for(var i=1;i<=3;i++){ setTimeout(function(){ console.log(i); },0); };答案：4 4 4。
看面试题时，发现了一道较为经典的面试题，代码如下
for(var i=1;i<=3;i++){
setTimeout(function(){
console.log(i);
},0);
};
//答案：4 4 4。
很多初学者，可能会以为答案会是：1 2 3，但仔细研究，你会发现这道题涉及了异步、作用域、闭包。由于网上有很多类似的内容，故我这里就不再赘述，附上链接如下：
https://www.cnblogs.com/destinyruru/p/5823760.html
https://www.cnblogs.com/wangwenhui/p/7657654.html
解决：
var arr = [1,2,3];
for(let i = 0; i < arr.length; i++) {
setTimeout(function() {
console.log(i);
},0);
}
var arr = [1,2,3];
for(var i = 0; i < arr.length; i++) {
(function(){
var j = i;
setTimeout( function timer(){
console.log( j );
}, 0);
})();
}
参考链接：https://www.cnblogs.com/171220-barney/p/8552370.html
另外：对一堆a标签做点击事件也涉及闭包问题
window.onload = function () {
var aLink = document.getElementsByTagName("a");
var last = aLink[15]; // 福建
for (var i = 0; i < aLink.length; i++) {
// 错误方式
/*aLink[i].onclick=function(){
alert(aLink[i-1].innerText);
}*/
// 正确方式
(function (j) {
aLink[j].onclick = function () {
// 点击后 将其他背景色去除
if (last) {
last.classList.remove("selected");
}
aLink[j].classList.add("selected");
last = aLink[j];
document.getElementById("citySelected").innerText = aLink[j].innerText;
}
})(i)
}
}
当然也可以通过this解决
window.onload = function() {
var aLink = document.getElementsByTagName("a");
var last = aLink[15]; // 福建
for(var i = 0; i < aLink.length; i++) {
aLink[i].onclick = function() {
// 去除样式
if(last) {
last.classList.remove("selected");
}
// 用this表示当前对象，可避免用闭包，减少代码量
this.classList.add("selected");
last = this;
document.getElementById("citySelected").innerText = this.innerText;
}
}
}
以下为a标签点击事件demo：
<!DOCTYPE html>
<html lang="en"> <head>
<meta charset="UTF-8">
<title>Title</title>
<style>
body {
-webkit-font-smoothing: antialiased;
background-color: #fff;
font: 12px/1.5 Microsoft YaHei, tahoma, arial, Hiragino Sans GB, \\5b8b\4f53, sans-serif;
color: #666;
} #ttbar-mycity {
margin-left: 200px;
} .shortcut_btn {
position: relative;
z-index: 21;
} li {
list-style: none;
display: list-item;
text-align: -webkit-match-parent;
} .fl {
float: left;
height: 30px;
line-height: 30px;
color: #999;
} ul {
display: block;
list-style-type: disc;
margin-block-start: 1em;
margin-block-end: 1em;
margin-inline-start: 0px;
margin-inline-end: 0px;
padding-inline-start: 40px;
} .cw-icon {
height: 28px;
line-height: 28px;
} .dt {
padding-left: 7px;
padding-right: 7px;
width: 45px;
} .cw-icon {
overflow: hidden;
position: relative;
z-index: 1;
float: left;
border: 1px solid #e3e4e5;
} .iconfont {
font-family: iconfont, sans-serif;
font-style: normal;
-webkit-text-stroke-width: .2px;
-moz-osx-font-smoothing: grayscale;
} .dd {
left: 0;
width: 300px;
padding: 10px;
} .dropdown-layer {
top: 30px;
} .dd-spacer {
display: none !important;
} .ui-areamini-content-list {
overflow: hidden;
} .item {
float: left;
width: 60px;
padding: 2px 0;
} .item a {
float: left;
padding: 0 8px;
color: #999;
text-decoration: none;
} #ttbar-mycity .item a.selected {
background-color: #f10215;
color: #fff;
}
</style>
<script>
window.onload = function() {
var aLink = document.getElementsByTagName("a");
var last = aLink[15]; // 福建
for(var i = 0; i < aLink.length; i++) {
aLink[i].onclick = function() {
// 去除样式
if(last) {
last.classList.remove("selected");
}
// 用this表示当前对象，可避免用闭包，减少代码量
this.classList.add("selected");
last = this;
document.getElementById("citySelected").innerText = this.innerText;
}
}
}
</script>
</head> <body>
<ul class="fl" clstag="h|keycount|head|topbar_01">
<li class="shortcut_btn dropdown" id="ttbar-mycity">
<div class="dt cw-icon ui-areamini-text-wrap" style=""><i class="iconfont"></i><span class="ui-areamini-text" data-id="16" title="福建" id="citySelected">福建</span>
</div>
<div class="dd dropdown-layer">
<div class="dd-spacer"></div>
<div class="ui-areamini-content-wrap" style="left: auto;">
<div class="ui-areamini-content">
<div class="ui-areamini-content-list">
<div class="item">
<a data-id="1" name="city" href="javascript:void(0)">北京</a>
</div>
<div class="item">
<a data-id="2" name="city" href="javascript:void(0)">上海</a>
</div>
<div class="item">
<a data-id="3" name="city" href="javascript:void(0)">天津</a>
</div>
<div class="item">
<a data-id="4" name="city" href="javascript:void(0)">重庆</a>
</div>
<div class="item">
<a data-id="5" name="city" href="javascript:void(0)">河北</a>
</div>
<div class="item">
<a data-id="6" name="city" href="javascript:void(0)">山西</a>
</div>
<div class="item">
<a data-id="7" name="city" href="javascript:void(0)">河南</a>
</div>
<div class="item">
<a data-id="8" name="city" href="javascript:void(0)">辽宁</a>
</div>
<div class="item">
<a data-id="9" name="city" href="javascript:void(0)">吉林</a>
</div>
<div class="item">
<a data-id="10" name="city" href="javascript:void(0)">黑龙江</a>
</div>
<div class="item">
<a data-id="11" name="city" href="javascript:void(0)">内蒙古</a>
</div>
<div class="item">
<a data-id="12" name="city" href="javascript:void(0)">江苏</a>
</div>
<div class="item">
<a data-id="13" name="city" href="javascript:void(0)">山东</a>
</div>
<div class="item">
<a data-id="14" name="city" href="javascript:void(0)">安徽</a>
</div>
<div class="item">
<a data-id="15" name="city" href="javascript:void(0)">浙江</a>
</div>
<div class="item">
<a data-id="16" name="city" href="javascript:void(0)" class="selected">福建</a>
</div>
<div class="item">
<a data-id="17" name="city" href="javascript:void(0)">湖北</a>
</div>
<div class="item">
<a data-id="18" name="city" href="javascript:void(0)">湖南</a>
</div>
<div class="item">
<a data-id="19" name="city" href="javascript:void(0)">广东</a>
</div>
<div class="item">
<a data-id="20" name="city" href="javascript:void(0)">广西</a>
</div>
<div class="item">
<a data-id="21" name="city" href="javascript:void(0)">江西</a>
</div>
<div class="item">
<a data-id="22" name="city" href="javascript:void(0)">四川</a>
</div>
<div class="item">
<a data-id="23" name="city" href="javascript:void(0)">海南</a>
</div>
<div class="item">
<a data-id="24" name="city" href="javascript:void(0)">贵州</a>
</div>
<div class="item">
<a data-id="25" name="city" href="javascript:void(0)">云南</a>
</div>
<div class="item">
<a data-id="26" name="city" href="javascript:void(0)">西藏</a>
</div>
<div class="item">
<a data-id="27" name="city" href="javascript:void(0)">陕西</a>
</div>
<div class="item">
<a data-id="28" name="city" href="javascript:void(0)">甘肃</a>
</div>
<div class="item">
<a data-id="29" name="city" href="javascript:void(0)">青海</a>
</div>
<div class="item">
<a data-id="30" name="city" href="javascript:void(0)">宁夏</a>
</div>
<div class="item">
<a data-id="31" name="city" href="javascript:void(0)">新疆</a>
</div>
<div class="item">
<a data-id="52993" name="city" href="javascript:void(0)">港澳</a>
</div>
<div class="item">
<a data-id="32" name="city" href="javascript:void(0)">台湾</a>
</div>
<div class="item">
<a data-id="84" name="city" href="javascript:void(0)">钓鱼岛</a>
</div>
<div class="item">
<a data-id="53283" name="city" href="javascript:void(0)">海外</a>
</div>
</div>
</div>
</div>
</div>
</li>
</ul>
</body> </html>
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