JS事件绑定陷阱与解决-优快云博客

<!DOCTYPE html>
<html>
	<head>
		<title></title>
		<style>
			ul{margin:0;padding:0;}
			li{list-style:none;height:50px;}
			li:nth-child(even){background: #baf}
			li:nth-child(odd){background: #fab}
		</style>
	</head>
	<body>
		<ul>
			<li></li>
			<li></li>
			<li></li>
			<li></li>
		</ul>
		<script>
			var lis = document.getElementsByTagName('li');
			console.log(lis)
			for(var i = 0;i<lis.length;i++){
				lis[i].onclick = function(){
					console.log(i)
				}
			}
		</script>
	</body>
</html>

不管点击哪一个li标签，输出都是4
onclick委托了但是没有触发，触发时因为代码自上而下运行，i已经变成了4，所以每个事件输出结果都是4.
正确做法1
将var 换成let

<!DOCTYPE html>
<html>
	<head>
		<title></title>
		<style>
			ul{margin:0;padding:0;}
			li{list-style:none;height:50px;}
			li:nth-child(even){background: #baf}
			li:nth-child(odd){background: #fab}
		</style>
	</head>
	<body>
		<ul>
			<li></li>
			<li></li>
			<li></li>
			<li></li>
		</ul>
		<script>
			var lis = document.getElementsByTagName('li');
			console.log(lis)
			for(let i = 0;i<lis.length;i++){
				lis[i].onclick = function(){
					console.log(i)
				}
			}
		</script>
	</body>
</html>

正确做法2

<!DOCTYPE html>
<html>
	<head>
		<title></title>
		<style>
			ul{margin:0;padding:0;}
			li{list-style:none;height:50px;}
			li:nth-child(even){background: #baf}
			li:nth-child(odd){background: #fab}
		</style>
	</head>
	<body>
		<ul>
			<li></li>
			<li></li>
			<li></li>
			<li></li>
		</ul>
		<script>
			var lis = document.getElementsByTagName('li');
			for(var i = 0;i<lis.length;i++){
				console.log(i);
				(function(j){
					lis[j].onclick = function(){
						console.log(j)
					}
				})(i)
				
			}
		</script>
	</body>
</html>