javascript - ES6 for 循环 let 改 var 作用域出错如何解决？
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问题描述
问 题
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
* {
padding: 0;
margin: 0;
}
div.slider {
width: 730px;
height: 454px;
position: relative;
overflow: scroll;
}
div.slider li.slider-panel {
position: absolute;
transition-duration: 0.5s;
transition-property: all;
}
div.slider ul.slider-extra {
position: absolute;
text-align: center;
width: 730px;
height: 0;
cursor: default;
bottom: 26px;
}
div.slider li.slider-nav {
display: inline-block;
width: 18px;
height: 18px;
line-height: 18px;
border-radius: 50%;
background-color: #3e3e3e;
color: white;
font-size: 12px;
cursor: pointer;
}
div.slider-page a.slider-prev {
position: absolute;
font-size: 22px;
height: 62px;
width: 28px;
top: 200px;
background-color: rgba(0, 0, 0, 0.2);
text-align: center;
line-height: 62px;
color: white;
text-decoration: none;
}
div.slider-page a.slider-next {
position: absolute;
font-size: 22px;
height: 62px;
width: 28px;
top: 200px;
right: 0;
background-color: rgba(0, 0, 0, 0.2);
text-align: center;
line-height: 62px;
color: white;
text-decoration: none;
}
</style>
</head>
<body>
<div class="slider">
<ul class="slider-main">
<li class="slider-panel"><img src="1.jpg" alt=""></li>
<li class="slider-panel"><img src="2.jpg" alt=""></li>
<li class="slider-panel"><img src="3.jpg" alt=""></li>
<li class="slider-panel"><img src="4.jpg" alt=""></li>
<li class="slider-panel"><img src="5.jpg" alt=""></li>
<li class="slider-panel"><img src="6.jpg" alt=""></li>
</ul>
<ul class="slider-extra">
<li class="slider-nav">1</li>
<li class="slider-nav">2</li>
<li class="slider-nav">3</li>
<li class="slider-nav">4</li>
<li class="slider-nav">5</li>
<li class="slider-nav">6</li>
</ul>
<div class="slider-page">
<a href="" class="slider-prev"><</a>
<a href="" class="slider-next">></a>
</div>
</div>
<script>
window.onload = function() {
var sliderPanels = document.querySelector("ul.slider-main").getElementsByTagName("li");
var sliderNavs = document.querySelector("ul.slider-extra").getElementsByTagName("li");
var sliderPage = document.querySelector("div.slider-page");
var sliderPageButtons = document.querySelector("div.slider-page").getElementsByTagName("a");
var timer = null;
var currentPoint;
/*init function*/
changeImg(0);
autoChange(0);
/*slider navigator dots*/
for (var i = 0; i < sliderNavs.length; i++) {
sliderNavs[i].onmouseover = function() {
if (timer) clearInterval(timer);
changeImg(i);
};
sliderNavs[i].onmouseout = function() {
autoChange(currentPoint);
};
sliderPanels[i].onmouseover = function() {
if (timer) clearInterval(timer);
sliderPage.style.display = "";
};
sliderPage.onmouseover = function() {
sliderPage.style.display = "";
};
sliderPanels[i].onmouseout = function() {
autoChange(currentPoint);
sliderPage.style.display = "none";
};
}
/*page slider button*/
sliderPageButtons[0].onmouseover = function() {
if (timer) clearInterval(timer);
};
sliderPageButtons[1].onmouseover = function() {
if (timer) clearInterval(timer);
};
sliderPageButtons[0].onclick = function() {
event.preventDefault();
currentPoint--;
if (currentPoint < 0) currentPoint = 5;
changeImg(currentPoint);
};
sliderPageButtons[1].onclick = function() {
event.preventDefault();
currentPoint++;
if (currentPoint >= sliderNavs.length) currentPoint = 0;
changeImg(currentPoint);
};
/*change images function*/
function changeImg(id) {
for (var j = 0, len = sliderNavs.length; j < len; j++) {
sliderNavs[j].style.backgroundColor = "#3e3e3e";
sliderPanels[j].style.opacity = 0;
}
sliderNavs[id].style.backgroundColor = "red";
sliderPanels[id].style.opacity = 1;
currentPoint = id;
}
/*auto change images function*/
function autoChange(currentPoint) {
timer = setInterval(function() {
currentPoint++
if (currentPoint >= sliderNavs.length) currentPoint = 0;
changeImg(currentPoint);
}, 1000);
}
};
</script>
</body>
</html>
上面是全部代码，其中第一个for循环处：
/*slider navigator dots*/
for (let /*在这里*/ i = 0; i < sliderNavs.length; i++) {
sliderNavs[i].onmouseover = function() {
if (timer) clearInterval(timer);
changeImg(i);
};
如果改为let 则不会报错，如果是var 则出现如下错误：
3js轮播3.html:162 Uncaught TypeError: Cannot read property 'style' of undefined
请问为了改为var而不是let，我应该如果修改代码？
解决方案
因为使用let定义的变量只在当前的块级作用域有效，所以，使用let就能够避免你的for循环的函数在循环结束之后执行。但是，使用var就不同了。使用var定义的变量，是不存在块级作用域的。所以，当你使用var的时候，其实循环已经执行完了，你的i的值就是sliderNavs.length，当然就会报错。所以，你需要一个闭包来保存i的值。
所以，把你的for循环的代码改成这样：
/*slider navigator dots*/
for (var i = 0; i < sliderNavs.length; i++) {
(function ( i ) {
sliderNavs[i].onmouseover = function() {
if (timer) clearInterval(timer);
changeImg(i);
};
sliderNavs[i].onmouseout = function() {
autoChange(currentPoint);
};
sliderPanels[i].onmouseover = function() {
if (timer) clearInterval(timer);
sliderPage.style.display = "";
};
sliderPage.onmouseover = function() {
sliderPage.style.display = "";
};
sliderPanels[i].onmouseout = function() {
autoChange(currentPoint);
sliderPage.style.display = "none";
};
})( i );
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