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

should.js

Using the should.js library with should.fail

var should = require('should')
it('should fail', function(done) {
  try {
      new ErrorThrowingObject();
      // Force the test to fail since error wasn't thrown
       should.fail('no error was thrown when it should have been')
  }
  catch (error) {
   // Constructor threw Error, so test succeeded.
   done();
  }
});

Alternative you can use the should throwError

(function(){
  throw new Error('failed to baz');
}).should.throwError(/^fail.*/)

Chai

And with chai using the throw api

var expect = require('chai').expect
it('should fail', function(done) {
  function throwsWithNoArgs() {
     var args {} // optional arguments here
     new ErrorThrowingObject(args)
  }
  expect(throwsWithNoArgs).to.throw
  done()
});