有趣的JavaScript原生数组函数

最新推荐文章于 2024-08-01 10:46:16 发布

转载最新推荐文章于 2024-08-01 10:46:16 发布 · 339 阅读

本文深入探讨了JavaScript中数组的各种原型方法，包括循环、判断、连接、修改等实用技巧，并对比了相似方法间的区别。

在JavaScript中，创建数组可以使用Array构造函数，或者使用数组直接量[],后者是首选方法。Array对象继承自Object.prototype,对数组执行typeof操作符返回object而不是array。然而，[] instanceof Array也返回true。也就是说，类数组对象的实现更复杂，例如strings对象、arguments对象，arguments对象不是Array的实例，但有length属性，并能通过索引取值，所以能像数组一样进行循环操作。
在本文中，我将复习一些数组原型的方法，并探索这些方法的用法。

循环：.forEach
判断：.some和.every
区分.join和.concat
栈和队列的实现：.pop, .push, .shift,和 .unshift
模型映射：.map
查询：.filter
排序：.sort
计算：.reduce和.reduceRight
复制：.slice
强大的.splice
查找：.indexOf
操作符：in
走近.reverse

Array

循环:`.forEach`

这是JavaScript中最简单的方法，但是IE7和IE8不支持此方法。
.forEach 有一个回调函数作为参数，遍历数组时，每个数组元素均会调用它，回调函数接受三个参数：

value:当前元素
index:当前元素的索引
array:要遍历的数组
此外，可以传递可选的第二个参数，作为每次函数调用的上下文（this）.

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">[<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'_'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'t'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'a'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'n'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'i'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'f'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">']'</span>].forEach(function (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">value</span>, index, array) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.push(String.fromCharCode(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">value</span>.charCodeAt() + index + <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>))
}, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">out</span> = [])

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">out</span>.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">join</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">''</span>)
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- 'awesome'</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li></ul>

后文会提及.join，在这个示例中，它用于拼接数组中的不同元素，效果类似于out[0] + '' + out[1] + '' + out[2] + '' + out[n]。
不能中断.forEach循环，并且抛出异常也是不明智的选择。幸运的事我们有另外的方式来中断操作。

判断：`.some`和`.every`

如果你用过.NET中的枚举，这两个方法和.Any(x => x.IsAwesome) 、 .All(x => x.IsAwesome)类似。
和.forEach的参数类似，需要一个包含value，index，和array三个参数的回调函数，并且也有一个可选的第二个上下文参数。MDN对.some的描述如下：

some将会给数组里的每一个元素执行一遍回调函数，直到回调函数返回true。如果找到目标元素，some立即返回true，否则some返回false。回调函数只对已经指定值的数组索引执行；它不会对已删除的或未指定值的元素调用。

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">max</span> = -Infinity
satisfied = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">23</span>].some(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> (<span class="hljs-title" style="box-sizing: border-box;">value</span>, <span class="hljs-title" style="box-sizing: border-box;">index</span>, <span class="hljs-title" style="box-sizing: border-box;">array</span>) {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">value</span> > <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">max</span>) <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">max</span> = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">value</span>
    <span class="hljs-constant" style="box-sizing: border-box;">return</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">value</span> < <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>
})

console.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">log</span>(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">max</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- 12</span>

satisfied<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- true</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li></ul>

注意，当回调函数的value < 10时，中断函数循环。.every的运行原理和.some类似，但回调函数是返回false而不是true。

区分`.join`和`.concat`

.join和.concat 经常混淆。.join(separator)以separator作为分隔符拼接数组元素，并返回字符串形式，如果没有提供separator，将使用默认的,。.concat会创建一个新数组，作为源数组的浅拷贝。

.concat常用用法：array.concat(val, val2, val3, valn)
.concat返回一个新数组
array.concat()在没有参数的情况下，返回源数组的浅拷贝。
浅拷贝意味着新数组和原数组保持相同的对象引用，这通常是好事。例如：

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">var <span class="hljs-operator" style="box-sizing: border-box;">a</span> = { foo: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'bar'</span> }
var b = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-operator" style="box-sizing: border-box;">a</span>]
var c = b.concat()

console.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">log</span>(b === c)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- false</span>

b[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>] === <span class="hljs-operator" style="box-sizing: border-box;">a</span> && c[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>] === <span class="hljs-operator" style="box-sizing: border-box;">a</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- true</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

栈和队列的实现：`.pop`, `.push`, `.shift`和 `.unshift`

每个人都知道.push可以再数组末尾添加元素，但是你知道可以使用[].push('a', 'b', 'c', 'd', 'z')一次性添加多个元素吗？
.pop 方法是.push 的反操作，它返回被删除的数组末尾元素。如果数组为空，将返回void 0 (undefined),使用.pop和.push可以创建LIFO (last in first out)栈。

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">Stack</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>._stack = []
}

Stack.prototype.next = <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>._stack.pop()
}

Stack.prototype.add = <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>._stack.push.apply(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>._stack, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">arguments</span>)
}

stack = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> Stack()
stack.add(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>)

stack.next()
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- 3</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>

相反，可以使用.shift和 .unshift创建FIFO (first in first out)队列。

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">Queue</span> () {</span>
    this.<span class="hljs-title" style="box-sizing: border-box;">_queue</span> = []
}

Queue.prototype.next = <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> () {</span>
    <span class="hljs-constant" style="box-sizing: border-box;">return</span> this.<span class="hljs-title" style="box-sizing: border-box;">_queue</span>.shift()
}

Queue.prototype.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">add</span> = <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> () {</span>
    <span class="hljs-constant" style="box-sizing: border-box;">return</span> this.<span class="hljs-title" style="box-sizing: border-box;">_queue</span>.unshift.apply(this.<span class="hljs-title" style="box-sizing: border-box;">_queue</span>, arguments)
}

queue = <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">new</span> Queue()
queue.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">add</span>(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>)

queue.next()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- 1</span>
Using .shift (<span class="hljs-operator" style="box-sizing: border-box;">or</span> .pop) is <span class="hljs-operator" style="box-sizing: border-box;">an</span> easy way <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">to</span> loop through <span class="hljs-operator" style="box-sizing: border-box;">a</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">set</span> <span class="hljs-operator" style="box-sizing: border-box;">of</span> array elements, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> draining <span class="hljs-operator" style="box-sizing: border-box;">the</span> array <span class="hljs-operator" style="box-sizing: border-box;">in</span> <span class="hljs-operator" style="box-sizing: border-box;">the</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">process</span>.

list = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>]

<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">while</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">item</span> = list.shift()) {
    console.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">log</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">item</span>)
}

list<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- []</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li><li style="box-sizing: border-box; padding: 0px 5px;">18</li><li style="box-sizing: border-box; padding: 0px 5px;">19</li><li style="box-sizing: border-box; padding: 0px 5px;">20</li><li style="box-sizing: border-box; padding: 0px 5px;">21</li><li style="box-sizing: border-box; padding: 0px 5px;">22</li><li style="box-sizing: border-box; padding: 0px 5px;">23</li><li style="box-sizing: border-box; padding: 0px 5px;">24</li><li style="box-sizing: border-box; padding: 0px 5px;">25</li><li style="box-sizing: border-box; padding: 0px 5px;">26</li><li style="box-sizing: border-box; padding: 0px 5px;">27</li></ul>

模型映射：`.map`

.map为数组中的每个元素提供了一个回调方法，并返回有调用结果构成的新数组。回调函数只对已经指定值的数组索引执行；它不会对已删除的或未指定值的元素调用。

Array.prototype.map 和上面提到的.forEach、.some和 .every有相同的参数格式：.map(fn(value, index, array), thisArgument)

<code class="hljs coffeescript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">values = [<span class="hljs-reserved" style="box-sizing: border-box;">void</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">null</span>, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">''</span>]
values[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>] = <span class="hljs-reserved" style="box-sizing: border-box;">void</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>
result = values.map(<span class="hljs-reserved" style="box-sizing: border-box;">function</span>(value, index, array){
    <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">console</span>.log(value)
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> value
})

<span class="hljs-regexp" style="color: rgb(0, 136, 0); box-sizing: border-box;">//</span> <- [<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">undefined</span>, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">null</span>, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">''</span>, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">undefined</span> × <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">undefined</span>]</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

undefined × 3很好地解释了.map不会对已删除的或未指定值的元素调用，但仍然会被包含在结果数组中。.map在创建或改变数组时非常有用，看下面的示例：

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// casting</span>
[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'2'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'30'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'9'</span>].map(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(value)</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">parseInt</span>(value, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>)
})
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// 1, 2, 30, 9</span>

[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">97</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">119</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">101</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">115</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">111</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">109</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">101</span>].map(<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">String</span>.fromCharCode).join(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">''</span>)
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- 'awesome'</span>

<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// a commonly used pattern is mapping to new objects</span>
items.map(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(item)</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> {
        id: item.id,
        name: computeName(item)
    }
})</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li></ul>

查询：`.filter`

filter对每个数组元素执行一次回调函数，并返回一个由回调函数返回true的元素组成的新数组。回调函数只会对已经指定值的数组项调用。

通常用法：.filter(fn(value, index, array), thisArgument),跟C#中的LINQ表达式和SQL中的where语句类似，.filter只返回在回调函数中返回true值的元素。

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">[<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">null</span>, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">''</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>].filter(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(value)</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> value
})
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- [1]</span>

[<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">null</span>, <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">false</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">''</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>].filter(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(value)</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> !value
})
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- [void 0, null, false, '']</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li></ul>

排序：`.sort(compareFunction)`

如果没有提供compareFunction，元素会被转换成字符串并按照字典排序。例如，”80”排在”9”之前，而不是在其后。

跟大多数排序函数类似，Array.prototype.sort(fn(a,b))需要一个包含两个测试参数的回调函数,其返回值如下：

a在b之前则返回值小于0
a和b相等则返回值是0
a在b之后则返回值小于0

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">80</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>].sort()
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- [10, 3, 5, 6, 80, 9]</span>

[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">80</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>].sort(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(a, b)</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> a - b
})
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- [3, 5, 6, 9, 10, 80]</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

计算：`.reduce`和`.reduceRight`

这两个函数比较难理解，.reduce会从左往右遍历数组，而.reduceRight则从右往左遍历数组，二者典型用法：.reduce(callback(previousValue,currentValue, index, array), initialValue)。
previousValue 是最后一次调用回调函数的返回值，initialValue则是其初始值，currentValue是当前元素值，index是当前元素索引，array是调用.reduce的数组。
一个典型的用例，使用.reduce的求和函数。

<code class="hljs javascript has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Array</span>.prototype.sum = <span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span> {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.reduce(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(partial, value)</span> {</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> partial + value
    }, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>)
};

[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>].sum()
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- 28</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

如果想把数组拼接成一个字符串，可以用.join实现。然而，若数组值是对象，.join就不会按照我们的期望返回值了，除非对象有合理的valueOf或toString方法，在这种情况下，可以用.reduce实现：

<code class="hljs cs has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">function concat (input) {
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> input.reduce(function (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">partial</span>, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">value</span>) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">partial</span>) {
            <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">partial</span> += <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">', '</span>
        }
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">partial</span> + <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">value</span>
    }, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">''</span>)
}

concat([
    { name: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'George'</span> },
    { name: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Sam'</span> },
    { name: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'Pear'</span> }
])
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- 'George, Sam, Pear'</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>

复制：`.slice`

和.concat类似，调用没有参数的.slice()方法会返回源数组的一个浅拷贝。.slice有两个参数：一个是开始位置和一个结束位置。
Array.prototype.slice 能被用来将类数组对象转换为真正的数组。

<code class="hljs avrasm has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">Array<span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.prototype</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.slice</span><span class="hljs-preprocessor" style="color: rgb(68, 68, 68); box-sizing: border-box;">.call</span>({ <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'a'</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'b'</span>, length: <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> })
// <- [<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'a'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'b'</span>]</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

这对.concat不适用，因为它会用数组包裹类数组对象。

<code class="hljs perl has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">Array.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">prototype</span>.concat.call({ <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'a'</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'b'</span>, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">length</span>: <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> })
// <- [{ <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'a'</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'b'</span>, <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">length</span>: <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span> }]</code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li></ul>

此外，.slice的另一个通常用法是从一个参数列表中删除一些元素，这可以将类数组对象转换为真正的数组。

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-title" style="box-sizing: border-box;">format</span> (<span class="hljs-title" style="box-sizing: border-box;">text</span>, <span class="hljs-title" style="box-sizing: border-box;">bold</span>) {</span>
    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (bold) {
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">text</span> = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'<b>'</span> + <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">text</span> + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'</b>'</span>
    }
    var values = Array.prototype.slice.call(arguments, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>)

    values.forEach(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> (<span class="hljs-title" style="box-sizing: border-box;">value</span>) {</span>
        <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">text</span> = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">text</span>.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">replace</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'%s'</span>, <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">value</span>)
    })

    <span class="hljs-constant" style="box-sizing: border-box;">return</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">text</span>
}

<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">format</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'some%sthing%s %s'</span>, <span class="hljs-constant" style="box-sizing: border-box;">true</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'some'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'other'</span>, <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'things'</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- <b>somesomethingother things</b></span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li></ul>

强大的`.splice`

.splice 是我最喜欢的原生数组函数，只需要调用一次，就允许你删除元素、插入新的元素，并能同时进行删除、插入操作。需要注意的是，不同于`.concat和.slice,这个函数会改变源数组。

<code class="hljs mel has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">var <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">source</span> = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">11</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">13</span>]
var spliced = <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">source</span>.splice(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">4</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>)

console.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">log</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">source</span>)
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 ,13]</span>

spliced
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- [8, 8, 8, 8]</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li></ul>

正如你看到的，.splice会返回删除的元素。如果你想遍历已经删除的数组时，这会非常方便。

<code class="hljs php has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> source = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">11</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">12</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">13</span>]
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> spliced = source.splice(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">9</span>)

spliced.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">forEach</span>(<span class="hljs-function" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">function</span> <span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(value)</span> {</span>
    console.log(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'removed'</span>, value)
})
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- removed 10</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- removed 11</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- removed 12</span>
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- removed 13</span>

console.log(source)
<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">// <- [1, 2, 3, 8, 8, 8, 8, 8, 9]</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li></ul>

查找：`.indexOf`

利用.indexOf 可以在数组中查找一个元素的位置，没有匹配元素则返回-1。我经常使用.indexOf的情况是当我有比较时，例如：a === 'a' || a === 'b' || a === 'c'，或者只有两个比较，此时，可以使用.indexOf：['a', 'b', 'c'].indexOf(a) !== -1。
注意，如果提供的引用相同，.indexOf也能查找对象。第二个可选参数用于指定开始查找的位置。

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">var <span class="hljs-operator" style="box-sizing: border-box;">a</span> = { foo: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'bar'</span> }
var b = [<span class="hljs-operator" style="box-sizing: border-box;">a</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>]

console.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">log</span>(b.indexOf(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- -1</span>

console.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">log</span>(b.indexOf({ foo: <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'bar'</span> }))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- -1</span>

console.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">log</span>(b.indexOf(<span class="hljs-operator" style="box-sizing: border-box;">a</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- 0</span>

console.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">log</span>(b.indexOf(<span class="hljs-operator" style="box-sizing: border-box;">a</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>))<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- -1</span>

b.indexOf(<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- 1</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li><li style="box-sizing: border-box; padding: 0px 5px;">8</li><li style="box-sizing: border-box; padding: 0px 5px;">9</li><li style="box-sizing: border-box; padding: 0px 5px;">10</li><li style="box-sizing: border-box; padding: 0px 5px;">11</li><li style="box-sizing: border-box; padding: 0px 5px;">12</li><li style="box-sizing: border-box; padding: 0px 5px;">13</li><li style="box-sizing: border-box; padding: 0px 5px;">14</li><li style="box-sizing: border-box; padding: 0px 5px;">15</li><li style="box-sizing: border-box; padding: 0px 5px;">16</li><li style="box-sizing: border-box; padding: 0px 5px;">17</li></ul>

如果你想从后向前搜索，可以使用.lastIndexOf。

操作符：`in`

在面试中新手容易犯的错误是混淆.indexOf和in操作符:

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">var <span class="hljs-operator" style="box-sizing: border-box;">a</span> = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span>]

<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> <span class="hljs-operator" style="box-sizing: border-box;">in</span> <span class="hljs-operator" style="box-sizing: border-box;">a</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- true, but because of the 2!</span>

<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">5</span> <span class="hljs-operator" style="box-sizing: border-box;">in</span> <span class="hljs-operator" style="box-sizing: border-box;">a</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- false</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li><li style="box-sizing: border-box; padding: 0px 5px;">5</li><li style="box-sizing: border-box; padding: 0px 5px;">6</li><li style="box-sizing: border-box; padding: 0px 5px;">7</li></ul>

问题是in操作符是检索对象的键而非值。当然，这在性能上比.indexOf快得多。

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">var <span class="hljs-operator" style="box-sizing: border-box;">a</span> = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>]

<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span> <span class="hljs-operator" style="box-sizing: border-box;">in</span> <span class="hljs-operator" style="box-sizing: border-box;">a</span> === !!<span class="hljs-operator" style="box-sizing: border-box;">a</span>[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>]<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// <- true</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

走近`.reverse`

该方法将数组中的元素倒置。

<code class="hljs livecodeserver has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;">var <span class="hljs-operator" style="box-sizing: border-box;">a</span> = [<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">7</span>, <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>]

<span class="hljs-operator" style="box-sizing: border-box;">a</span>.reverse()<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">
// [8, 7, 1, 1]</span></code><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul><ul class="pre-numbering" style="box-sizing: border-box; position: absolute; width: 50px; top: 0px; left: 0px; margin: 0px; padding: 6px 0px 40px; border-right-width: 1px; border-right-style: solid; border-right-color: rgb(221, 221, 221); list-style: none; text-align: right; background-color: rgb(238, 238, 238);"><li style="box-sizing: border-box; padding: 0px 5px;">1</li><li style="box-sizing: border-box; padding: 0px 5px;">2</li><li style="box-sizing: border-box; padding: 0px 5px;">3</li><li style="box-sizing: border-box; padding: 0px 5px;">4</li></ul>

.reverse 会修改数组本身。