Missing require.main inside of scripts loaded inside of isolateModules callback

[Alexsey]

🐛 Bug Report

If some script would be required inside of jest.isolateModules callback then require.main would be null inside of it. Thank is not how NodeJS works and not how the same script would work if it would be loaded outside of jest.isolateModules

To Reproduce

Steps to reproduce the behavior:

Consider a super simple script that would just use require.main.require index.js

const path = require('path')

module.exports = () => require.main.require(path.resolve(__dirname, './package.json'))

And a test for it

let foo
jest.isolateModules(() =>
  foo = require('./index')
)

test('test', () => {
  expect(foo()).toEqual({
    dependencies: { jest: '^25.1.0' },
    name: 'jest-main-require-bug',
    scripts: { test: 'jest' },
    version: '1.0.0'
  })
})

Expected behavior

The test should pass. And it will, if only 2 and 4 lines of index.test.js that wrap require inside of jest.isolateModules would be removed

Link to repl or repo (highly encouraged)

https://github.com/Alexsey/jest-require-main-bug

envinfo

npx: installed 1 in 3.765s

  System:
    OS: Windows 10 10.0.18362
    CPU: (4) x64 Intel(R) Core(TM) i5-6500 CPU @ 3.20GHz
  Binaries:
    Node: 13.7.0 - C:\Program Files\nodejs\node.EXE
    npm: 6.13.6 - C:\Program Files\nodejs\npm.CMD
  npmPackages:
    jest: ^25.1.0 => 25.1.0

[SimenB]

require.main is implemented by recursively walking up all .parent and returning the first one: https://github.com/facebook/jest/blob/084508290f4a90b28c3190021d3358ab1f753c3f/packages/jest-runtime/src/index.ts#L1018-L1033

See #5618 for some info on why this algorithm was chosen

Logic for parent: https://github.com/facebook/jest/blob/084508290f4a90b28c3190021d3358ab1f753c3f/packages/jest-runtime/src/index.ts#L756-L762

This breaks due to the way isolateModules is implemented, which is by making moduleRegistry a fake empty one: https://github.com/facebook/jest/blob/084508290f4a90b28c3190021d3358ab1f753c3f/packages/jest-runtime/src/index.ts#L533-L544

Followed by https://github.com/facebook/jest/blob/084508290f4a90b28c3190021d3358ab1f753c3f/packages/jest-runtime/src/index.ts#L330-L341

I'm not sure how to best untangle this so require.main is still set to the test module, but otherwise we ignore cached modules...

Would you be able to dig a bit into this an potentially provide a PR?

//cc @rogeliog in case you have any ideas

[jeysal]

Looks like the same problem will exist with .parent?
I think we need to drop the "swapping out the module registry" approach and instead introduce an optional "first check the isolated module registry if it exists" somewhat similar to how we first check for mocks before real modules.
We can then enable that logic for certain types of lookups (like standard require) but not others (like .parent).

[SimenB]

@jeysal yeah, agreed. The current approach is a bit too naive

[flozender]

Can I help with this? Or does #10610 fix this?

[SimenB]

Yes please!

#10610 does not fix it since _isolatedModuleRegistry will still take preference. So we need to somehow keeping track of the main module. Possibly by assigning it as an instance variable mentioned here: #5618 (comment)

You solved the issue of "first require might not be test file" in #10610, so that should not be an issue

[github-actions]

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