12.html

<!DOCTYPE HTML>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
        <title>自定义属性</title>
        <style>
            li{
                list-style:none;
                width:106px;
                height:115px;
                float:left;
                margin-right:20px;
                background:url('img/default.png');
            }
        </style>
        <script>
            window.onload = function(){
                var aBtn = document.getElementsByTagName('input');
                //for里面有函数的时候，先不用i
                for(var i=0;i<aBtn.length;i++){
                    aBtn[i].index = i; //自定义属性(添加索引值)
                    
                    aBtn[i].onclick = function (){
                        //alert(i);  //这里是3
                        alert(this.index);
                    }
                }
            }
        </script>
    </head>
    <body>
        <input type="button" value="btn1" />
        <input type="button" value="btn2" />
        <input type="button" value="btn3" />
    </body>
</html>