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<h1 id="箭头函数"><a href="#箭头函数" class="headerlink" title="箭头函数"></a>箭头函数</h1><h2 id="基础语法"><a href="#基础语法" class="headerlink" title="基础语法"></a>基础语法</h2><blockquote>
<p>(参数1,参数2,…参数N) =>{函数声明}<br>(参数1,参数2,…参数N) => 表达式(单一)<br>//相当于：(参数1,参数2,…参数N) =>{return 表达式;}</p>
<p>//当只有一个参数时，圆括号是可选的:<br>(单一参数) => {函数声明}<br>单一参数 => {函数声明} </p>
<p>//无参数的函数应该写成一对圆括号<br>() => {函数声明} </p>
</blockquote>
<h2 id="高级语法"><a href="#高级语法" class="headerlink" title="高级语法"></a>高级语法</h2><blockquote>
<p>//加括号的函数体返回对象字面表达式<br>参数 =>({foo:bar}) </p>
<p>//支持剩余参数和默认参数<br>(参数1,参数2,…rest) => {函数声明}<br>(参数1 = 默认值1,参数2,…参数N = 默认值N) => {函数声明}</p>
<p>//同样支持参数列表解构<br>let f = ([a,b] = [1,2],{x:c} = {x: a + b}) => a + b +c;<br>f(); //6</p>
</blockquote>
<h2 id="描述"><a href="#描述" class="headerlink" title="描述"></a>描述</h2><p>引入箭头函数有两个方面的作用：更简短的函数并且不绑定 <code>this</code> </p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> a = [<span class="string">'fiftyfifty'</span>,<span class="string">'done'</span>,<span class="string">'you'</span>,<span class="string">'goodjob'</span>];</span><br><span class="line">a.map(<span class="function"><span class="keyword">function</span>(<span class="params">a</span>)</span>{</span><br><span class="line"> <span class="keyword">return</span> a.length;</span><br><span class="line">}); <span class="comment">//[10,4,3,7]</span></span><br><span class="line"></span><br><span class="line">a.map(a) => {</span><br><span class="line"> <span class="keyword">return</span> a.length;</span><br><span class="line">} <span class="comment">//[10,4,3,7]</span></span><br><span class="line"></span><br><span class="line">a.map(<span class="function"><span class="params">a</span> =></span> a.length); <span class="comment">//[10,4,3,7]</span></span><br></pre></td></tr></table></figure>
<h2 id="不绑定-this"><a href="#不绑定-this" class="headerlink" title="不绑定 this"></a>不绑定 <code>this</code></h2><p>在箭头函数出现之前，每个新定义的函数都有自己的 <code>this</code> 值(在构造函数的情况下是一个新对象，在严格模式的函数调用中为 <code>undefined</code>,如果该函数被称为“对象方法” 则为基础对象等 )</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line">funtion Person(){</span><br><span class="line"> <span class="comment">//Person() 构造函数定义 `this` 作为自己的实例</span></span><br><span class="line"> <span class="keyword">this</span>.age = <span class="number">0</span>;</span><br><span class="line"></span><br><span class="line"> setInterval(<span class="function"><span class="keyword">function</span> <span class="title">growUp</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="comment">//在非严格模式，growUp()函数定义 `this` 作为全局对象,</span></span><br><span class="line"> <span class="comment">//在与Person() 构造函数中定义的 `this` 并不相同</span></span><br><span class="line"> <span class="keyword">this</span>.age++;</span><br><span class="line"> },<span class="number">1000</span>);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> p = <span class="keyword">new</span> Person();</span><br></pre></td></tr></table></figure>
<p>在 <code>ECMAScript 3/5</code> 中，通过将 <code>this</code> 值分配给封闭的变量，可以解决 <code>this</code> 问题。</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">Person</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> that = <span class="keyword">this</span>;</span><br><span class="line"> <span class="keyword">this</span>.age = <span class="number">0</span>;</span><br><span class="line"> setIntervar(<span class="function"><span class="keyword">function</span> <span class="title">growUp</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="comment">//回调引用的是 `that` 变量，其值是预期的对象</span></span><br><span class="line"> that.age++;</span><br><span class="line"> },<span class="number">1000</span>);</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<p>或者，可以创建绑定函数，以便将预先分配的 <code>this</code> 值传递到绑定的目标函数上</p>
<p>箭头函数不会创建自己的 <code>this</code> ,它会从自己的作用域链的上一层继承 <code>this</code> .因此，在下面的代码中，传递给 <code>setInterval</code> 的函数内的 <code>this</code> 与封闭函数中的 <code>this</code> 相同</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">Person</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">this</span>.age = <span class="number">0</span>;</span><br><span class="line"> setInterval(<span class="function"><span class="params">()</span> =></span>{</span><br><span class="line"> <span class="keyword">this</span>.age++; <span class="comment">//|this| 正确指向 person 对象</span></span><br><span class="line"> },<span class="number">1000</span>);</span><br><span class="line">}</span><br><span class="line"><span class="keyword">var</span> p = <span class="keyword">new</span> Person();</span><br></pre></td></tr></table></figure>
<h2 id="通过-call-或者-apply-调用"><a href="#通过-call-或者-apply-调用" class="headerlink" title="通过 call 或者 apply 调用"></a>通过 <code>call</code> 或者 <code>apply</code> 调用</h2><p>由于箭头函数没有自己的 <code>this</code> 指针，通过 <code>call()</code> 或者 <code>apply()</code> 方法调用一个函数时，只能传递参数</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> adder = {</span><br><span class="line"> base:<span class="number">1</span>,</span><br><span class="line"> add:<span class="function"><span class="keyword">function</span>(<span class="params">a</span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> f = <span class="function"><span class="params">v</span> =></span> v + <span class="keyword">this</span>.base;</span><br><span class="line"> <span class="keyword">return</span> f(a);</span><br><span class="line"> },</span><br><span class="line"></span><br><span class="line"> addThruCall: <span class="function"><span class="keyword">function</span>(<span class="params">a</span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> f = <span class="function"><span class="params">v</span> =></span> v + <span class="keyword">this</span>.base;</span><br><span class="line"> <span class="keyword">var</span> b = {</span><br><span class="line"> base: <span class="number">2</span></span><br><span class="line"> };</span><br><span class="line"> <span class="keyword">return</span> f.call(b,a);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line"><span class="built_in">console</span>.log(adder.add(<span class="number">1</span>)); <span class="comment">//2</span></span><br><span class="line"><span class="built_in">console</span>.log(adder.addThruCall(<span class="number">1</span>)); <span class="comment">//2 不是3</span></span><br></pre></td></tr></table></figure>
<h2 id="不绑定-arguments"><a href="#不绑定-arguments" class="headerlink" title="不绑定 arguments"></a>不绑定 <code>arguments</code></h2><p>箭头函数不绑定 <code>arguments 对象</code>，在下例中， <code>arguments</code> 只是引用了封闭作用域内的 <code>arguments</code></p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> <span class="built_in">arguments</span> = [<span class="number">1</span>,<span class="number">2</span>,<span class="number">3</span>];</span><br><span class="line"><span class="keyword">var</span> arr = <span class="function"><span class="params">()</span> =></span> <span class="built_in">arguments</span>[<span class="number">0</span>];</span><br><span class="line">arr(); <span class="comment">//1</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">foo</span>(<span class="params">n</span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> f = <span class="function"><span class="params">()</span> =></span> <span class="built_in">arguments</span>[<span class="number">0</span>] + n; <span class="comment">//隐式绑定 foo 函数的 arguments 对象，arguments[0] 是 n</span></span><br><span class="line"> <span class="keyword">return</span> f();</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">foo(<span class="number">1</span>); <span class="comment">//2</span></span><br></pre></td></tr></table></figure>
<p>大多数情况下，使用剩余参数是相较使用 <code>arguments</code> 对象的更好选择</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">foo</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> f = <span class="function">(<span class="params">...args</span>) =></span> args[<span class="number">0</span>];</span><br><span class="line"> <span class="keyword">return</span> f(<span class="number">2</span>);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line">foo(<span class="number">1</span>); <span class="comment">//2</span></span><br></pre></td></tr></table></figure>
<h2 id="像方法一样使用箭头函数"><a href="#像方法一样使用箭头函数" class="headerlink" title="像方法一样使用箭头函数"></a>像方法一样使用箭头函数</h2><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="meta">'use strict'</span>;</span><br><span class="line"><span class="keyword">var</span> obj = {</span><br><span class="line"> i:<span class="number">10</span>,</span><br><span class="line"> b:<span class="function"><span class="params">()</span> =></span> <span class="built_in">console</span>.log(<span class="keyword">this</span>.i,<span class="keyword">this</span>),</span><br><span class="line"> c:<span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="built_in">console</span>.log(<span class="keyword">this</span>.i,<span class="keyword">this</span>);</span><br><span class="line"> }</span><br><span class="line">}</span><br><span class="line">obj.b();</span><br><span class="line"><span class="comment">//undefined</span></span><br><span class="line">obj.c();</span><br><span class="line"><span class="comment">//10,Object{...}</span></span><br></pre></td></tr></table></figure>
<h2 id="使用-new-操作符"><a href="#使用-new-操作符" class="headerlink" title="使用 new 操作符"></a>使用 <code>new</code> 操作符</h2><p>箭头函数不能用作构造器，和 <code>new</code> 一起用会抛出错误</p>
<blockquote>
<p>var Foo = () =>{};<br>var foo = new Foo(); //TypeError:Foo is not a constructor</p>
</blockquote>
<h2 id="使用-prototype-属性"><a href="#使用-prototype-属性" class="headerlink" title="使用 prototype 属性"></a>使用 <code>prototype</code> 属性</h2><p>箭头函数没有 <code>prototype</code> 属性</p>
<blockquote>
<p>var Foo = () =>{}<br>console.log(Foo.prototype); //undefined</p>
</blockquote>
<h2 id="函数体"><a href="#函数体" class="headerlink" title="函数体"></a>函数体</h2><p>箭头函数可以有一个简写体或者是常见的块体<br>在一个简写体重，只需要一个表达式，并附加一个隐式的返回值，在块体中，必须使用明确的 <code>return</code> 语句</p>
<blockquote>
<p>var func = x => x * x;<br>//简写函数 省略return </p>
<p>var func = (x,y) => {return x+y;};<br>//常规编写，明确的返回值</p>
</blockquote>
<h2 id="返回对象字面量"><a href="#返回对象字面量" class="headerlink" title="返回对象字面量"></a>返回对象字面量</h2><p>使用 <code>params =>{object:literal}</code> 这种简单的语法返回对象字面量是行不通的<br>记得用圆括号把字面量包起来 </p>
<blockquote>
<p>var func = () => ({foo:1});</p>
</blockquote>
<h2 id="换行"><a href="#换行" class="headerlink" title="换行"></a>换行</h2><p>箭头函数在参数和箭头之间不能换行</p>
<h2 id="解析顺序"><a href="#解析顺序" class="headerlink" title="解析顺序"></a>解析顺序</h2><p>虽然箭头函数中的箭头不是运算符，但箭头函数具有与常规函数不同的特殊运算符优先级解析规则</p>
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">let</span> callback;</span><br><span class="line">callback = callback || <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{}; <span class="comment">//ok</span></span><br><span class="line">callback = callback || <span class="function">(<span class="params">(</span>) =></span>{}); <span class="comment">//ok</span></span><br><span class="line"></span><br><span class="line"><span class="comment">//空的箭头函数返回 undefined</span></span><br><span class="line"><span class="keyword">let</span> empty = <span class="function"><span class="params">()</span> =></span> {}</span><br><span class="line"></span><br><span class="line">(<span class="function"><span class="params">()</span> =></span> <span class="string">'foobar'</span>)();</span><br><span class="line"><span class="comment">//Return "foobar"</span></span><br><span class="line"><span class="comment">//这是一个立即执行函数表达式</span></span><br></pre></td></tr></table></figure>
<h2 id="箭头函数也可以使用条件（三元）运算符："><a href="#箭头函数也可以使用条件（三元）运算符：" class="headerlink" title="箭头函数也可以使用条件（三元）运算符："></a>箭头函数也可以使用条件（三元）运算符：</h2><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> simple = <span class="function"><span class="params">a</span> =></span> a > <span class="number">15</span> ? <span class="number">15</span> : a;</span><br><span class="line">simple(<span class="number">16</span>); <span class="comment">//15</span></span><br><span class="line">simple(<span class="number">10</span>); <span class="comment">//10</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">let</span> max = <span class="function">(<span class="params">a,b</span>) =></span> a > b ? a : b;</span><br></pre></td></tr></table></figure>
<h2 id="箭头函数内定义的变量及其作用域"><a href="#箭头函数内定义的变量及其作用域" class="headerlink" title="箭头函数内定义的变量及其作用域"></a>箭头函数内定义的变量及其作用域</h2><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//常规写法</span></span><br><span class="line"><span class="keyword">var</span> greeting = <span class="function"><span class="params">()</span> =></span> { <span class="keyword">let</span> now = <span class="keyword">new</span> <span class="built_in">Date</span>();<span class="keyword">return</span>(<span class="string">"Good"</span> + ((now.getHours() > <span class="number">17</span>) ? <span class="string">"evening."</span> : <span class="string">"day."</span>));}</span><br><span class="line"><span class="built_in">console</span>.log(now); <span class="comment">// ReferenceError: now is not defined 标准的let作用域</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 参数括号内定义的变量是局部变量（默认参数）</span></span><br><span class="line"><span class="keyword">var</span> greeting = <span class="function">(<span class="params">now = <span class="keyword">new</span> <span class="built_in">Date</span>(</span>)) =></span> <span class="string">"Good"</span> + ((now.getHours() > <span class="number">17</span>) ? <span class="string">"evening."</span> : <span class="string">"day."</span>);</span><br><span class="line"><span class="built_in">console</span>.log(now); <span class="comment">// ReferenceError: now is not defined</span></span><br><span class="line"></span><br><span class="line"><span class="comment">//对比：函数体内{} 不使用var 定义的变量是全局变量</span></span><br><span class="line"><span class="keyword">var</span> greeting = <span class="function"><span class="params">()</span> =></span> {now = <span class="keyword">new</span> <span class="built_in">Date</span>(); <span class="keyword">return</span> (<span class="string">"Good"</span> + ((now.getHours() > <span class="number">17</span>) ? <span class="string">"evening."</span> : <span class="string">"day."</span>));}</span><br><span class="line"><span class="built_in">console</span>.log(now); <span class="comment">// Fri Dec 22 2017 10:01:00 GMT+0800 (中国标准时间)</span></span><br><span class="line"></span><br><span class="line"><span class="comment">//对比：函数体内用{} 用var定义的变量是局部变量</span></span><br><span class="line"><span class="keyword">var</span> greeting = <span class="function"><span class="params">()</span> =></span>{<span class="keyword">var</span> now = <span class="keyword">new</span> <span class="built_in">Date</span>(); <span class="keyword">return</span> (<span class="string">"Good"</span> + ((now.getHours() > <span class="number">17</span>) ? <span class="string">"evening."</span> : <span class="string">"day."</span>));}</span><br><span class="line"><span class="built_in">console</span>.log(now); <span class="comment">// ReferenceError: now is not defined</span></span><br></pre></td></tr></table></figure>
<h2 id="箭头函数也可以使用闭包"><a href="#箭头函数也可以使用闭包" class="headerlink" title="箭头函数也可以使用闭包"></a>箭头函数也可以使用闭包</h2><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">//标准的闭包函数</span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">A</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">var</span> i = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">return</span> <span class="function"><span class="keyword">function</span> <span class="title">b</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">return</span> (++i);</span><br><span class="line"> };</span><br><span class="line">};</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> v = A();</span><br><span class="line">v(); <span class="comment">//1</span></span><br><span class="line">v(); <span class="comment">//2</span></span><br><span class="line"></span><br><span class="line"><span class="comment">//箭头函数体的闭包 (i=0 是默认参数)</span></span><br><span class="line"><span class="keyword">var</span> Add = <span class="function">(<span class="params">i=<span class="number">0</span></span>) =></span> {<span class="keyword">return</span> <span class="function">(<span class="params">(</span>) =></span> (++i))};</span><br><span class="line"><span class="keyword">var</span> v = Add();</span><br><span class="line">v(); <span class="comment">//1</span></span><br><span class="line">v(); <span class="comment">//2</span></span><br><span class="line"></span><br><span class="line"><span class="comment">//因为只有一个返回，return 以及括号也可以省略</span></span><br><span class="line"><span class="keyword">var</span> Add = <span class="function">(<span class="params">i=<span class="number">0</span></span>) =></span> (++i);</span><br></pre></td></tr></table></figure>
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<div class="post-toc-content"><ol class="nav"><li class="nav-item nav-level-1"><a class="nav-link" href="#箭头函数"><span class="nav-number">1.</span> <span class="nav-text">箭头函数</span></a><ol class="nav-child"><li class="nav-item nav-level-2"><a class="nav-link" href="#基础语法"><span class="nav-number">1.1.</span> <span class="nav-text">基础语法</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#高级语法"><span class="nav-number">1.2.</span> <span class="nav-text">高级语法</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#描述"><span class="nav-number">1.3.</span> <span class="nav-text">描述</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#不绑定-this"><span class="nav-number">1.4.</span> <span class="nav-text">不绑定 this</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#通过-call-或者-apply-调用"><span class="nav-number">1.5.</span> <span class="nav-text">通过 call 或者 apply 调用</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#不绑定-arguments"><span class="nav-number">1.6.</span> <span class="nav-text">不绑定 arguments</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#像方法一样使用箭头函数"><span class="nav-number">1.7.</span> <span class="nav-text">像方法一样使用箭头函数</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#使用-new-操作符"><span class="nav-number">1.8.</span> <span class="nav-text">使用 new 操作符</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#使用-prototype-属性"><span class="nav-number">1.9.</span> <span class="nav-text">使用 prototype 属性</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#函数体"><span class="nav-number">1.10.</span> <span class="nav-text">函数体</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#返回对象字面量"><span class="nav-number">1.11.</span> <span class="nav-text">返回对象字面量</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#换行"><span class="nav-number">1.12.</span> <span class="nav-text">换行</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#解析顺序"><span class="nav-number">1.13.</span> <span class="nav-text">解析顺序</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#箭头函数也可以使用条件（三元）运算符："><span class="nav-number">1.14.</span> <span class="nav-text">箭头函数也可以使用条件（三元）运算符：</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#箭头函数内定义的变量及其作用域"><span class="nav-number">1.15.</span> <span class="nav-text">箭头函数内定义的变量及其作用域</span></a></li><li class="nav-item nav-level-2"><a class="nav-link" href="#箭头函数也可以使用闭包"><span class="nav-number">1.16.</span> <span class="nav-text">箭头函数也可以使用闭包</span></a></li></ol></li></ol></div>
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if(String(hnum).length ==1 ){hnum = "0" + hnum;} minutes = (now - grt ) / 1000 /60 - (24 * 60 * dnum) - (60 * hnum);
mnum = Math.floor(minutes); if(String(mnum).length ==1 ){mnum = "0" + mnum;}
seconds = (now - grt ) / 1000 - (24 * 60 * 60 * dnum) - (60 * 60 * hnum) - (60 * mnum);
snum = Math.round(seconds);
if(String(snum).length ==1 ){snum = "0" + snum;}
document.getElementById("timeDate").innerHTML = dnum+" 天 ";
document.getElementById("times").innerHTML = hnum + " 小时 " + mnum + " 分 " + snum + " 秒";
}
setInterval("createtime()",250);
</script>
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