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<blockquote class="blockquote-center"><p>JavaScript中的函数运行在它们被定义的作用域里，而不是它们被执行的作用域里。</p>
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<p>在JS中函数作为普通对象进行传递</p>
<hr>
<a id="more"></a>
<h2 id="什么是闭包-属于一种特殊的作用域，称为-静态作用域"><a href="#什么是闭包-属于一种特殊的作用域，称为-静态作用域" class="headerlink" title="什么是闭包?(属于一种特殊的作用域，称为 静态作用域)"></a>什么是闭包?(属于一种特殊的作用域，称为 静态作用域)</h2><p>简单来说，闭包是指可以访问另一个函数作用域变量的函数，一般是定义在外层函数中的内层函数。(函数和声明该函数的词法环境的组合)子函数所在的父函数的作用域不会被释放。</p>
<h2 id="为什么需要闭包？"><a href="#为什么需要闭包？" class="headerlink" title="为什么需要闭包？"></a>为什么需要闭包？</h2><p>使用闭包主要是为了设计私有的变量和方法。闭包的优点可以避免全局变量的污染，让变量始终保存在内存中，缺点是会常驻内存，增大内存的使用
量，使用不当会造成内存泄漏。</p>
<h2 id="特点"><a href="#特点" class="headerlink" title="特点"></a>特点</h2><ol>
<li>定义外层函数，封装被保护的局部变量。 </li>
<li>定义内层函数，执行对外部函数变量的操作。 </li>
<li>外层函数返回内层函数的对象，并且外层函数被调用，结果保存在一个全局的变量中。</li>
</ol>
<h2 id="特性"><a href="#特性" class="headerlink" title="特性"></a>特性</h2><ol>
<li>函数嵌套函数，内层函数被返回。</li>
<li>函数内部可以引用外部的参数和变量，以及外部函数能访问的所有变量和函数。但是，外部函数却不能够访问定义在内部函数中的变量和函数。</li>
<li>参数和变量不会被垃圾回收机制回收。</li>
</ol>
<h2 id="实例"><a href="#实例" class="headerlink" title="实例"></a>实例</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></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> add = (<span class="function"><span class="keyword">function</span> (<span class="params"></span>) </span>{</span><br><span class="line"> <span class="keyword">var</span> counter = <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="params"></span>) </span>{<span class="keyword">return</span> counter += <span class="number">1</span>;}</span><br><span class="line">})();</span><br><span class="line">add();</span><br><span class="line">add();</span><br><span class="line">add();</span><br><span class="line"><span class="comment">// add()调用过后应当销毁其变量，但其内层函数被返回了，并且还保留着对变量的引用，所以没有销毁还保留在内存当中。</span></span><br></pre></td></tr></table></figure>
<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="keyword">var</span> arr=[<span class="string">'one'</span>,<span class="string">'two'</span>,<span class="string">'three'</span>]</span><br><span class="line"><span class="keyword">for</span>(<span class="keyword">var</span> i =<span class="number">0</span>;i<arr.length;i++){</span><br><span class="line">setTimeout(<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(i)</span><br><span class="line">},i*<span class="number">1000</span>)</span><br><span class="line">}</span><br><span class="line"><span class="comment">// 打印3次3 执行setTimeout时for循环已经结束此时的i的值为3</span></span><br></pre></td></tr></table></figure>
<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> arr=[<span class="string">'one'</span>,<span class="string">'two'</span>,<span class="string">'three'</span>]</span><br><span class="line"><span class="keyword">for</span>(<span class="keyword">var</span> i =<span class="number">0</span>;i<arr.length;i++){</span><br><span class="line"><span class="comment">// 匿名闭包 封闭每个变量</span></span><br><span class="line">(<span class="function"><span class="keyword">function</span>(<span class="params">index</span>)</span>{</span><br><span class="line"> setTimeout(<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(arr[index])</span><br><span class="line">},index*<span class="number">1000</span>)</span><br><span class="line">})(i)</span><br><span class="line">} </span><br><span class="line"><span class="comment">// one two three</span></span><br></pre></td></tr></table></figure>
<p>避免使用过多的闭包，可以用let关键词，每个闭包都绑定了块作用域的变量，这意味着不再需要额外的闭包。</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="keyword">var</span> arr=[<span class="string">'one'</span>,<span class="string">'two'</span>,<span class="string">'three'</span>]</span><br><span class="line"><span class="keyword">for</span>(<span class="keyword">let</span> i =<span class="number">0</span>;i<arr.length;i++){</span><br><span class="line">setTimeout(<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(arr[i])</span><br><span class="line">},i*<span class="number">1000</span>)</span><br><span class="line">}</span><br></pre></td></tr></table></figure>
<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="comment">// setTimeout(code, milliseconds, param1, param2, ...)</span></span><br><span class="line"><span class="comment">// setTimeout(function, milliseconds, param1, param2, ...)</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">while</span> (i++ < <span class="number">3</span>) {</span><br><span class="line"> setTimeout(<span class="built_in">console</span>.log, <span class="number">0</span>, i)</span><br><span class="line">}</span><br><span class="line"><span class="comment">// 1 2 3 这个也是闭包</span></span><br></pre></td></tr></table></figure>
<h2 id="经典问题"><a href="#经典问题" class="headerlink" title="经典问题"></a>经典问题</h2><p>多个子函数的[[scope]]都是同时指向父级，是完全共享的。因此当父级的变量对象被修改时，所有子函数都受到影响。</p>
<p>解决:</p>
<ul>
<li>变量可以通过 函数参数的形式 传入，避免使用默认的[[scope]]向上查找</li>
<li>使用setTimeout包裹，通过第三个参数传入</li>
<li>使用 块级作用域，让变量成为自己上下文的属性，避免共享</li>
</ul>
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<p><span>发布时间:</span>2018年06月11日 - 02:06</p>
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