Test for expected failure in Mocha

Question

Using Mocha, I am attempting to test whether a constructor throws an error. I haven\'t been able to do this using the expect syntax, so I\'d like to do the following:

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Answer 1

If you don't want to wrap a whole lot of source into the expect parameter, or if you have many arguments to pass and it just gets ugly, you can still do this with the original syntax just fine by leveraging the done argument that is provided (but was originally ignored):

it('should throw exception when instantiated', function(done: Done) {
  try {
    new ErrorThrowingObject();
    done(new Error(`Force the test to fail since error wasn't thrown`));
  }
  catch (error) {
    // Constructor threw Error, so test succeeded.
    done();
  }
}

Because you're using done here, it allows you go execute arbitrary code above it in the try, then specify exactly where in your source you'd like to record the failure.

Normally, someone may be tempted to throw or assert(false), but these will both be caught by the catch of the try, and cause you to do some meta-checking to determine if the error you caught was the expected error from your test, or if it was the final determination that your test failed. That's just a mess.