MyTest-test.js

describe('This is a test', function() {
  it('is only a test', function() {
    var SomeEmitter = require('../SomeEmitter');
    var emitter = new SomeEmitter;

    emitter.on('foo').emit('bar');

    expect(emitter.on).toBeCalledWith('foo');
    expect(emitter.emit).toBeCalledWith('bar');
  });
});

SomeEmitter.js

var SomeEmitter = require.generateMock('../SomeEmitter');

SomeEmitter.prototype.on.mockReturnThis();
SomeEmitter.prototype.emit.mockReturnThis();

module.exports = SomeEmitter;