worker.

// pseudo-code
import {writeFile} from 'fs/promises';

await writeFile(
  './module-eval.js', 
 `import something from 'somewhere';
console.log(something.doSomething());
`,
  'utf8'
);

const worker = new Worker('./module-eval.js', {type: 'module'});

Note: This assumes top-level await support.

[hax]

We are now at 2023, and still no {type:"module"} support, and it waste three engineers one hour to struggle and finally I find this issue. 😒

[aduh95]

And of course those three engineers are going to invest their time to contribute a fix, or they are simply going to waste everyone’s time complaining it’s 2023?

[isaacs]

A lot reasons to complain about it being 2023, to be fair.

[avivkeller]

new (require('worker_threads').Worker)(
  `
  import { join } from 'path';
  console.log(join('hello', 'world'));
`,
  { eval: true, type: 'module' },
);

import { Worker } from 'worker_threads';

new Worker(
  `
  import { join } from 'path';
  console.log(join('hello', 'world'));
`,
  { eval: true, type: 'module' },
);

AFAICT both these work, so I've closed this issue. LMK if I'm missing something.

[targos]

I'll have a look. Please don't close my issues unless there's an obvious reason (like a PR that fixes it)

[targos]

It also works without type: 'module'. This is unexpected.

[aduh95]

It also works without type: 'module'. This is unexpected.

Not really, type: 'module' is completely ignored – it's not an option the constructor supports. The snippet works unless you run it with the --no-experimental-detect-module CLI flag.

[targos]

I know it's ignored. What I find unexpected is that module detection happens here (in the eval path)

[aduh95]

I don't think it's unexpected (at least I don't find it surprinsing), string inputs being affected was mentioned in the original PR that added the flag (#50096 (comment)). However, what I find surprising is that the snippet is also broken if you pass --input-type=module in the CLI flags:

$ node <<'EOF'                    
import { Worker } from 'worker_threads';

new Worker(
  `
  import { join } from 'path';
  console.log(join('hello', 'world'));
`,
  { eval: true },
);
EOF
hello/world
$ node --input-type=module <<'EOF'                               
import { Worker } from 'worker_threads';

new Worker(
  `
  import { join } from 'path';
  console.log(join('hello', 'world'));
`,
  { eval: true },
);
EOF

node:internal/event_target:1090
  process.nextTick(() => { throw err; });
                           ^
[worker eval]:2
  import { join } from 'path';
  ^^^^^^

SyntaxError: Cannot use import statement outside a module
    at makeContextifyScript (node:internal/vm:185:14)
    at node:internal/process/execution:107:22
    at [worker eval]-wrapper:6:24
    at runScript (node:internal/process/execution:101:62)
    at evalScript (node:internal/process/execution:136:3)
    at MessagePort.<anonymous> (node:internal/main/worker_thread:167:9)
    at [nodejs.internal.kHybridDispatch] (node:internal/event_target:816:20)
    at MessagePort.<anonymous> (node:internal/per_context/messageport:23:28)

Node.js v23.0.0-pre