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为什么加入注释的第一条（var pos = parseInt(div.style.left));还有关于div定位的那些语句之后，函数就无效了？

<!DOCTYPE html>
<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
        <style type="text/css">
            div {
                width:90px;
                height:95px;
                background-color:red;
                position:absolute;
                top:50px;
        </style>
        <script src="http://libs.baidu.com/jquery/1.9.0/jquery.js" type="text/javascript"></script>
        <title>jQuery动画特效</title>
    </head>
    
    <body>
    <div id="move"></div>
    <input type="button" id="right" value="右移"></input>
    <input type="button" id="left" value="左移"></input>
    <script>
             var btnR = $("#right");
             var btnL = $("#left");
             var div = $("#move");
             //var pos =parseInt(div.style.left);
            btnR.bind("click",function(){
                div.animate({
                    left:"+=50px"
                    //left:pos + 50 + "px"
                },
                200);
            });
            btnL.bind("click",function(){
                div.animate({
                    left:"-=50px"
                    //left:pos + 50 + "px"
                },
                200);
            });
            
                
    </script>    
    </body>
</html>