Documentation is not clear about how to use source maps in custom transformers

[fcostarodrigo]

🐛 Bug Report

Documentation is not clear about how to use source maps in custom transformers.
Additionally, if you don't pass any source maps, jest will report a failed assertion in a random position in the original not transformed source code.

To Reproduce

Steps to reproduce the behavior:

1. Visit Custom transformers documentation page.
2. Create a custom transform following the documentation.
3. Create a test that fails.
4. See the jest output pointing at a random position in the original source code.

Expected behavior

Inform how the source map should be returned by a custom transformer.
If the code was transformed and the source map was not provided, don't just assume the lines and columns will match.

Link to repl or repo (highly encouraged)

https://repl.it/@fcostarodrigo/JestCustomTransformIssue

I am using CoffeeScript just as an example.

envinfo

  System:
    OS: Linux 5.4 Debian GNU/Linux 9 (stretch) 9 (stretch)
    CPU: (4) x64 Intel(R) Xeon(R) CPU @ 2.30GHz
  Binaries:
    Node: 12.18.2 - /usr/local/bin/node
    Yarn: 1.22.4 - /usr/local/bin/yarn
    npm: 6.14.5 - /usr/local/bin/npm
  npmPackages:
    jest: ^24.9.0 => 24.9.0 

[Mskat]

Can I grab it?

[thymikee]

@Mskat sure! :)

[fcostarodrigo]

There is a code snippet in https://jestjs.io/docs/next/code-transformation#writing-custom-transformers (next version doc), reading the types you can see the structure expected by the source map. But maybe if the code is transformed and no source map is provided, maybe jest shouldn't try to display the position in the file where the error occurs.

[SimenB]

A note about the shape of the return value of transformer in the docs (without going via "how to write") is probably still a good idea even if we've added some details to that page.

---

I don't think we should try to detect if source maps are used - code might be transformed in a way that does not change line/column position etc.. If you transform the code, you should return sourcemaps, and us assuming that code running is either original or provide source maps seems like a fine assumption to make

[SimenB]

#12566 (hopefully clearer now, also #12407)

[github-actions]

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