Spies

Mocha does not come equipped with spies, though libraries like Sinon provide this behaviour if desired. The following is an example of Mocha utilizing Sinon to test an EventEmitter:

var sinon = require('sinon')
  , EventEmitter = require('events').EventEmitter;

describe('EventEmitter', function(){
  describe('#emit()', function(){
    it('should invoke the callback', function(){
      var spy = sinon.spy()
        , emitter = new EventEmitter;

      emitter.on('foo', spy);
      emitter.emit('foo');
      spy.called.should.equal.true;
    })

    it('should pass arguments to the callbacks', function(){
      var spy = sinon.spy()
        , emitter = new EventEmitter;

      emitter.on('foo', spy);
      emitter.emit('foo', 'bar', 'baz');
      sinon.assert.calledOnce(spy);
      sinon.assert.calledWith(spy, 'bar', 'baz');
    })
  })
})

The following is the same test, performed without any special spy library, simply utilizing the Mocha done([err]) callback as a means to assert that the callback has occurred, otherwise resulting in a timeout.

describe('EventEmitter', function(){
  describe('#emit()', function(){
    it('should invoke the callback', function(done){
      var emitter = new EventEmitter;
      emitter.on('foo', done);
      emitter.emit('foo');
    })

    it('should pass arguments to the callbacks', function(done){
      var emitter = new EventEmitter;

      emitter.on('foo', function(a, b){
        a.should.equal('bar');
        b.should.equal('baz');
        done();
      });

      emitter.emit('foo', 'bar', 'baz');
    })
  })
})