使用js實現簡單放大鏡的效果

<div id="small">
    <div id="pic1">
        <img src="62-130501163925.jpg" alt="">
    </div>
    <div id="mask"></div>
</div>
<div id="big">
    <div id="pic2">
        <img src="62-130501163925.jpg" alt="">
    </div>
</div>

#small{
    width: 500px;
    height: 312px;
    position: absolute;
    left: 20px;
    top: 20px;
}
#pic1{
    position: absolute;
    left: 0px;
    top: 0px;
}
#pic1 img{
    width: 100%;
    height: 100%;
}
#big{
    width: 200px;
    height: 200px;
    position: absolute;
    right: 50px;
    top: 50px;
    border: 1px solid blue;
    overflow: hidden;
}
#pic2{
    width: 1000px;
    height: 625px;
    position: absolute;
    left: 0;
    top: 0;
}
#pic2 img{
    width: 100%;
    height: 100%;
}
#mask{
    width: 100px;
    height: 100px;
    background: black;
    opacity: 0.3;/*讓遮罩層看起來透明*/
    filter: alpha(opacity = 30);/*相容不同的瀏覽器*/
    position: absolute;
    display: none;
}

window.onload = function(){//文檔內容載入完之後再執行
    //當滑鼠移入小圖片，顯示遮罩層和放大的區域
    $('small').onmouseenter = function(){
        $('mask').style.display = 'block';
        $('big').style.display='block';
    }
        //滑鼠移出時，隱藏遮罩層和放大的區域
    $('small').onmouseleave = function(){
        $('mask').style.display='none';
        $('big').style.display="none";
    }
    //滑鼠移動
    $('small').onmousemove = function(ev){
        var e = ev || window.event;
        //計算滑鼠的位置，並讓滑鼠顯示在遮罩層的中間
        var l = e.clientX - $('small').offsetLeft - 50;
        var t = e.clientY - $('small').offsetTop -50;
        //別讓遮罩層移出圖片
        if(l <= 0){
            l = 0;
        }
        if(l >= 500 - 100){
            l = 400;
        }
        if(t <= 0){
            t = 0;
        }
        if(t >= 312 - 100){
            t = 212;
        }
        //遮罩層的移動
        $('mask').style.left = l + 'px';
        $('mask').style.top = t + 'px';
        //通過遮罩層移動，來計算出放大後圖片的顯示區域
        $("pic2").style.left = -$("mask").offsetLeft * 2 + 'px';
        $("pic2").style.top = -$("mask").offsetTop * 2 + 'px';
    }
}
//為了更容容易的獲取id
function $(id){
    return document.getElementById(id);
}