Chai: Add custom error validation on .throw

[kharandziuk]

With node built-in assert I can do something like:

assert.throws(
  () => {
    throw new Error('Wrong value');
  },
  function(err) { // custom function instead of just instanceof check
    if ( (err instanceof Error) && /value/.test(err) ) {
      return true;
    }
  },
  'unexpected error'
);

Is it make sense to add this feature in Chai?

[keithamus]

Hey @kharandziuk thanks for the issue.

If you're looking to just test the error constructor and/or message, you can already do that with .throw:

assert.throws(fn, Error, /value/);
assert.throws(fn, ReferenceError, /missing/);

expect(fn).to.throw(Error, /value/);
expect(fn).to.throw(ReferenceError, /missing/);

If you're looking for something more fine grained, for the expect/should API we have satisfy which does something very similar to your example. For example:

expect(fn).to.throw().and.to.satisfy((err) => {
  return error.code === 404;
});

Sadly it seems like assert doesn't include satisfy - we should look into adding it though.

How do either of these methods work for you?

[kharandziuk]

This one:

expect(fn).to.throw().and.to.satisfy((err) => {
  return error.code === 404;
});

is actually what I'm looking for. Does it make sense to add this sample in documentation of .throw?

[keithamus]

Great! The bdd method description of throw has a small note about how to chain but it does not have a useful example. We could definitely extend the documentation.

BTW, satisfy is a bit of an escape hatch - it would likely be much cleaner to use other assertions to do the same thing (well, you'd get a lot better error messages when it fails). Example:

expect(fn).to.throw(Error).which.has.property('code', 404);

// You can also use multiple chains!
expect(fn).to.throw(Error).which.has.property('code').that.is.within(399, 500);

I'll switch issue over to a docs issue, and if you'd like to raise a PR to clarify the documentation for these kinds of usecases, that'd be awesome! The docs for .throws are all in the code over here

[kharandziuk]

Created a PR