淘宝、天猫等电商网站浏览图片时可以查看大图的功能；
思路很简单，两张图片，一张较小渲染在页面上，一张较大是细节图，随鼠标事件调整它的 left & top;
// index.html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<link rel="stylesheet" href="./css/style.css">
<title>放大镜</title>
</head>
<body>
<div id="container">
<div id="small-box">
<!-- #mark 防止用户点击并拖动图片加的一个遮罩层 -->
<div id="mark"></div>
<div id="float-box"></div>
<img src="./img/small.jpg" >
</div>
<div id="big-box">
<img src="./img/big.jpg" >
</div>
</div>
<script src="./js/render.js"></script>
</body>
</html>
// style.css
* {
margin: 0;
padding: 0;
}
#container {
display: block;
/* 图片宽高 */
width: 450px;
height: 450px;
margin: 50px;
position: relative;
border: 1px solid #ccc;
}
#small-box {
position: relative;
z-index: 1;
}
#float-box {
display: none;
width: 200px;
height: 150px;
position: absolute;
background-color: #ffc;
border: 1px solid #ccc;
filter: alpha(opacity = 50); /* 兼容 ie 写法*/
opacity: 0.5;
}
#mark {
position: absolute;
display: block;
/* 图片宽高 */
width: 450px;
height: 450px;
background-color: #fff;
filter: alpha(opacity = 0);
opacity: 0;
z-index: 10;
}
#big-box {
display: none;
position: absolute;
top: 0;
left: 460px;
/* 此处宽高比例同 #float-box */
width: 480px;
height: 360px;
overflow: hidden;
border: 1px solid #ccc;
z-index: 1;
}
#big-box img {
position: absolute;
z-index: 5;
}
// render.js
window.onload = function() {
let container = document.querySelector(‘#container‘);
let smallBox = document.querySelector(‘#small-box‘);
let mark = document.querySelector(‘#mark‘);
let floatBox = document.querySelector(‘#float-box‘);
let bigBox = document.querySelector(‘#big-box‘);
let bigBoxImage = document.querySelectorAll(‘img‘)[1];
mark.onmouseover = function() {
floatBox.style.display = ‘block‘;
bigBox.style.display = ‘block‘;
}
mark.onmouseout = function() {
floatBox.style.display = ‘none‘;
bigBox.style.display = ‘none‘;
}
mark.onmousemove = function(e) {
let _event = e || window.event; // 兼容性
// console.log(_event);
// console.log(_event.clientX,container.offsetLeft, smallBox.offsetLeft, floatBox.offsetWidth)
let left = _event.clientX - container.offsetLeft - smallBox.offsetLeft - floatBox.offsetWidth / 2;
let top = _event.clientY - container.offsetTop - smallBox.offsetTop - floatBox.offsetHeight / 2;
// 处理边界
if (left < 0) {
left = 0;
} else if (left > (mark.offsetWidth - floatBox.offsetWidth)) {
left = mark.offsetWidth - floatBox.offsetWidth;
}
if (top < 0) {
top = 0;
} else if (top > (mark.offsetHeight - floatBox.offsetHeight)) {
top = mark.offsetHeight - floatBox.offsetHeight;
}
floatBox.style.left = left + ‘px‘;
floatBox.style.top = top + ‘px‘;
// 求百分比
let percentX = left / (mark.offsetWidth - floatBox.offsetWidth);
let percentY = top / (mark.offsetHeight - floatBox.offsetHeight);
//方向相反，小图片鼠标移动方向与大图片相反，故而是负值
bigBoxImage.style.left = - percentX * (bigBoxImage.offsetWidth - bigBox.offsetWidth) + "px";
bigBoxImage.style.top = - percentY * (bigBoxImage.offsetHeight - bigBox.offsetHeight) + "px";
}
}
offsetLeft 获取的是相对于父对象的左边距；left 获取或设置相对于 具有定位属性(position定义为relative)的父对象 的左边距；
这是这个代码的要点之一，另外一个就是去要计算其比例。根据对应比例，进行代码的显示。
另外，小图片和大图片的显示，移动方向是相反的，所以比例前面会乘以一个负号。这个需要注意。
原文：https://www.cnblogs.com/cc-freiheit/p/10430484.html