<!doctype html>
<html ng-app="plunker">

<head>
  <script src="http://code.jquery.com/jquery-1.9.1.min.js"></script>
  <script src="http://ajax.googleapis.com/ajax/libs/angularjs/1.1.5/angular.min.js"></script>
  <script src="//netdna.bootstrapcdn.com/twitter-bootstrap/2.3.0/js/bootstrap.min.js"></script>
  <script src="script.js"></script>
  <link href="//netdna.bootstrapcdn.com/twitter-bootstrap/2.3.0/css/bootstrap-combined.min.css" rel="stylesheet">
  <link rel="stylesheet" href="style.css" />
</head>

<body ng-controller="AppController">

  <button class="btn">
    <i ng-class="{'icon-autoscroll': autoScroll, 'icon-autoscroll-disabled': !autoScroll}"></i> button
  </button>

  <button class="btn">
    <i ng-class="{true: 'icon-autoscroll', false: 'icon-autoscroll-disabled'}[autoScroll]"></i> button
  </button>

  <button class="btn">
    <i ng-class="autoScroll ? 'icon-autoscroll' : 'icon-autoscroll-disabled'"></i> button
  </button>

  <button ng-click="autoScroll = !autoScroll">Toggle autoScroll</button>

</body>

</html>
// Code goes here
var app = angular.module('plunker', []);
app.controller('AppController',
    [
      '$scope',
      function($scope) {
        $scope.autoScroll = true;
      }
    ]
  );

/* Styles go here */
.icon-autoscroll {
  display: inline-block;
  background-color: green;
}
.icon-autoscroll-disabled {
  display: inline-block;
  background-color: red;
}