title: Using Karma / Mocha get Uncaught TypeError -> this[opt] is not a function
body: <p>I'm just installing Karma / Mocha trying to work it with React. I get a mysterious error saying that this[opt] is not a function. Not sure what to make of it. </p>
  
  <p>I have the following Karma Config file:</p>
  
  <pre><code>let webpackConfig = require('./webpack.config.js');
  module.exports = function(config) {
    config.set({
      browsers: ['Chrome'],
      singleRun: true,
      frameworks: ['mocha'],
      files: ['app/tests/**/*.test.jsx'],
      preprocessors: {
        'app/tests/**/*.test.jsx': ['webpack', 'sourcemap']
      },
      reporters: ['mocha'],
      client: {
        mocha: {
          timout: '5000'
        }
      },
      webpack: webpackConfig,
      webpackServer: {
        noInfo: true
      }
    });
  };
  </code></pre>
  
  <p>with the simple test: </p>
  
  <pre><code>'use strict'
  let expect = require('expect');
  
  describe('App', () =&gt; {
    it('should properly run tests', () =&gt; {
      expect(1).toBe(1);
    });
  });
  </code></pre>
  
  <p>The browser starts with the test but I get the following error: </p>
  
  <p>Chrome 52.0.2743 (Mac OS X 10.11.3) ERROR
    Uncaught TypeError: this[opt] is not a function
    at /Users/bpr/Documents/ReactTimer/node_modules/mocha/mocha.js:124</p>