主要是利用了浏览器的渲染机制。
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<html lang="en"><head><meta charset="UTF-8"><meta name="viewport"content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0"><meta http-equiv="X-UA-Compatible" content="ie=edge"><title>Document</title><style>.content {background: blue;width: 200px;overflow: hidden;height: 0;}</style></head><body><span class="btn">测试</span><div class="content"><p>111111111111</p><p>222222222222</p><p>222222222222</p><p>222222222222</p><p>222222222222</p><p>222222222222</p><p>222222222222</p><p>222222222222</p><p>222222222222</p></div><script>const btn = document.querySelector('.btn');const content = document.querySelector('.content')btn.onmouseenter = function () {content.style.height = 'auto';const {height} = content.getBoundingClientRect();console.log(height);//有回流，但是没有绘制，所以不会闪一下，界面看不出变化，但是拿到了高度content.style.height = 0;//不加content.offsetHeight，默认是直接渲染height + 'px'，不会出现从0->height的动画content.style.transition = 'height 0.5s';//读取几何属性会造成强制回流，从而触发渲染content.offsetHeight;content.style.height = height + 'px';}btn.onmouseleave = function () {content.style.height = 0;}</script></body>
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