<body>
<div id="div1">div1</div>
<div id="div2">div2</div>
<div id="div3">div3</div>
<div class="cls1">div4</div>
<div class="cls1">div5</div>
<input type="text" name="username">
<script>
//根据元素名称获取元素对象,返回是一个数组
var divs = document.getElementsByTagName("div");
alert(divs.length);
//根据Class属性获取元素,返回是一个数组
var div_cls = document.getElementsByClassName("cls1");
alert(div_cls.length);
//根据name属性来获取元素,返回一个数组
var username = document.getElementsByName("username");
alert(username.length);
</script>
</body>