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finish the work of day18
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‎18 - AddingUpTimesWithReduce/README.md

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# 18 使用reduce进行时间累加
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> 本篇作者：©[大史快跑Dashrun](https://github.com/dashrun)——Chinasoft Frontend Web Developer
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> 简介：[JavaScript30](https://javascript30.com) 是 [Wes Bos](https://github.com/wesbos) 推出的一个 30 天挑战。项目免费提供了 30 个视频教程、30 个挑战的起始文档和 30 个挑战解决方案源代码。目的是帮助人们用纯 JavaScript 来写东西，不借助框架和库，也不使用编译器和引用。现在你看到的是这系列指南的第 18 篇。完整指南在 [GitHub](https://github.com/soyaine/JavaScript30)，喜欢请 Star 哦♪(^∇^*)
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> 创建时间：2017-08-25
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最后更新：2017-08-29
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## 挑战任务
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初始文件`index-start.html`中提供了一个包含多个列表项的无序列表元素，每一个列表项均添加了`data-time`属性，该属性用**分**和**秒**表示了时间。要求将所有的时间累加在一起，并用`时:分:秒`来表示计算的结果。
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## 实现效果
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![结果展示](https://github.com/dashrun/vanilla-javascript-30/blob/master/day18-AddingUpTimesWithReduce/effects.png)
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## 基本思路
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1.取得所有`li`中`data-time`属性的值，将时间换算为秒并累加求得总时间（单位：秒）;<br>
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2.手动计算将总时间转化为新的格式“XX小时XX分XX秒”;<br>
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3.将结果显示在页面上。
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## 过程指南(以非ES6版本为例)
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1.取得所有`li`标签
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```js
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var oLi = document.getElementsByTagName('li');
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```
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2.遍历`li`元素节点，取得每个`data-time`的值并以：为界将其分解为含有两个元素的数组,每个数组中含有两项，第一项为表示分钟的字符串，第二项为表示秒的字符串，将两者进行运算转化为表示秒的数字，并添加进新的数组。
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```js
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for( var i = 0, len = oLi.length; i < len; i++){
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var timeItem = oLi[i].dataset['time'].split(':');
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//将时间转换为秒
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times.push(parseInt(timeItem[0],10)*60+parseInt(timeItem[1],10));
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}
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```
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3.将新数组`times`中各项累加
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```js
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//方法1.因为times为数组类型，故可以直接使用reduce函数进行累加
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return times.reduce(function(a,b){
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return a+b;
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},0);
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//方法2.不熟悉reduce函数的也可通过for循环遍历数组各项进行结果累加
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```
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4.总时间格式转换
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```js
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//总时间对60取余即为不足1分钟的秒数
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var sec = seconds % 60;
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//总时间除以3600并向下取整为小时数
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var hour = Math.floor(seconds/3600);
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//总时间减去前两项即可获得分钟数
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var min = (seconds - 3600*hour - sec)/60;
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```
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5.将结果打印在界面上即可

‎18 - AddingUpTimesWithReduce/effects.png

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‎18 - AddingUpTimesWithReduce/index-finished-Dashrun-es5.html

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<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8">
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<title>Videos</title>
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</head>
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<body>
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<p>总播放时间为：<span id="totaltime"></span></p>
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<ul class="videos">
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<li data-time="5:43">
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Video 1
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</li>
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<li data-time="2:33">
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Video 2
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</li>
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<li data-time="3:45">
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Video 3
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</li>
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<li data-time="0:47">
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Video 4
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</li>
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<li data-time="5:21">
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Video 5
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</li>
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<li data-time="6:56">
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Video 6
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</li>
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<li data-time="3:46">
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Video 7
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</li>
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<li data-time="5:25">
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Video 8
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</li>
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<li data-time="3:14">
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Video 9
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</li>
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<li data-time="3:31">
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Video 10
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</li>
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<li data-time="5:59">
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Video 11
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</li>
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<li data-time="3:07">
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Video 12
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</li>
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<li data-time="11:29">
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Video 13
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</li>
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<li data-time="8:57">
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Video 14
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</li>
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<li data-time="5:49">
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Video 15
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</li>
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<li data-time="5:52">
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Video 16
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</li>
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<li data-time="5:50">
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Video 17
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</li>
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<li data-time="9:13">
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Video 18
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</li>
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<li data-time="11:51">
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Video 19
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</li>
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<li data-time="7:58">
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Video 20
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</li>
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<li data-time="4:40">
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Video 21
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</li>
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<li data-time="4:45">
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Video 22
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</li>
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<li data-time="6:46">
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Video 23
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</li>
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<li data-time="7:24">
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Video 24
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</li>
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<li data-time="7:12">
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Video 25
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</li>
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<li data-time="5:23">
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Video 26
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</li>
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<li data-time="3:34">
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Video 27
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</li>
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<li data-time="8:22">
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Video 28
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</li>
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<li data-time="5:17">
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Video 29
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</li>
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<li data-time="3:10">
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Video 30
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</li>
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<li data-time="4:43">
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Video 31
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</li>
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<li data-time="19:43">
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Video 32
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</li>
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<li data-time="0:47">
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Video 33
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</li>
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<li data-time="0:47">
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Video 34
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</li>
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<li data-time="3:14">
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Video 35
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</li>
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<li data-time="3:59">
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Video 36
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</li>
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<li data-time="2:43">
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Video 37
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</li>
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<li data-time="4:17">
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Video 38
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</li>
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<li data-time="6:56">
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Video 39
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</li>
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<li data-time="3:05">
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Video 40
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</li>
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<li data-time="2:06">
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Video 41
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</li>
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<li data-time="1:59">
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Video 42
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</li>
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<li data-time="1:49">
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Video 43
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</li>
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<li data-time="3:36">
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Video 44
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</li>
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<li data-time="7:10">
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Video 45
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</li>
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<li data-time="3:44">
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Video 46
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</li>
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<li data-time="3:44">
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Video 47
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</li>
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<li data-time="4:36">
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Video 48
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</li>
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<li data-time="3:16">
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Video 49
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</li>
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<li data-time="1:10">
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Video 50
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</li>
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<li data-time="6:10">
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Video 51
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</li>
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<li data-time="2:14">
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Video 52
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</li>
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<li data-time="3:44">
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Video 53
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</li>
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<li data-time="5:05">
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Video 54
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</li>
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<li data-time="6:03">
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Video 55
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</li>
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<li data-time="12:39">
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Video 56
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</li>
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<li data-time="1:56">
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Video 57
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</li>
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<li data-time="4:04">
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Video 58
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</li>
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</ul>
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<script>
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//计算总时间
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var totaltime = timeTransAndReduce();
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//格式化时间并显示
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timeFormatter(totaltime);
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function timeTransAndReduce(){
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var times = [];
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var oLi = document.getElementsByTagName('li');
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//构建时间数组
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for( var i = 0, len = oLi.length; i < len; i++){
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var timeItem = oLi[i].dataset['time'].split(':');
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times.push(parseInt(timeItem[0],10)*60+parseInt(timeItem[1],10));//将时间转换为秒
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}
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//累加时间并将结果返回
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return times.reduce(function(a,b){
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return a+b;
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},0);
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}
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//修改总时间表达式
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function timeFormatter(seconds) {
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var sec = seconds % 60;
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var hour = Math.floor(seconds/3600);
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var min = (seconds - 3600*hour - sec)/60;
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document.getElementById('totaltime').innerHTML = hour+'小时'+min+'分'+sec+'秒';
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}
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</script>
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</body>
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</html>

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