gistfile1.js

// SomeEmitter.js

function SomeEmitter() {}
SomeEmitter.prototype.on = function() {};
SomeEmitter.prototype.emit = function() {};

module.exports = SomeEmitter;

gistfile2.js

// __mocks__/SomeEmitter.js

/**
 * This is a manual mock that Jest will look for before it
 * tries to automatically generate a mock itself.
 * 
 * For more details:
 * http://facebook.github.io/jest/docs/manual-mocks.html
 */

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

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

module.exports = SomeEmitter;

gistfile3.js

// __tests__/MyThingThatUsesSomeEmitter-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');
  });
});