缩小搜索范围

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
    span{
        height:300px;
        300px;
        background:#F00;
        margin: 10px;
    }
</style>
</head>

<body>
    <div id='mydiv'>
        <span id='span'>点击我变成蓝色</span>
        <span id='span2'>点击我变成蓝色</span>
        <span id='span3'>点击我变成蓝色</span>
        <span id='span4'>点击我变成蓝色</span>
    </div>
   
</body>
</html>
<script>
    function $tag(tag,id){
        var dom = document.getElementById(id);
        return dom.getElementsByTagName(tag);
    }

    //缩小搜索范围
    //先根据id 再根据tag
  var spans = $tag('span','mydiv');
  for(var i=0;i<spans.length;i++) {
	      spans[i].onclick = function(){
			     this.style.background= 'blue';
			  }
	  }
  

</script>
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