Option to disable built-in enhanced formatting of Node AssertionError

[Zalathar]

🚀 Feature Proposal

It should be possible to disable Jest's built-in handling of Node's AssertionError, to allow better integration with other tools that also enhance assertion error messages.

Motivation

I've been trying to configure Jest and power-assert to work well together, so that I can use assert in my code and tests, and see power-assert's enhanced output when running tests under Jest.

The biggest obstacle is that Jest already has special enhanced handling of AssertionError failures. Normally that's good, but when combined with power-assert it means that Jest's (understandably) less-useful enhanced output takes priority, and the more-useful power-assert output gets pushed down and is harder to see.

It would therefore be nice to have the option to disable the Jest-side enhancement, and have it treat AssertionError in the same way as all other errors.

Example

When the user enables power-assert support in their Jest and Babel configuration, they would also choose to disable Jest's special treatment of AssertionError, to give more helpful test failure output.

Pitch

The enhanced output is built into jest-circus (and the Jasmine runner). Custom environments can't cleanly undo the enhancements, and there's currently no reasonable way to opt out of them.

[Zalathar]

Side note: It's almost possible to get rid of the enhanced formatting, by using a custom environment to get rid of every part of the formatted error message prior to \nMessage:\n .

In practice this doesn't quite work cleanly, because the actual error message is full of chalk formatting codes around every line, so the necessary string manipulation becomes a lot harder.

[jeysal]

Also stumbled over this at some point and had to make https://github.com/spockjs/spockjs/blob/master/packages/assertion-post-processor-regular-errors/src/index.ts
Maybe we could move adding the handler into the environment so that a custom one can just not do it.

[Zalathar]

Another possibility would be to add the original error as a property of the custom message object.

https://github.com/facebook/jest/blob/33e2d5ce454554363198c8d8a673793222955dbc/packages/jest-circus/src/formatNodeAssertErrors.ts#L64

This would make it easier for a custom environment to cleanly undo the enhanced formatting.

[SimenB]

Yeah, I don't want the complexity of allowing an environment to influence this. Exposing the original error is something we want to do anyways (#4547, #7594), so I think that's a better approach. Would you need this in a reporter or somewhere else? How did you integrate power-assert?

[Zalathar]

How did you integrate power-assert?

I added babel-preset-power-assert to my Babel config, which takes care of (a) rewriting 'assert' imports to 'power-assert' imports, and (b) rewriting assert calls to capture intermediate values and pass them along.

Internally, power-assert takes care of calling the underlying assert module, intercepting the thrown AssertionError, and then replacing its message with an enhanced message and re-throwing.

[SimenB]

Right, so you really just need it in the reporter. At that point, we might consider supporting it in the built-in reporter as well

[Zalathar]

If I could keep using the default reporter, and just set a reporter option to tell it to use the original message instead of the one enhanced by formatNodeAssertErrors.ts, that would be convenient.

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