test_runner: document how to mock external libs

[mehulkar]

Affected URL(s)

https://nodejs.dev/en/api/v18/test/#mocking

Description of the problem

I don't see any docs here on how to mock an external library imported from node modules. It would be useful to see that!

[cjihrig]

Right now you should be able to mock individual parts of modules. However, if you're looking to mock an entire module in one call, that is not yet possible. You can read about the current state of that in https://github.com/orgs/nodejs/discussions/47959.

[mehulkar]

Turns out my specific issue is because my src was compiled to CJS using tsup, and I guess somewhere along the way the mock doesn't work. Not sure if that's a bug with Node's mocking or not, but I'll leave this issue open for documentation regardless

[cjihrig]

Not sure if that's a bug with Node's mocking or not

Can you provide a minimal reproduction? Based on the information provided in this issue it's impossible to tell.

I'll leave this issue open for documentation regardless

Without addressing the previous point, it's also not clear what would need to be documented.

[mehulkar]

Here's a minimal repro: https://github.com/mehulkar/node-test-runner-mocking

This is the output I see from running the test command from `main`:

``` ➜ node-test-runner git:(main) npm run test

node-test-runner-test@0.0.0 test
npm run build && node --test tests/index.test.mjs

node-test-runner-test@0.0.0 build
tsup

CLI Building entry: src/index.ts
CLI Using tsconfig: tsconfig.json
CLI tsup v7.2.0
CLI Using tsup config: /Users/mehulkar/dev/vercel/playground/node-test-runner/tsup.config.ts
CLI Target: es2022
CLI Cleaning output folder
ESM Build start
CJS Build start
DTS Build start
CJS dist/index.js 103.13 KB
CJS ⚡️ Build success in 352ms
ESM dist/index.mjs 103.12 KB
ESM ⚡️ Build success in 352ms
DTS ⚡️ Build success in 480ms
DTS dist/index.d.mts 78.00 B
DTS dist/index.d.ts 78.00 B
✖ testtesttest (465.910583ms)
AssertionError [ERR_ASSERTION]: Expected values to be strictly equal:

0 !== 1

  at TestContext.<anonymous> (file:///Users/mehulkar/dev/vercel/playground/node-test-runner/tests/index.test.mjs:16:10)
  at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
  at async Test.run (node:internal/test_runner/test:572:9)
  at async startSubtest (node:internal/test_runner/harness:202:3) {
generatedMessage: true,
code: 'ERR_ASSERTION',
actual: 0,
expected: 1,
operator: 'strictEqual'

}

ℹ tests 1
ℹ suites 0
ℹ pass 0
ℹ fail 1
ℹ cancelled 0
ℹ skipped 0
ℹ todo 0
ℹ duration_ms 552.657958

✖ failing tests:

✖ testtesttest (465.910583ms)
AssertionError [ERR_ASSERTION]: Expected values to be strictly equal:

0 !== 1

  at TestContext.<anonymous> (file:///Users/mehulkar/dev/vercel/playground/node-test-runner/tests/index.test.mjs:16:10)
  at process.processTicksAndRejections (node:internal/process/task_queues:95:5)
  at async Test.run (node:internal/test_runner/test:572:9)
  at async startSubtest (node:internal/test_runner/harness:202:3) {
generatedMessage: true,
code: 'ERR_ASSERTION',
actual: 0,
expected: 1,
operator: 'strictEqual'

}

</details>

If I remove tsup/typescript entirely it works. I _think_ there are some configs in both `tsconfig.json` and `tsup.config.js` that could make this better, but I haven't read through each of those and understood how tsup works yet.

[mehulkar]

I also added a branch that removes tsup and uses a --loader=@swc-node/register/esm instead and the tests seem to pass with that: mehulkar/node-test-runner-mocking#1

[mehulkar]

it's also not clear what would need to be documented.

It would just be helpful for the page to have an example where an external module is mocked out. At least for me, on reading that page, my first question was "does it support that", and when it didn't just work in my setup, I was left wishing for more docs that addressed this use case.

[cjihrig]

It looks like the issue is that you're bundling everything into a single file. So, you are mocking got.head() from node_modules but your test is actually using got.head() from dist/index.js.