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<h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a><strong>前言</strong></h2><p><strong>这在我们平常操作 DOM 的时候司空见惯。就比如：我通过获取一个元素来改变其样式，自然而然的反映到 HTML 页面中。</strong></p>
<p><strong>但是，我们操作 JS 对象的时候，本质上操作的是 JS 堆内存，为什么会反映到页面中呢？就是因为浏览器存在这个 DOM 的映射机制。</strong></p>
<h2 id="1-什么是-DOM-的映射机制？"><a href="#1-什么是-DOM-的映射机制？" class="headerlink" title="1. 什么是 DOM 的映射机制？"></a>1. 什么是 DOM 的映射机制？</h2><p><strong>我们使用 JS 从页面获取到的元素对象，或者自己手动创建的已经插入页面的元素对象，与页面中的 HTML 元素是绑定在一起的。也就是说修改其中一个，另一个也会跟着自动修改。这就是 DOM 的映射机制。</strong></p>
<h2 id="2-形成映射的几种情形"><a href="#2-形成映射的几种情形" class="headerlink" title="2. 形成映射的几种情形"></a>2. 形成映射的几种情形</h2><h3 id="改变元素对象的属性"><a href="#改变元素对象的属性" class="headerlink" title="改变元素对象的属性"></a><strong>改变元素对象的属性</strong></h3><p><strong>这是我们最常用到的一种情形。当我们需要为元素添加自定义属性、或者修改属性等，就可以从页面中获取到元素对象，然后对其进行修改，就能够自动反映到 HTML 页面元素上。</strong></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="comment">//=> 修改从页面中获取的元素样式</span></span><br><span class="line">Div.style.color = <span class="string">'red'</span>;</span><br><span class="line"></span><br><span class="line"><span class="comment">//=> 修改已经插入页面的元素的属性</span></span><br><span class="line"><span class="keyword">var</span> p = <span class="built_in">document</span>.createElement(<span class="string">'p'</span>);</span><br><span class="line">box.appendChild(p);</span><br><span class="line">p.dataset.index = <span class="number">1</span>;</span><br></pre></td></tr></table></figure>
<p><strong>这两种方式得到的元素对象，修改其属性，都能够直接反映到页面中，不需要再次插入页面中。</strong></p>
<h2 id="在元素内部继续添加元素"><a href="#在元素内部继续添加元素" class="headerlink" title="在元素内部继续添加元素"></a>在元素内部继续添加元素</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="comment">//=> 在其内部插入标签或文本</span></span><br><span class="line"><span class="keyword">var</span> list = Div.getElementsByTagName(<span class="string">'li'</span>);</span><br><span class="line"><span class="built_in">console</span>.log(list); <span class="comment">// 空的元素集合</span></span><br><span class="line">Div.innerHTML(<span class="string">'<li></li>'</span>);</span><br><span class="line"><span class="built_in">console</span>.log(list); <span class="comment">// 有一个元素集合</span></span><br><span class="line"></span><br><span class="line"><span class="comment">//=> 添加自己创建的元素对象，同样原理</span></span><br><span class="line">Div.appendChild(p);</span><br></pre></td></tr></table></figure>
<p><strong>在容器中的数据绑定前，我们获取容器中元素，得到一个空的元素集合，容器数据绑定后，我们不需要重新获取，DOM 的映射机制会帮我们把新增加的元素映射到之前获取的空集合中，让其变为有元素的集合。</strong></p>
<h2 id="在页面中追加已有元素"><a href="#在页面中追加已有元素" class="headerlink" title="在页面中追加已有元素"></a>在页面中追加已有元素</h2><figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">list = Div.getElementsByTagName(<span class="string">'li'</span>)[<span class="number">0</span>];</span><br><span class="line">Div.appendChild(list);</span><br></pre></td></tr></table></figure>
<p><code>appendChild</code> <strong>在追加元素对象的时候，如果这个元素在容器中已经存在，此时并不是克隆一份新的追加到末尾，而是把原有的元素移动到末尾。</strong></p>
<blockquote>
<p>其根本原因在于，同一个元素在页面中，只能够有一个位置。把 JS 元素对象插入页面中某个位置，实际上就是把其绑定的 HTML 元素移动到那个位置上。</p>
</blockquote>
<p><strong>这里的元素已经存在有两种情形：</strong></p>
<ul>
<li><strong>元素是从页面中获取到的</strong></li>
<li><strong>创建的元素已经添加过一次，再次添加时</strong><br><strong>因此，就无需手动移除原先的元素，再进行添加。直接插入即可。</strong></li>
</ul>
<h2 id="3-特殊情况"><a href="#3-特殊情况" class="headerlink" title="3. 特殊情况"></a><strong>3. 特殊情况</strong></h2><p><strong><code>querySelectAll</code> 获取的集合是静态集合（staticNodeList），不存在上述所谓的映射机制，基于这个方法，数据绑定完成后需要重新获取一次才可以。</strong><br><figure class="highlight js"><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></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> box = <span class="built_in">document</span>.querySelectorAll(<span class="string">'#box'</span>);</span><br><span class="line"><span class="keyword">var</span> box1 = <span class="built_in">document</span>.getElementById(<span class="string">'box'</span>);</span><br><span class="line"><span class="built_in">console</span>.log(box); <span class="comment">//=> 获取到的是 NodeList 对象</span></span><br><span class="line"><span class="built_in">console</span>.dir(box1); <span class="comment">//=> 而这里获取到的是 HTMLElement 的实例</span></span><br></pre></td></tr></table></figure></p>
<p><strong>这个特殊的 NodeList 不存在与 HTML 页面元素的映射，而且没有很多 HTMLElement 实例才拥有的方法。因此，使用这个方法的时候每次操作时，都应该重新获取。</strong></p>
<p><strong>另外，jQuery 中获取的元素同样不存在这些映射，实现映射需要使用其内部的方法，而且要使用原生 HTMLElement 实例的方法，需要通过后面加 <code>[0]</code> 的方法转换为原生元素对象。</strong></p>
<p><strong>另外，由于存在这种映射机制，在使用循环 DOM 中，改变了 DOM 的顺序，那么循环原本的顺序也就改变了。</strong><br><figure class="highlight js"><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></pre></td><td class="code"><pre><span class="line">[].forEach.call(children, (item)=> {</span><br><span class="line"> parent.appendChild(item);</span><br><span class="line">});</span><br><span class="line"><span class="comment">//=> 这样做你会发现，循环完成后 children 改变了，不符合预期</span></span><br></pre></td></tr></table></figure></p>
<p><strong>因此，真实项目中，不应该过分依赖于 DOM 的映射机制。</strong></p>
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