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class="post-meta-date-updated" datetime="2022-03-22T22:48:18.000Z" title="更新于 2022-03-22 22:48:18">2022-03-22</time></span><span class="post-meta-categories"><span class="post-meta-separator">|</span><i class="fas fa-inbox fa-fw post-meta-icon"></i><a class="post-meta-categories" href="/categories/HTML/">HTML</a><i class="fas fa-angle-right post-meta-separator"></i><i class="fas fa-inbox fa-fw post-meta-icon"></i><a class="post-meta-categories" href="/categories/HTML/javscript/">javscript</a></span></div><div class="meta-secondline"><span class="post-meta-separator">|</span><span class="post-meta-wordcount"><i class="far fa-file-word fa-fw post-meta-icon"></i><span class="post-meta-label">字数总计:</span><span class="word-count">401</span><span class="post-meta-separator">|</span><i class="far fa-clock fa-fw post-meta-icon"></i><span class="post-meta-label">阅读时长:</span><span>1分钟</span></span><span class="post-meta-separator">|</span><span class="post-meta-pv-cv" data-flag-title="ES5当中var只有全局作用域和函数作用域,注意变量提升在var中的坑"><i class="far fa-eye fa-fw post-meta-icon"></i><span class="post-meta-label">阅读量:</span><span id="busuanzi_value_page_pv"><i class="fa-solid fa-spinner fa-spin"></i></span></span></div></div></div></header><main class="layout" id="content-inner"><div id="post"><article class="post-content" id="article-container"><h3 id="前置知识"><a href="#前置知识" class="headerlink" title="前置知识"></a>前置知识</h3><ol><li>在ES5当中,变量的作用域只有全局作用域和函数作用域之分的,也就是说,你使用 var 关键字创建的变量,要么是全局都可以使用的,要么就是函数内部可以使用的</li><li>在ES5当中,var定义的变量是会变量提升的</li><li>在ES6当中,let是块级作用域,不会变量提升</li></ol><h3 id="于是乎产生了一次错误"><a href="#于是乎产生了一次错误" class="headerlink" title="于是乎产生了一次错误"></a>于是乎产生了一次错误</h3><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"><script></span><br><span class="line"> <span class="keyword">var</span> data=<span class="string">'动感超人'</span></span><br><span class="line"> <span class="keyword">if</span>(data){</span><br><span class="line"> <span class="keyword">var</span> flag=<span class="literal">true</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="variable language_">console</span>.<span class="title function_">log</span>(flag);<span class="comment">//输出结果为true</span></span><br><span class="line"></script></span><br></pre></td></tr></table></figure><h4 id="这一段代码一看就是true"><a href="#这一段代码一看就是true" class="headerlink" title="这一段代码一看就是true"></a>这一段代码一看就是true</h4><ol><li><p>在js当中,除了 <strong>null</strong> <strong>undefined</strong> <strong>“”</strong> <strong>NaN</strong> <strong>false</strong> 这四个转化为布尔值为false,其他均为true</p></li><li><p>所以data=”动感超人”转化为布尔值为true~所以执行var flag=true;</p></li><li><p>由于var只有全局作用域和函数作用域,所以var flag为<strong>全局变量</strong></p><h4 id="代码改改"><a href="#代码改改" class="headerlink" title="代码改改"></a>代码改改</h4></li></ol><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"><script></span><br><span class="line"> <span class="keyword">var</span> data=<span class="string">''</span></span><br><span class="line"> <span class="keyword">if</span>(data){</span><br><span class="line"> <span class="keyword">var</span> flag=<span class="literal">true</span>;</span><br><span class="line"> }</span><br><span class="line"> <span class="variable language_">console</span>.<span class="title function_">log</span>(flag);</span><br><span class="line"> </script></span><br></pre></td></tr></table></figure><h4 id="这一段代码呢"><a href="#这一段代码呢" class="headerlink" title="这一段代码呢?"></a>这一段代码呢?</h4><p class="note warn-note">结果为undefined</p><p></p><h3 id="分析"><a href="#分析" class="headerlink" title="分析"></a>分析</h3><h4 id="执行流程"><a href="#执行流程" class="headerlink" title="执行流程"></a>执行流程</h4><p>1.js引擎执行js代码前先看看有没有可以变量提升,函数提升),上面代码转换下实际上是这样子在执行的过程中</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"><script></span><br><span class="line"> <span class="keyword">var</span> data;<span class="comment">//变量提升</span></span><br><span class="line"> <span class="keyword">var</span> flag;<span class="comment">//变量提升</span></span><br><span class="line"> data=<span class="string">''</span><span class="comment">//赋值</span></span><br><span class="line"> <span class="keyword">if</span>(data){<span class="comment">//转布尔值为false</span></span><br><span class="line"> flag=<span class="literal">true</span>;<span class="comment">//条件不成立,不执行赋值</span></span><br><span class="line"> }</span><br><span class="line"> <span class="variable language_">console</span>.<span class="title function_">log</span>(flag);<span class="comment">//没有赋值,又为全局变量,不报错,但是输出为undefined</span></span><br><span class="line"> </script></span><br></pre></td></tr></table></figure><h3 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h3><h4 id="施主-要小心-小心-再小心-阿弥陀佛"><a href="#施主-要小心-小心-再小心-阿弥陀佛" class="headerlink" title="施主,要小心,小心,再小心~阿弥陀佛"></a>施主,要小心,小心,再小心~阿弥陀佛</h4><p><img src="https://dreamos.oss-cn-beijing.aliyuncs.com/gitblog/20220316215624.png"></p></article><div class="post-copyright"><div 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class="announcement_content">不断更新中,有问题请留言回复(会通过邮箱提醒~)</div></div><div class="sticky_layout"><div class="card-widget" id="card-toc"><div class="item-headline"><i class="fas fa-stream"></i><span>目录</span><span class="toc-percentage"></span></div><div class="toc-content"><ol class="toc"><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%89%8D%E7%BD%AE%E7%9F%A5%E8%AF%86"><span class="toc-number">1.</span> <span class="toc-text">前置知识</span></a></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E4%BA%8E%E6%98%AF%E4%B9%8E%E4%BA%A7%E7%94%9F%E4%BA%86%E4%B8%80%E6%AC%A1%E9%94%99%E8%AF%AF"><span class="toc-number">2.</span> <span class="toc-text">于是乎产生了一次错误</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#%E8%BF%99%E4%B8%80%E6%AE%B5%E4%BB%A3%E7%A0%81%E4%B8%80%E7%9C%8B%E5%B0%B1%E6%98%AFtrue"><span class="toc-number">2.1.</span> <span class="toc-text">这一段代码一看就是true</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E4%BB%A3%E7%A0%81%E6%94%B9%E6%94%B9"><span class="toc-number">2.2.</span> <span class="toc-text">代码改改</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#%E8%BF%99%E4%B8%80%E6%AE%B5%E4%BB%A3%E7%A0%81%E5%91%A2"><span class="toc-number">2.3.</span> <span class="toc-text">这一段代码呢?</span></a></li></ol></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E5%88%86%E6%9E%90"><span class="toc-number">3.</span> <span class="toc-text">分析</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#%E6%89%A7%E8%A1%8C%E6%B5%81%E7%A8%8B"><span class="toc-number">3.1.</span> <span class="toc-text">执行流程</span></a></li></ol></li><li class="toc-item toc-level-3"><a class="toc-link" href="#%E6%80%BB%E7%BB%93"><span class="toc-number">4.</span> <span class="toc-text">总结</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" 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