jasmine's spyOn() + React.js + ES6

component.js

import React from 'react';
import _ from 'lodash';
import PubSub from 'pubsub-js';

class Example extends React.Component{
  bar(e){
    console.log('Bar called')
  }
  render() {
    return(
      <button ref="foo" onClick={this.bar}>Hey</button>
    )
  }
};

export default Example;

test.js

describe('Example Test', function() {

var React = require('react/addons');
var TestUtils = React.addons.TestUtils;
var Example = require('./Example.jsx');
var example;

beforeEach(function() {
  spyOn(Example.prototype, 'bar');
  example = TestUtils.renderIntoDocument(<Example />);
});

it('should exist', function() {
  expect(TestUtils.isCompositeComponent(example)).toBeTruthy();
});

it('should call bar() onClick', function() {
  var button = React.findDOMNode(pagination.refs.foo);
  TestUtils.Simulate.click(button);
  expect( example.bar ).toHaveBeenCalled();
});

});