[Bug]: Worker class of jest-worker does not support ESM worker modules

[mrazauskas]

Version

v27.4.7

Steps to reproduce

An exception is raised, if I try to initialise a worker with an ESM module: new Worker('path/to/ESMworker.mjs'). Looks like jest-worker is simply trying to require the module and that does not work:
https://github.com/facebook/jest/blob/d1bc333c549ffb53691e1ba68b16a21915fc5fdc/packages/jest-worker/src/workers/processChild.ts#L113-L114

https://github.com/facebook/jest/blob/d1bc333c549ffb53691e1ba68b16a21915fc5fdc/packages/jest-worker/src/workers/processChild.ts#L96-L97

https://github.com/facebook/jest/blob/d1bc333c549ffb53691e1ba68b16a21915fc5fdc/packages/jest-worker/src/index.ts#L32-L33

I was trying to refactor the code using the requireOrImportModule from jest-utils. Unfortunately the private method this._bindExposedWorkerMethods() of the Worker class has to become async, but it is called in the constructor. Async calls are not possible in constructors.

Good news: Worker is a named export. If I get it right, this makes it possible to export something else from jest-worker. So perhaps an asynchronous createWorker() factory function could be a non-breaking solution?

import {createWorker} from 'jest-worker';

const workerPath = new URL('./testWorker.js', import.meta.url).pathname;

const worker = await createWorker(workerPath);

@SimenB Does it look acceptable for you? Feels like I could solve it this way.

Expected behavior

Would be great to support ESM worker modules in jest-worker package.

Actual behavior

jest-worker package does not support ESM worker modules.

Additional context

No response

Environment

System:
    OS: macOS 10.15.7
    CPU: (8) x64 Intel(R) Core(TM) i7-3615QM CPU @ 2.30GHz
Binaries:
    Node: 17.3.1 - ~/.nvm/versions/node/v17.3.1/bin/node
    Yarn: 2.4.3 - /usr/local/opt/node@16/bin/yarn
    npm: 8.3.0 - ~/.nvm/versions/node/v17.3.1/bin/npm
npmPackages:
    jest: workspace:* => 27.4.7

[mrazauskas]

@SimenB Would it be possible to include this issue in Jest 28 milestone? I really need it for one work-in-progress test runner (;

Might be there is better solution, but it seemed like having async createWorker('path/toWorker') factory instead of new Worker('path/toWorker') would be enough. Perhaps this breaking change could be introduced now and the real ESM support could be implemented later.

The problem is that constructor of the class cannot be asynchronous. Async createWorker() makes it possible to await import() the worker module in order to detect exported methods.

[SimenB]

@SimenB Does it look acceptable for you? Feels like I could solve it this way.

yes! factory pattern is how we've solved other "class does require in constructor, we need await import"

[SimenB]

@mrazauskas wanna send a PR for this one? 🙂

[mrazauskas]

Yes, I would love to. Next week (;

[SimenB]

yay!

[mrazauskas]

This is so old. Probably nobody needs it anymore. Closing.

[github-actions]

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