我们在浏览购物网站的商品详情页时，通常都会有一个展示商品图片的放大镜，如下所示：

GIF 2022-4-26 17-19-08.gif

下面我们就来仿一下这个功能。

首先是结构的搭建：

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<div id="container">
    <div id="left">
        <img src="img.jpg" alt="" style="width: 100%;">
        <span></span>
    </div>
    <div id="right">
        <img style="position: absolute;" src="bigimg.jpg" alt="">
    </div>
</div>

然后做亿点样式：

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* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

#left {
    left: 50px;
    top: 50px;
    position: relative;
    width: 400px;
    height: 400px;
    border: 1px solid black;
}

#left span {
    position: absolute;
    width: 250px;
    height: 250px;
    left: 0;
    top: 0;
    background-color: rgba(192, 149, 83, .6);
}

#right {
    position: absolute;
    left: 460px;
    top: 50px;
    width: 500px;
    height: 500px;
    border: 1px solid black;
    overflow: hidden;
}

现在我们的页面是这样：

image.png

接下来将 #left span 与 #right 设置为不显示，display: none;，然后为左边的 div 添加鼠标移入移出事件：

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let leftDiv = document.getElementById('left')
let rightDiv = document.getElementById('right')
let span = document.querySelector('#left span')
let bigImg = document.querySelector('#right img')

leftDiv.onmouseover = () => {
    span.style.display = 'block'
    rightDiv.style.display = 'block'
}

leftDiv.onmouseout = () => {
    span.style.display = 'none'
    rightDiv.style.display = 'none'
}

这样，当鼠标悬浮于 leftDiv 时，span 与 rightDiv 便会显现。

下面为leftDiv添加 onmousemove 事件：

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leftDiv.onmousemove = (e) => {
    let spanLeft = e.clientX - leftDiv.offsetLeft - span.offsetWidth / 2
    let spanTop = e.clientY - leftDiv.offsetTop - span.offsetHeight / 2
    let maxLeft = leftDiv.offsetWidth - span.offsetWidth
    let maxTop = leftDiv.offsetHeight - span.offsetHeight
    
    spanLeft = spanLeft < 0 ? 0 : (spanLeft > maxLeft ? maxLeft : spanLeft)
    spanTop = spanTop < 0 ? 0 : (spanTop > maxTop ? maxTop : spanTop)

    span.style.left = spanLeft + 'px'
    span.style.top = spanTop + 'px'

    let imgLeft = spanLeft * (bigImg.width - 500) / (400 - 250)
    let imgTop = spanTop * (bigImg.height - 500) / (400 - 250)

    bigImg.style.left = -imgLeft + 'px'
    bigImg.style.top = -imgTop + 'px'
}

最后为span更换一下指针样式，与购物网站更接近：

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#left span:hover {
    cursor: move;
}

效果如下：

最终效果展示

完整代码：

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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>仿购物网站放大</title>
    <style>
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        #left {
            left: 50px;
            top: 50px;
            position: relative;
            width: 400px;
            height: 400px;
            border: 1px solid black;
        }

        #left span {
            position: absolute;
            width: 250px;
            height: 250px;
            left: 0;
            top: 0;
            background-color: rgba(192, 149, 83, .6);
            display: none;
        }

        #left span:hover {
            cursor: move;
        }

        #right {
            display: none;
            position: absolute;
            left: 460px;
            top: 50px;
            width: 500px;
            height: 500px;
            border: 1px solid black;
            overflow: hidden;
        }
    </style>
</head>
<body>
    <div id="container">
        <div id="left">
            <img src="img.jpg" alt="" style="width: 100%;">
            <span></span>
        </div>
        <div id="right">
            <img style="position: absolute;" src="bigimg.jpg" alt="">
        </div>
    </div>
    <script>
        let leftDiv = document.getElementById('left')
        let rightDiv = document.getElementById('right')
        let span = document.querySelector('#left span')
        let bigImg = document.querySelector('#right img')

        leftDiv.onmouseover = () => {
            span.style.display = 'block'
            rightDiv.style.display = 'block'
        }

        leftDiv.onmouseout = () => {
            span.style.display = 'none'
            rightDiv.style.display = 'none'
        }

        leftDiv.onmousemove = (e) => {
            let spanLeft = e.clientX - leftDiv.offsetLeft - span.offsetWidth/2
            let spanTop = e.clientY - leftDiv.offsetTop - span.offsetHeight/2
            let maxLeft = leftDiv.offsetWidth - span.offsetWidth
            let maxTop = leftDiv.offsetHeight - span.offsetHeight
            spanLeft = spanLeft < 0 ? 0 : (spanLeft > maxLeft ? maxLeft : spanLeft)
            spanTop = spanTop < 0 ? 0 : (spanTop > maxTop ? maxTop : spanTop)
            
            span.style.left = spanLeft + 'px'
            span.style.top = spanTop + 'px'

            let imgLeft = spanLeft * (bigImg.width - 500) / (400 -250)
            let imgTop = spanTop * (bigImg.height - 500) / (400 -250)

            bigImg.style.left = -imgLeft + 'px'
            bigImg.style.top = -imgTop + 'px'
        }
    </script>
</body>
</html>