Jest + React + Gulp

gulpfile.js

gulp.task('jest', function () {
    var nodeModules = path.resolve('./node_modules');
    return gulp.src('app/scripts/**/__tests__')
        .pipe($.jest({
            scriptPreprocessor: nodeModules + '/gulp-jest/preprocessor.js',
            unmockedModulePathPatterns: [nodeModules + '/react']
        }));
});

Timer-test.js

// app/scripts/ui/__tests__/Timer-test.js

/** @jsx React.DOM */

jest.dontMock('../Timer');

describe('Timer', function () {
    var Timer = require('../Timer');
    var React = require('react/addons');
    var TestUtils = React.addons.TestUtils;
    var timer = null;

    beforeEach(function () {
        timer = TestUtils.renderIntoDocument(<Timer/>);
    });

    it('increments seconds elapsed with each tick', function () {
        expect(timer.state.secondsElapsed).toBe(0);
        timer.tick();
        expect(timer.state.secondsElapsed).toBe(1);
    });

    it('registers tick to run once each second', function () {
        expect(setInterval.mock.calls.length).toBe(1);
        expect(setInterval.mock.calls[0][0]).toBe(timer.tick);
        expect(setInterval.mock.calls[0][1]).toBe(1000);
    });
});

Timer.js

// app/scripts/ui/Timer.js

/** @jsx React.DOM */
var React = require('react');

var Timer = React.createClass({
  getInitialState: function() {
    return {secondsElapsed: 0};
  },
  tick: function() {
    this.setState({secondsElapsed: this.state.secondsElapsed + 1});
  },
  componentDidMount: function() {
    this.interval = setInterval(this.tick, 1000);
  },
  componentWillUnmount: function() {
    clearInterval(this.interval);
  },
  render: function() {
    return (
      <div>Seconds Elapsed: {this.state.secondsElapsed}</div>
    );
  }
});


module.exports = Timer;