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闭包来做动画的效果

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<!DOCTYPE html>
<html>
<head>
  <title>Using a closure in a timer interval callback</title>
  <meta charset="utf-8">
  <script src="../assert.js"></script>
  <link rel="stylesheet" type="text/css" href="../assert.css">
  <style>
  	#box1{
  		width: 100px;
  		height: 100px;
  		position: relative;
  		margin:5;
  		color: white;
  		font-weight: bolder;
      background-color:blue;
      margin-bottom: 100px;
  	}
  </style>
</head>
<body>
	<div id="box1">First Box</div> 
  <script>
    function animateIt(elementId) {
      var elem = document.getElementById(elementId);
      var tick = 0;
      var timer = setInterval(function(){
        if (tick < 100) {
          elem.style.left = elem.style.top = tick + "px";
          tick++;
        }
        else {
          clearInterval(timer);
          assert(tick === 100,
                 "Tick accessed via a closure.");
          assert(elem,
                 "Element also accessed via a closure.");
          assert(timer,
                 "Timer reference also obtained via a closure." );
        }
      }, 10);
    }
    animateIt("box1");
  </script>
</body>
</html>