title: Karma: cannot read property 'style' of undefined
body: <p>I'm trying to do unit tests on my Angularjs code using karma.
  This is my conf file:</p>
  
  <pre><code>module.exports = function(config) {
  config.set({
  
  // base path that will be used to resolve all patterns (eg. files, exclude)
  basePath: './',
  
  // frameworks to use
  // available frameworks: https://npmjs.org/browse/keyword/karma-adapter
  frameworks: ['jasmine'],
  
  
  // list of files / patterns to load in the browser
  files: [
    'http://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js',
    'https://ajax.googleapis.com/ajax/libs/angularjs/1.2.26/angular.min.js',
    'https://code.angularjs.org/1.2.26/angular-mocks.js',
    'test/unit/**/*.js',
    'js/app.js',
    'js/controllers/AffairesCtrl.js'
  ],
  
  
  
  // list of files to exclude
  exclude: [
  ],
  
  
  // preprocess matching files before serving them to the browser
  // available preprocessors: https://npmjs.org/browse/keyword/karma-preprocessor
  preprocessors: {
  },
  
  
  // test results reporter to use
  // possible values: 'dots', 'progress'
  // available reporters: https://npmjs.org/browse/keyword/karma-reporter
  reporters: ['progress'],
  
  
  // web server port
  port: 9876,
  
  
  // enable / disable colors in the output (reporters and logs)
  colors: true,
  
  
  // level of logging
  // possible values: config.LOG_DISABLE || config.LOG_ERROR || config.LOG_WARN || config.LOG_INFO || config.LOG_DEBUG
  logLevel: config.LOG_DEBUG,
  
  
  // enable / disable watching file and executing tests whenever any file changes
  autoWatch: true,
  
  
  // start these browsers
  // available browser launchers: https://npmjs.org/browse/keyword/karma-launcher
  browsers: ['Chrome'],
  
  
  // Continuous Integration mode
  // if true, Karma captures browsers, runs the tests and exits
  singleRun: false
  });
  };
  </code></pre>
  
  <p>I'm testing a function which has the following code:</p>
  
  <pre><code>document.getElementById('case').style.display="none";
  </code></pre>
  
  <p>When I run the test, I get this error: </p>
  
  <pre><code>TypeError: Cannot read property 'style' of undefined
  </code></pre>
  
  <p>But, if I replace the javascript code with jQuery code: </p>
  
  <pre><code>$("#case").css("display","none");
  </code></pre>
  
  <p>Then, the test is a success and the previous error disappear.</p>
  
  <p>Do you know why I get this error?</p>
  
  <p>Thank you!</p>