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  ES6笔记（解构赋值） - kaelinda

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          <h1>ES6笔记（解构赋值）</h1>

        <div class="read-more clearfix">
          <span class="date">2017/12/20</span>

          <span>posted in&nbsp;</span>

              <span class="posted-in"><a href='js.html'>JavaScript</a></span>


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      <h2 id="toc_0">解构赋值</h2>

<h3 id="toc_1">变量的解构：</h3>

<ul>
<li>解构赋值允许指定默认值。</li>
<li>解构赋值必须保证赋值方  被赋值方类型一直  但Item允许有遗漏或者多出。</li>
<li>默认值可以引用解构赋值的其他变量，但该变量必须已经声明</li>
</ul>

<pre><code class="language-text">let [x = 1, y = x] = [];     // x=1; y=1
let [x = y, y = 1] = [];     // ReferenceError
</code></pre>

<span id="more"></span><!-- more -->

<h3 id="toc_2">对象的 解构赋值：</h3>

<ul>
<li>对象的解构与数组有一个重要的不同。数组的元素是按次序排列的，变量的取值由它的位置决定；</li>
<li><p>而对象的属性没有次序，变量必须与属性同名，才能取到正确的值<br/>
（数据库、序列化）</p></li>
<li><p>对象的解构赋值的内部机制，是先找到同名属性，然后再赋给对应的变量。真正被赋值的是后者，而不是前者；前者是key,后者才是变量！</p>
<blockquote>
<p>注意，采用这种写法时，变量的声明和赋值是一体的。对于let和const来说，变量不能重新声明，所以一旦赋值的变量以前声明过，就会报错。</p>
</blockquote></li>
</ul>

<h3 id="toc_3">字符串的解构赋值</h3>

<blockquote>
<p>字符串也可以解构赋值。<br/>
这是因为此时，字符串被转换成了一个类似数组的对象<br/>
类似数组的对象都有一个length属性，因此还可以对这个属性解构赋值。</p>
</blockquote>

<pre><code class="language-javascript">let {length : len} = &#39;hello&#39;;
len // 5

let {toString: s} = 123;
s === Number.prototype.toString // true

let {toString: s} = true;
s === Boolean.prototype.toString // true

let { prop: x } = undefined; // TypeError
let { prop: y } = null; // TypeError
</code></pre>

<p>解构赋值时，如果等号右边是数值和布尔值，则会先转为对象。<br/>
undefined和null无法转为对象，所以对它们进行解构赋值，都会报错。</p>

<ul>
<li>不能使用圆括号的情况
<ul>
<li>变量声明语句中，不能带有圆括号</li>
<li>函数参数中，模式不能带有圆括号。</li>
<li>赋值语句中，不能将整个模式，或嵌套模式中的一层，放在圆括号之中</li>
</ul></li>
<li>可以使用圆括号的情况只有一种：赋值语句的<strong>非模式部分</strong>，可以使用圆括号。</li>
</ul>


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