ESM loaders cannot be defined via Worker option execArgv in v20

[cjihrig]

Version

20.0.0 (tested on main at 36e4e3d too)

Platform

macOS but probably all platforms

Subsystem

esm

What steps will reproduce the bug?

The following works in Node 18 and 19, but not 20:

// main.mjs
import { Worker } from 'node:worker_threads';

new Worker('./worker.js', {
  execArgv: ['--experimental-loader', './loader.mjs'],
});

// worker.js
'use strict';

async function main() {
  await import('node:fs');
}

main();

// loader.mjs
export function resolve (specifier, context, nextResolve) {
  throw new Error('boom');
}

Run: node main.mjs. In Node 18 and 19, the exception in the loader is thrown. In Node 20 it is not. The problem is not unique to throwing. I haven't been able to see console.log() or any other indication that the loader is being called.

How often does it reproduce? Is there a required condition?

100% of the time for me.

What is the expected behavior? Why is that the expected behavior?

I expect it to work as it did in earlier versions of Node. This is the expected behavior because right now it doesn't seem to work at all.

What do you see instead?

See the description of the bug above.

Additional information

Just a guess, but I'm assuming this is related to #44710.

[isaacs]

Another side effect of ES loaders to a separate thread: updating require.extensions in an es loader no longer has any effect, meaning that node --loader=ts-node/esm no longer functions properly to load both cjs and mjs typescript.

This makes it significantly more challenging to have loaders that work for both esm and cjs, effectively breaking the esm on-ramp for those of us supporting both modes, since now you need to know ahead of time what the module type is (or write the transpiled source to another path and return a short circuit url to that instead), rather than being able to have a single loader capable of handling both.

[isaacs]

Verified that node --loader=ts-node/esm foo.ts stopped working on 4667b07

[cjihrig]

cc: @nodejs/loaders

[JakobJingleheimer]

As stated in the ESM doc, async activity like console.log (which is specifically cited in the caveat) is not guaranteed to run.

Please try other methods, such as fs.writeSync to confirm.

I believe this is not a dupe of #47615 because it specifically provides execArgv, thus avoiding the fork-bomb.

[cjihrig]

@JakobJingleheimer I've tried fs.writeSync() and process._rawDebug() with no luck.

[mcollina]

I’m not sure how would it work in the current design. There is (by design) one loader process for the whole Node.js process. In the example above, we are asking Node.js to start another one just for that specific thread.

We need a new API to handle this case, something that:

1. allows us to create a "loaders worker", potentially exposing what we are already doing in core
2. pass that loader down to new Worker(path, { loader } ), so that the loader thread could be reused.

[cjihrig]

We need a new API to handle this case

What you are proposing seems like an explicit API. It seems like --experimental-loader passed to the Worker constructor should implicitly do that (without needing to expose more public APIs).

[targos]

It's interesting because it doesn't work --loader in the worker's execArgv, but it works fine if --loader is passed to the main script (the worker inherits the flag).

[GeoffreyBooth]

It’s interesting because it doesn’t work --loader in the worker’s execArgv, but it works fine if --loader is passed to the main script (the worker inherits the flag).

I don’t think of worker threads like child processes that can have their own Node flags. I feel like they should inherit whatever the parent process has, which is what @targos is describing here.

[cjihrig]

I don’t think of worker threads like child processes that can have their own Node flags.

That is the exact purpose of the execArgv option to the Worker constructor. Some flags are not supported there, but, for example, --require does appear to be. And the loaders docs do claim that they follow the pattern of --require

[GeoffreyBooth]

the loaders docs do claim that they follow the pattern of --require

Only in the sense that the way that chaining works for multiple invocations of --require is the same way that chaining works for multiple invocations of --loader. There’s not much else that loaders share in common with require.

Unless there’s some argument for why this should be considered a bug, I think we should just update the docs for https://nodejs.org/api/worker_threads.html#new-workerfilename-options to clarify that --loader is one of the flags not supported by that option.

[cjihrig]

I guess my question is, are there actually technical reasons why this can't be implemented?

[JakobJingleheimer]

I guess my question is, are there actually technical reasons why this can't be implemented?

I was about to ask this too. If it works from main, I would've expected it to work here too, but if not, I imagine it would be fairly simple to support from the esm worker 🤔 (unless we get stuck in Inception)

[GeoffreyBooth]

I guess my question is, are there actually technical reasons why this can’t be implemented?

In the current behavior, there’s a loaders thread and it works for both the main application thread and all worker threads (I think); within that loaders thread there’s shared scope that the loaders can use, such as to keep caches and such. If we support execArgv then that would require spinning up a new loaders thread specific to that worker thread, because the --loader argument passed to execArgv might be different than that used for the root process. This presents a few issues:

• There are now multiple loaders threads, with multiple scopes, and so loader authors would need to be aware of this possibility. We would probably get calls for ways to communicate between loaders threads.
• The number of loaders threads would increase linearly with the number of worker threads spawned with execArgv, doubling the memory/processing requirements of each worker thread.

In short, it’s a lot of added complexity for both Node and for loader authors, and for what? I don’t see the benefit. If you want to create a new process with different loaders, spawn a child process.

[mcollina]

@GeoffreyBooth I agree with your assessment with supporting execArgv: it will cost too much memory and CPU time. I still think allowing to provide a custom loader for each worker is very useful for all sorts of use cases (mocking, transpiling, ...). Processes are more costly and harder to manage.

IMHO, we need a new API to handle this case, something that:

1. allows us to create a "loaders worker", potentially exposing what we are already doing in core.
2. pass that loader down to new Worker(path, { loader } ), so that the loader thread could be reused across multiple workers.

[GeoffreyBooth]

I still think allowing to provide a custom loader for each worker is very useful for all sorts of use cases

What would a specific use case be? When would you want to do something that only a loader can do, that you wouldn’t want applied to the main application thread?

Keep in mind there’s also the registerLoader API in progress, that allows for registering loaders after startup.

[mcollina]

What would a specific use case be? When would you want to do something that only a loader can do, that you wouldn’t want applied to the main application thread?

CLI tools can't really set Node.js options. registerLoader would solve all the same use cases of execArgv in this case. One can always set up a "jump" worker to get things done.
I didn't know it was in the works, is there a tracking issue/pr?

[cjihrig]

Looks like the PR is #46826. I believe that would work for my use case.

I still think execArgv should be an option, but I also don't care enough to push for it if there is an alternative that works for me.

[GeoffreyBooth]

I believe that would work for my use case.

What is your use case?

CLI tools can’t really set Node.js options.

We’ve been discussing adding support for .env files: #43973 (comment). This would be a shorter-term solution for being able to set NODE_OPTIONS from a file. Farther down the road I think we’d like to support a new field in package.json (or possibly even a new config file) that would do the same thing.

[cjihrig]

What is your use case?

Using loaders in a worker without requiring users to specify CLI flags. Using a shebang is not really an option for me either, and ideally I don't want to spawn a child process just to hide the CLI flag. The fact that the loader thread is shared between workers and the main thread is not a problem for me.

[GeoffreyBooth]

Using loaders in a worker without requiring users to specify CLI flags.

Can you explain a bit more? Is this a CLI tool? What does it do? Why does it create worker threads?

[cjihrig]

I can't go into too much detail yet unfortunately, as it is for work and still unreleased. There is a CLI, but also an SDK. The loader itself is just used for hot module reloading, so I think registerLoader() will work just fine.

[GeoffreyBooth]

The loader itself is just used for hot module reloading, so I think registerLoader() will work just fine.

If you find a good general solution for hot module reloading, please share it.

The registerLoader PR is by a new contributor and I’m not sure he’s working on it any more, so if you have time to help get it finished, that would be greatly appreciated.

[cjihrig]

I will take a look at the registerLoader PR and can take it over if the current author is no longer interested in working on it.

[targos]

In the current behavior, there’s a loaders thread and it works for both the main application thread and all worker threads (I think)

That's wrong. Each worker thread has its own loaders thread. Without evidence of the contrary, I think we should consider that there is a bug somewhere.

[targos]

Found the bug: when a worker is created with execArgv, its child loaders worker doesn't inherit the CLI arguments, so we have:

main thread: []
main thread loaders: []
worker thread: ['--experimental-loader', './loader.mjs']
worker thread loaders: []

[bradchristensen]

✨ Edit: For those looking for a workaround in the meantime, @hi-ogawa has a working solution described here:

vitest-dev/vitest#5757 (comment)

---

@GeoffreyBooth thanks for championing this for us! I've just tried out the newly released v22.2.0 as you suggested, with the following simplified repro, in which the script tries to spawn a worker thread from its own file path:

import { fileURLToPath } from "node:url";
import { Worker, isMainThread } from "node:worker_threads";

const THREAD_COUNT = 2;

if (isMainThread) {
  console.log("Main thread running");

  const workers = [...new Array(THREAD_COUNT)].map(() => {
    const worker = new Worker(fileURLToPath(import.meta.url));

    // Capture threadId immediately after spawning, while the worker is alive
    const threadId = worker.threadId;

    worker.on("exit", (exitCode: number) => {
      console.log(`Worker ${threadId} exited with code ${exitCode}`);
    });
    return worker;
  });

  // The real application would post messages to the worker threads here
} else {
  console.log("Doing work");
}

Unfortunately I found that for both Node v22.1.0 and v22.2.0, executing this with npx tsx src/index.ts has the same result - it fails and logs the following:

Main thread running

node:internal/event_target:1090
  process.nextTick(() => { throw err; });
                           ^
TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".ts" for /app/src/index.ts

I was executing this in an ESM package (with { "type": "module" }), but changing it to "type": "commonjs" didn't appear to change anything.

I'm not familiar with the internals here but I'm wondering if this is a consequence of not being able to modify require.extensions as per @isaacs' comment above, or something similar (e.g. would it work as expected with a custom loader for regular .js files, and it's just .ts that it doesn't recognise now?)

[SynthLuvr]

I'm getting ERR_UNKNOWN_FILE_EXTENSION for node v20.13.1. For node v22.2.0, I'm getting timeouts before the scripts are even run.

Edit: For node v22.1.0 I also get ERR_UNKNOWN_FILE_EXTENSION. So the error changed between v22.1.0 and v22.2.0.

[daniel-nagy]

Update

I was missing something, you need to import the code that registers the hooks inside the worker. Once I did that the hooks started working.

---

Am I missing something, or are single thread hooks not working in Node v22.2.0?

According to the changelog, this landed in v22.2.0

[8fbf6628d6] - module: have a single hooks thread for all workers (Gabriel Bota) #52706

However, I can't seem to make this work. I'm trying to add a loader for HTTP imports, using the example in the Nodejs docs (modified for HTTP instead of HTTPS). It works on the main thread but when I spawn a worker I get the following error.

Error [ERR_UNSUPPORTED_ESM_URL_SCHEME]: Only URLs with a scheme in: file, data, and node are supported by the default ESM loader. Received protocol 'http:'

I actually do want different loaders for the main thread and the worker thread. My use case is for React 19 server components. But I can probably workaround the single hooks thread limitation if it worked.

[daniel-nagy]

I see most people here are trying to use tsx to import a worker that is a TypeScript file. That still does not work. Node will throw an error

TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".tsx"

The workaround suggested here does work. However, when I pass conditions to my worker that does not seem to work with tsImport. e.g.

const worker = new TsWorker(new URL("./ClientRenderer.tsx", import.meta.url), {
  execArgv: ["--conditions", "node"], // <-- doesn't seem to be working with suggested workaround.
});

[Guihgo]

If you are using tsx you can run work threads as follow:

1. Remove "type": "module", from package.json
2. Imports without extension .ts or .js

• Use: import {some} from "./my_dir/my_file"
• Don't use: import {some} from "./my_dir/my_file.js"

3. Create the Worker using tsx with execArgv param

const worker = new Worker(require.resolve(workerPath), { execArgv: ["--require", "tsx/cjs"] })

Like if its help you 👍

---

the 'bug' was reported at privatenumber/tsx#354 (comment) too.

[SystemParadox]

Remove type: module?? That's really counterproductive.

[daniel-nagy]

BTW, the --conditions issue is a separate issue with Node itself #50885

[GeoffreyBooth]

If you are using tsx you can run work threads as follow:

This approach has tsx transpile everything to CommonJS and run it that way. You won’t be able to import ESM dependencies if you do this, at least without --experimental-require-module. Your code will run as CommonJS, not as ESM.

[GeoffreyBooth]

FYI, #52706 landed in 22.2.0 as an attempt to have a single hooks thread that applies to all application threads, including the main thread. It has some issues that we’re addressing in follow-ups, but in 22.3.0 or 22.4.0 there should hopefully be a cleaner way to handle this, where if you register hooks (such as tsx) at startup then they will automatically apply to all worker threads. So node --import=tsx app.js should work and apply tsx to any worker threads that your code creates, without needing to specify anything related to tsx in execArgv.

[proxybee]

✨ Edit: For those looking for a workaround in the meantime, @hi-ogawa has a working solution described here:

vitest-dev/vitest#5757 (comment)

@GeoffreyBooth thanks for championing this for us! I've just tried out the newly released v22.2.0 as you suggested, with the following simplified repro, in which the script tries to spawn a worker thread from its own file path:

import { fileURLToPath } from "node:url";
import { Worker, isMainThread } from "node:worker_threads";

const THREAD_COUNT = 2;

if (isMainThread) {
  console.log("Main thread running");

  const workers = [...new Array(THREAD_COUNT)].map(() => {
    const worker = new Worker(fileURLToPath(import.meta.url));

    // Capture threadId immediately after spawning, while the worker is alive
    const threadId = worker.threadId;

    worker.on("exit", (exitCode: number) => {
      console.log(`Worker ${threadId} exited with code ${exitCode}`);
    });
    return worker;
  });

  // The real application would post messages to the worker threads here
} else {
  console.log("Doing work");
}

Unfortunately I found that for both Node v22.1.0 and v22.2.0, executing this with npx tsx src/index.ts has the same result - it fails and logs the following:

Main thread running

node:internal/event_target:1090
  process.nextTick(() => { throw err; });
                           ^
TypeError [ERR_UNKNOWN_FILE_EXTENSION]: Unknown file extension ".ts" for /app/src/index.ts

I was executing this in an ESM package (with { "type": "module" }), but changing it to "type": "commonjs" didn't appear to change anything.

I'm not familiar with the internals here but I'm wondering if this is a consequence of not being able to modify require.extensions as per @isaacs' comment above, or something similar (e.g. would it work as expected with a custom loader for regular .js files, and it's just .ts that it doesn't recognise now?)

I guess your take works but when running typescript ensure your tsconfig is properly set and you build with tsc before running node, I was able to run the above with this tsconfig, it is just the index.ts so this works.
{
"compilerOptions": {
"rootDir": "./",
"outDir": "build",
"target": "esnext",
"module": "NodeNext",
"lib": ["es2020"],
"moduleResolution":"NodeNext",
},
"include": [
".ts",
".js"
],
"exclude": [
"package.json"
]
}

[bradchristensen]

@proxybee it's true that is certainly worth considering, if it fits your use case, but then you wouldn't be using a loader at all, which is the topic being discussed here 🙂

[GeoffreyBooth]

There’s a discussion at nodejs/loaders#203 regarding whether registering hooks should apply automatically to new Worker calls that spawn worker threads. Please comment there if you have any opinions to share. cc @cjihrig

[alshdavid]

I have been doing this while I wait for a solution:

// package.json
{
  "type": "module",
  "imports": {
    "#worker": {
      "source": "./cmd/worker.ts",
      "default": "./cmd/worker.js"
    }
  },
  // ...
}

// cmd/bin.ts
export function spawnWorker() {
  let workerPath = url.fileURLToPath(import.meta.resolve('#worker'));
  if (workerPath.endsWith('.ts')) {
    return new Worker(`import('tsx/esm/api').then(({ register }) => { register(); import('${workerPath}') })`, { eval: true })
  } else {
    return new Worker(workerPath)
  }
}

spawnWorker()

Then run it with:

node --conditions="source" --import tsx ./cmd/bin.ts

My long term plan here is to eventually replace tsx with Node's built-in --experimental-strip-types (though this pattern works already with Node 22.6+, it's not yet usable)

[SynthLuvr]

I have been doing this while I wait for a solution:

// package.json
{
  "type": "module",
  "imports": {
    "#worker": {
      "source": "./cmd/worker.ts",
      "default": "./cmd/worker.js"
    }
  },
  // ...
}

// cmd/bin.ts
export function spawnWorker() {
  let workerPath = url.fileURLToPath(import.meta.resolve('#worker'));
  if (workerPath.endsWith('.ts')) {
    return new Worker(`import('tsx/esm/api').then(({ register }) => { register(); import('${workerPath}') })`, { eval: true })
  } else {
    return new Worker(workerPath)
  }
}

spawnWorker()

Then run it with:

node --conditions="source" --import tsx ./cmd/bin.ts

My long term plan here is to eventually replace tsx with Node's built-in --experimental-strip-types (though this pattern works already with Node 22.6+, it's not yet usable)

I've tried this workaround but cannot get it working. Still stuck with this issue

[alshdavid]

I've tried this workaround but cannot get it working. Still stuck with this issue

This is how I am currently using it links: [1] [2] [3]

I'm using Node 20 and tsx.
Eventually, I will replace tsx with --experimental-strip-types (when .ts extensions can be rewritten to .js by tsc)

[SynthLuvr]

This is how I am currently using it links: [1] [2] [3]

I've see the solution but it doesn't work for my use case. I still need a fix for the problem described in this ticket.

[SynthLuvr]

In which version(s) is this fixed?

[SynthLuvr]

I tried on v20.17.0 and v22.9.0 and couldn't get it working. I think this ticket was closed prematurely.

[abetoots]

although @alshdavid already works, path aliases won't work. the tsx loader registered through eval doesn't pick up the paths defined in tsconfig.json. another solution is to do it in an actual file then import your ts scripts from there:

//worker.mts
import { fileURLToPath,} from "node:url";

const loader= fileURLToPath(import.meta.resolve("./dev-worker-ts-resolver.mjs"));

const scriptPath = './path-to-typescript-script.ts'

const worker = new Worker(loader, { workerData: { scriptPath } });

worker.on("message", (message) => {
  console.log(`message from worker: ${message}`);
});

//dev-worker-ts-resolver.mjs
import { register } from "tsx/esm/api";
import { workerData } from "node:worker_threads";

register();

if(workerData.scriptPath){
  await import(workerData.scriptPath);
}