Error in snapshotSerializer test doesn't have useful stack trace

[Daniel15]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
If a snapshot serializer's test method throws an exception, the stack trace doesn't point to it at all. This can make it difficult to determine where the exception is coming from.

Example:
Serializer:

const DummySerializer = {
  test: val => { throw new Error('Where is this error?') },
  print: val => '',
};

Config:

{
  "snapshotSerializers": [
    "<rootDir>/dummySerializer.js"
  ],
  "testRegex": ".*-test\\.js$"
}

Test:

test('hello world', () => {
  expect('foo').toMatchSnapshot();
})

Output:

 FAIL  ./add-test.js
  ● hello world

    Where is this error?
      
      at Object.<anonymous>.test (add-test.js:2:17)

add-test.js:2 is simply the expect('foo').toMatchSnapshot() call.

repl.it demo: https://repl.it/HKH4

What is the expected behavior?
Stack trace should point to the test method in the snapshot serializer.

Please provide your exact Jest configuration and mention your Jest, node, yarn/npm version and operating system.
I encountered this internally at Facebook, but can also repro on Jest v19.0.2 using repl.it.

[Nargonath]

I'd be happy to take a look at it and see if I can come up with a solution. 👍

[Nargonath]

Alright I'm going to try to report back on my current state on the matter. I'm not very much a Jest user at this point so bear with me if I misunderstood some of the OP information. Nevertheless I'm eager to learn so feel free to expose my mistakes, thanks. 👍

I added the provided test in this file: packages/jest-snapshot/src/__tests__/matcher.test.js alongside with an expect.addSnapshotSerializer() call to use the DummySerializer provided. You can see it here: Nargonath@a765803#diff-0daa7895e0d6a3b4644dd17dcd510eccR30. I'm not sure whether that's the best spot for it or not. You tell me, perhaps you preferred it in its own file.

I dug around in the project. I haven't been working on stack traces a lot but my first shot at the problem would be that the problem source is an Error.captureStackTrace() call that could be removing the interesting part from the stack trace. I may be wrong on that.

Plus I haven't been able to find where the test method of the serializer is actually called. I looked into toMatchSnapshot function and in the State match function. I found this: https://github.com/Nargonath/jest/blob/master/packages/jest-snapshot/src/State.js#L117 but it doesn't seem to be using the serializer test method, only serializing the argument given. Would you mind pointing me in the right direction please?

Thank you for your time.

[SimenB]

test is called by prettyFormat here https://github.com/facebook/jest/blob/e04e45102b3ebe486462d45db656f9b47f0e70e1/packages/pretty-format/src/index.js#L272 from this function in jest-snapshot: https://github.com/facebook/jest/blob/e04e45102b3ebe486462d45db656f9b47f0e70e1/packages/jest-snapshot/src/utils.js#L134-L144. You can stick a breakpoint there to trace upwards in the code

[Nargonath]

Thank you @SimenB ! I did look a bit in pretty-format source code but didn't notice anything. It seems I didn't look close enough. I'll continue digging. 👍

[nicolasiensen]

I was working on this issue a couple of weeks ago, but today I finally had enough time to finish it. Let me know what you think about the approach I used to fix the lack of details in the error messages.

[Nargonath]

I was working on the PR as well but I wasn't handling the problem the same way. I think your solution is elegant, probably better than mine. I'll add a review. 👍

[Nargonath]

I was going to add a property to the error object like doNotCapture or something alike so I could add a test here: https://github.com/facebook/jest/blob/0748e6f76394b58ce24aee42e08c96fb509e8744/packages/expect/src/index.js#L203 testing for the presence of that property. But like I said, your solution seems to be cleaner. Good job.