<!DOCTYPE html>
<html lang="zh-CN">
<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>Document</title>
<style>
table,th,td {
border-collapse: collapse;
border: 1px solid #333;
padding: 10px;
}
</style>
</head>
<body>
<table>
<tr><th>1</th><th>2</th><th>3</th></tr>
<tr><td>a</td><td>b</td><td>c</td></tr>
<tr><td>a</td><td>b</td><td>c</td></tr>
<tr><td>a</td><td>b</td><td>c</td></tr>
<tr><td>a</td><td>b</td><td>c</td></tr>
</table>
<script>
var tr = document.querySelectorAll('tr')
// 1. 先遍历获取每个元素
for(var i = 0; i < tr.length; i++) {
// 2. 给每个元素增加事件
tr[i].onmouseover = function () {
// 3. 排他，先对立去除所有元素的款式，for(var j = 0; j < tr.length; j++) {tr[j].style.backgroundColor = ''
}
// 4. 排他，再给以后项设置款式
this.style.background = 'hotpink'
}
}
</script>
</body>
</html>