Assertions on error properties

[dominykas]

Ran into a use case recently, where I'd like to assert on error codes and some more meta information.

I'm thinking about error([type], [message], [values]) / throw([type], [message], [values]), e.g.

expect(() => someMethod()).to.throw(CustomError, 'Oh no!', { code: 'E_NOENT' });

This would essentially do an extra expect(error).to.include(values); - there is no other way to check this when using to.throw() AFAIK?

Happy to PR if acceptable.

[mtharrison]

Sounds useful to me. Not sure if I'd be expecting includes() as you suggest or a deep equality check though.

[dominykas]

Yeah, I was thinking about deep vs includes(). But with error objects it may be hard to always assert the deep copy - there may be functions on it or other things you don't control, so a loose check feels more useful in practice. I couldn't come up with a nice way to make it optional - adding a fourth param would be pretty ugly? I also considered making the third param a function which receives the error thrown - but that feels overkill?

Another option would be to have something like error(type, [values], [options]), e.g.

expect(() => someMethod()).to.throw(CustomError, { message: 'Oh no!', code: 'E_NOENT' }, { strict: true });

Possibly leave out the { strict: true } as a future extension (if someone asks for it) and until then the second param is either a message or a values object for an includes() check?

[cjihrig]

I'll entertain a PR for this 😄

[dominykas]

@cjihrig any opinion on the API, before I jump into this? :)

[hueniverse]

I'll replicate our reject() interface style and just return the error for further, separate assertions.

[mtharrison]

So one would do the following?

const err = expect(() => someMethod()).to.throw(CustomError);
expect(err).to.include({ code: 'E_NOENT' });

[hueniverse]

Yes.

[lock]

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