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:

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

Is this possible?

Answer 1

With Chai throw (ES2016)

http://chaijs.com/api/bdd/#method_throw

For clarity... This works

it('Should fail if ...', done => {
    let ret = () => {
        MyModule.myFunction(myArg);
    };
    expect(ret).to.throw();
    done();
});

This doesn't work

it('Should fail if ...', done => {
    let ret = MyModule.myFunction(myArg);
    expect(ret).to.throw();
    done();
});

Answer 2

MarkJ's accepted answer is the way to go and way simpler than others here. Let me show example in real world:

function fn(arg) {
  if (typeof arg !== 'string')
    throw TypeError('Must be an string')

  return { arg: arg }
}

describe('#fn', function () {
  it('empty arg throw error', function () {
    expect(function () {
      new fn()
    }).to.throw(TypeError)
  })

  it('non-string arg throw error', function () {
    expect(function () {
      new fn(2)
    }).to.throw(TypeError)
  })

  it('string arg return instance { arg: <arg> }', function () {
    expect(new fn('str').arg).to.be.equal('str')
  })
})

Answer 3

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()
});