JavaScript面试总结

1.sort & split & join的用法

<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;">【网易<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2016</span>内推笔试题】<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> a = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"Hello World"</span>.split(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">""</span>).sort().<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>);alert(a); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//HWdelllor</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></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></ul>

知识点：

①sort( )方法：按升序排列数组项。

【说明】

　　如果调用该方法时没有使用参数，将按字母顺序对数组中的元素进行排序，即按照字符编码的顺序进行排序。（空格=32，0=48，A-65，a-97） 
　　为了实现排序，sort( )方法会调用每个数组项的toString( )转型方法，然后比较得到的字符串，以确定如何排序。

即使数组的每一项都是数值，sort( )方法比较的也是字符串。

<code class="language-javascript hljs  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;">例如：<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> values = [<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;">1</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;">10</span>,<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">15</span>];values.sort();alert(values); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//0,1,10,15,5</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>

②split() 方法：把一个字符串分割成字符串数组。

【语法】

<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;">stringObject.<span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">split</span>(separator,howmany)</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></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></ul>

separator：必需。字符串或正则表达式，从该参数指定的地方分割 stringObject。 
howmany：可选。该参数可指定返回的数组的最大长度。如果设置了该参数，返回的子串不会多于这个参数指定的数组。如果没有设置该参数，整个字符串都会被分割，不考虑它的长度。

<code class="language-javascript hljs  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;">例如：<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> str=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"How are you doing today?"</span>document.write(str.split(<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;">"<br />"</span>);document.write(str.split(<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;">"<br />"</span>)document.write(str.split(<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;">3</span>))OUT:How,are,you,doing,today?H,o,w, ,a,r,e, ,y,o,u, ,d,o,i,n,g, ,t,o,d,a,y,?How,are,you</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></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></ul>

③join() 方法：把数组中的所有元素放入一个字符串。

【语法】

<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;">arrayObject.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">join</span>(separator)</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></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></ul>

separator：可选。指定要使用的分隔符。如果省略该参数，则使用逗号作为分隔符。

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2.forEach的用法

<code class="language-javascript hljs  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;">【网易<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2016</span>内推笔试题】<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> arr = [{a:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>},{}];arr.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-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(item,idx)</span>{</span>    item.b = idx;});console.log(arr); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//[{a:1,b:0},{b:1}]</span><span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//解析：</span>forEach对数组的每一项运行item.b = idx;数组的第<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>项是{a:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>}，运行item.b = idx相当于给该项{a:<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>}添加b属性，值为<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span>；数组的第<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>项是{ }，运行item.b = idx相当于给该项{ }添加b属性，值为<span class="hljs-number" style="color: rgb(0, 102, 102); 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></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>

知识点：

forEach() 方法：对数组的每一项运行给定函数。没有返回值。

【语法】

<code class="language-javascript hljs  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>.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-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(item,index,array)</span>{</span>    <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//执行某些操作</span>}[,作用域对象]);</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></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></ul>

该方法接收2个参数：要在每一项上运行的函数和（可选的）运行该函数的作用域对象——影响this的值。

传入这些方法中的函数会接收3个参数： 
数组项的值item、该项在数组中的位置index和数组对象本身array。

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3.闭包

<code class="language-javascript hljs  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;">【网易<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2016</span>内推笔试题】<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> a = <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;">var</span> func = (<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;">var</span> a = <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</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;">()</span>{</span>        a++;        alert(a);    }})();func(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//4</span>func(); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//5</span>【解析】(<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-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//立即执行函数，这里是块级作用域（私有作用域）</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></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>

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4.原型 ？

<code class="hljs autohotkey 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;">【网易<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2016</span>内推笔试题】 （多选题）以下<span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">a</span>的原型是b的有： <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">A</span><span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">A</span>.var <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">a</span> = Object.create(b)<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">;</span>B.function <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">A</span>{}C.var <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">a</span> = {}D.function <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">A</span>(proto){this.prototype=proto<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></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>

知识点：

原型

　　每个函数都有一个prototype(原型)属性，这个属性是一个指针，指向一个对象(原型对象)，这个原型对象中的属性和方法是所有对象实例共享的。

*  一个对象的真正原型是被对象内部的[[Prototype]]属性(property)所持有。浏览器支持非标准的访问器proto。

---

5.this（apply、call、bind）

<code class="language-javascript hljs  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;">【网易<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2016</span>内推笔试题】 （多选题）<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> a={name:<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"jobs"</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-title" style="box-sizing: border-box;">getName</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>.name;}问：以下值为jobs的有？ ABA.getName.call(a);B.getName.apply(a);C.getName.bind(a);D.getName(a);      <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></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>

知识点：

①apply与call方法

它们的作用是改变函数的调用对象，它的第一个参数表示改变后的调用这个函数的对象，即this指第一个参数。指明函数为哪个对象的方法。 
后面参数： 
call是独立传递参数； 
apply以数组传递参数。

②bind方法

bind方法会创建一个函数的实例，其this值会被绑定到传给bind( )函数的值。

<code class="language-javascript hljs  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;">例如：window.color = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"red"</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> o = {color:<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"blue"</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-title" style="box-sizing: border-box;">sayColor</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">()</span>{</span>    alert(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.color);}<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> objectSayColor = sayColor.bind(o); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//这一步并不会输出"blue"</span>objectSayColor();  <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//blue</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>

sayColor( )调用bind( )并传入对象o，创建了objectSayColor( )函数。objectSayColor( )函数的this值等于o，因此即使是在全局作用域中调用这个函数，也会看到”blue”。

---

6.this（全局&new构造器）

Out1： 
 
Person(“p1”)时，this指向全局对象，name值改为p1； 
p没有name属性。

Out2： 
people 
p2 
new Person(“p1”)时，this指向对象p，全局变量name值不变； 
p.name值为p2。

---

8.DOM

<code class="language-html hljs  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;">【网易2016内推笔试题】 （多选题）<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">input</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">id</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"userName"</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">name</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"userName"</span>/></span>问：对应DOM节点是userNameElement，以下能获取输入框值的是：ACA.userNameElement.value;B.userNameElement.getAttribute("value");C.userNameElement["value"];D.userName.getValue();</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>

【解析】

B错，在此，value不是input标签的属性。这里只可以获取id和class。 
D错，没有这个函数。

<code class="language-html hljs  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-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">button</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">id</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"myBtn"</span>></span>点击这里<span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">button</span>></span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">input</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">id</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"userName"</span> <span class="hljs-attribute" style="box-sizing: border-box; color: rgb(102, 0, 102);">name</span>=<span class="hljs-value" style="box-sizing: border-box; color: rgb(0, 136, 0);">"userName"</span>></span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"><<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span><span class="javascript" style="box-sizing: border-box;">    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> user = document.getElementById(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"userName"</span>);    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> myBtn = document.getElementById(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"myBtn"</span>)    myBtn.onclick=<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>        console.log(user.value);        console.log(user[<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"value"</span>]);        console.log(user.getAttribute(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"value"</span>));        console.log(user.getValue());    };</span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">script</span>></span><span class="hljs-tag" style="color: rgb(0, 102, 102); box-sizing: border-box;"></<span class="hljs-title" style="box-sizing: border-box; color: rgb(0, 0, 136);">body</span>></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>

Out： 

---

9.JS动画

<code class="language-javascript hljs  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;">【网易<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2016</span>内推笔试题】 （多选题）实现JS动画的有：BCDA.keyframes   <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//CSS3动画</span>B.setIntervalC.setTimeOutD.requestAnimationFrame</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>

知识点：

①CSS3动画

②requestAnimationFrame

---

10.闭包&全局变量

<code class="language-javascript hljs  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;">【腾讯<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2016</span>笔试题】 <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> x=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</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;">()</span> {</span>    x=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>;    y=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">6</span>;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> z=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">3</span>;})();console.log(x==y); <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//true</span>console.log(z); <span class="hljs-comment" style="color: rgb(136, 0, 0); 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><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>

---

11.BOM浏览器对象模型

JS中关闭当前的窗口的方法是：window.close();

---

12.null

console.log(typeof null); 运行的结果是在控制台输出什么？ 
答案： 输出object。 在JavaScript中，null值表示一个空对象指针，而这正是使用typeof操作符检测null值时会返回“object”的原因。

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13.关键字let

在ECMAScript6中，定义一个变量只在所处的代码块起作用的关键字是？ 
答案： 关键字let。 ES6 新增了 let 命令，用来声明变量。它的用法类似于 var，但是所声明的变量，只在 let 命令所在的块级作用域（花括号{ }）起作用。

<code class="language-javascript hljs  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;">//例1</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> arr = [ ];<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> i=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> ; i<<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span> ; i++){    arr[i]=<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>        alert(i);    }}arr[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>]();<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//Out：10</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>

如果用var声明的变量，i的值会影响到各个块里面的i，等循环完后i等于10，所有块的i都变成了10。

<code class="language-javascript hljs  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> arr = [ ];<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">for</span>(<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">let</span> i=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">0</span> ; i<<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">10</span> ; i++){    arr[i]=<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>        alert(i);    }}arr[<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span>]();<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//Out：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>

for循环含有{ }，也就是含有了块级作用域，每个变量i都只是在自己的作用域起作用，例如：第10次循环中的i的值不会影响到第9次循环。

<code class="language-javascript hljs  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;">//例2</span><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> a=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</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;">()</span>{</span>    alert(a);    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">let</span> a=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;})();<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//Out:报错a未定义</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>

用let关键字来定义a，这样a在代码块内就不会提升了。那为什么报错了呢？因为用let声明的变量，在其块级作用域内是封闭的，是不会受到外面的全局变量a影响的，并且要先声明再使用，所以a的值既不是1（因为不受外面的影响），也不是undefined（因为先声明再使用），更不是2，未声明定义就使用，只有报错啦！

【注意】

1.同一个块级作用域内，不允许重复声明同一个变量。

<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-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//错误1 </span>{    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> a=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">let</span> a=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//报错，因为a已经用var声明过</span>}<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//错误2</span>{    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">let</span> a=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">1</span>;    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">let</span> a=<span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">2</span>;<span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//报错，因为a已经用let声明过</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>

2.函数内不能用let重新声明函数的参数。

<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-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;">say</span><span class="hljs-params" style="color: rgb(102, 0, 102); box-sizing: border-box;">(word)</span>{</span>    <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">let</span> word=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"hello Jack"</span>; <span class="hljs-comment" style="color: rgb(136, 0, 0); box-sizing: border-box;">//报错，用let重新声明word参数</span>    alert(word);}say(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"hello Lili"</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></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></ul>

【总结】

用let声明变量只在块级作用域起作用，适合在for循环使用，也不会出现变量提升现象。同一个代码块内，不可重复声明相同的变量，不可重复声明函数内的参数。

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14.instanceof

<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;">if</span>( [ ] <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">instanceof</span> <span class="hljs-built_in" style="color: rgb(102, 0, 102); box-sizing: border-box;">Object</span> ){      alert(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'hello'</span>);  }  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span>  {      alert(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'world'</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>

运行后结果是弹出什么？ 
答案：弹出”hello“，之所以[ ] instanceof Object返回true，是因为Array是object的子类。

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15.+操作符vs三元操作符

<code class="hljs rust 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;">console.<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">log</span>(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">'hello'</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-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"world"</span>:<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"me"</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></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></ul>

运行后输出什么？ 
答案：输出world，因为+操作符的优先级高于三元操作符，执行顺序分别是：（1<2）得到 true，然后"hello"+true = "hellotrue",最后进行三元操作符，得到"world"。

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16.作用域

<code class="language-javascript hljs  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> str1=str2=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"web"</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;">()</span>{</span>  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> str1=str2=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"前端"</span>;})();console.log(str1);</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>

运行后输出什么？ 
答案：输出“web”，代码中有两个str1，但是他们的都用var定义，处在不同的作用域，不会被覆盖，所以输出的str1是同一个作用域下的str1，所以是：“web”。

<code class="language-javascript hljs  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> str1=str2=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"web"</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;">()</span>{</span>  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> str1=str2=<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"前端"</span>;})();console.log(str2);</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>

答案：“前端”，虽然str2是匿名函数内部，但是并没有被var定义，所以会往上一级去寻找，找到并重新赋值成“前端”。

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17.关系运算符

<code class="language-javascript hljs  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;">if</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;">9</span> > <span class="hljs-number" style="color: rgb(0, 102, 102); box-sizing: border-box;">8</span> == <span class="hljs-literal" style="color: rgb(0, 102, 102); box-sizing: border-box;">true</span> ){      console.log(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"html5"</span>);  }  <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">else</span>  {      console.log(<span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"css3"</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>

答案：css3。先比较10和9，10 > 9是成立的，此时返回的是true。true和8进行比较运算，此时会出现数据类型的转换，true会转换成数字1，再与8进行比较，1自然是小于8的，因此此处返回的是false。最后比较false和true，这两者自然是不等的，执行else里面的语句，输出：css3。

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18.变量类型

<code class="language-javascript hljs  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> obj={};obj.name = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"first"</span>;<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">var</span> peo = obj;peo.name = <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"second"</span>;console.log(obj.name);</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></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></ul>

答案：输出second。

关于变量类型，分为了两大类，一种是引用类型，一种是值类型。

• 值类型相当于是重新创建了一个空间，然后将内容复制了一份放置到了空间当中。
• 引用类型相当于是重新创建了一份地址，两个地址（obj和peo）都是指向一个存储空间的，那么此时修改任意一个都会对另一个造成影响。

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19.BOM对象

（1）浏览器对象

在BOM对象中，能够获取浏览器信息的对象是？ 
答案：Navigator对象，Navigator 对象包含有关浏览器的信息，所有浏览器都支持该对象。

（2）客户端显示屏

在BOM对象中，能获取客户端显示屏的信息的对象是？ 
答案：Screen对象，Screen 对象包含有关客户端显示屏幕的信息。

（3）浏览器当前URL信息

在BOM对象中，能获取浏览器当前URL信息的对象是？ 
答案：Location对象，Location 对象是 Window 对象的一个部分，可通过 window.location 属性来访问。

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20.事件对象

（1）ondblclick双击事件

在事件对象中，表示对象被双击时发生的事件？ 
答案：ondblclick 事件会在对象被双击时发生。

（2）onblur失去焦点事件

在事件对象中，当对象失去焦点时会发生的事件是? 
答案：onblur 事件会在对象失去焦点时发生，常用在输入域里面。

（3）onchange 改变事件

在事件对象中，当在域的内容发生改变的时候会触发的事件是？ 
答案：onchange 事件会在域的内容改变时发生。常用在输入域，下拉选择，单选多选组件中。

（4）onkeyup键盘按键松开事件

在事件对象中，表示监听键盘按键松开的是事件是？ 
答案：onkeyup事件，onkeyup 事件会在键盘按键被松开时发生。

（5）onmousemove鼠标移动事件

在事件对象中，监听鼠标移动的事件是？ 
答案：onmousemove事件，每当用户把鼠标移动一个像素，就会发生一个 mousemove 事件。

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21.字符串与数组对象“+”

<code class="language-javascript hljs  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;">console.log([] + [] + <span class="hljs-string" style="color: rgb(0, 136, 0); box-sizing: border-box;">"foo"</span>.split(<span class="hljs-string" style="color: rgb(0, 136, 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></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></ul>

答案：f,o,o； 
详解：在[] + []的运算过程中，由于使用到了+号，两个数组对象均调用了自己自带的toString()方法，将[]转换成了字符串，两个字符串相加还是一个字符串。"foo".split("")，将字符串分割成字符串数组，为["f", "o", "o"]。最后一步，"" + ["f", "o", "o"]，这时候 + 左侧是字符串，右侧是数组对象，数组对象依旧会调用自身的原型方法，将数组转换为字符串。

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22.toFixed()方法

在JavaScript Number 对象中，可以四舍五入为指定小数位的数字的方法是？ 
答案：toFixed()方法。可以传入一个参数作为规定小数的位数，是 0 ~ 20 之间的值。

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23.getDay( )方法

在JavaScript Date对象中，能够获取今天是星期几的方法是？ 
答案：getDay()方法。

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24.eval( )方法

在javascript的全局函数中，把字符串作为脚本代码来执行的方法是？ 
答案：eval( ) 。eval() 函数计算 JavaScript 字符串，并把它作为脚本代码来执行。如果参数是一个表达式，eval() 函数将执行表达式。

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25.parseInt( )方法

在javascript全局函数中，解析一个字符串并返回一个整数的方法是？ 
答案：parseInt()。可解析一个字符串，并返回一个整数。 
注意： 只有字符串中的第一个数字会被返回。

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26.isNaN( )方法

在JavaScript 全局函数中，检测某个值是否是数字的函数是？ 
答案：isNaN()。如果参数值为 NaN 或字符串、对象、undefined等非数字值则返回 true, 否则返回 false。