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<ol>
<li>设置函数默认值 </li>
<li>结合解构赋值默认值使用</li>
<li>利用rest(…变量名)传入任意参数</li>
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<h2 id="…rest和扩展运算符"><a href="#…rest和扩展运算符" class="headerlink" title="…rest和扩展运算符"></a>…rest和扩展运算符</h2><p>rest参数（…变量名），用于获取函数的多余参数，rest参数搭配的变量是一个数组，该变量将多余的参数放入其中。
<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></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">...y</span>)</span>{</span><br><span class="line"> <span class="built_in">console</span>.log(y) <span class="comment">// [1,2,3,4]</span></span><br><span class="line">}</span><br><span class="line">foo(<span class="number">1</span>,<span class="number">2</span>,<span class="number">3</span>,<span class="number">4</span>)</span><br></pre></td></tr></table></figure></p>
<h2 id="利用扩展运算符（…数组）替代数组的apply方法"><a href="#利用扩展运算符（…数组）替代数组的apply方法" class="headerlink" title="利用扩展运算符（…数组）替代数组的apply方法"></a>利用扩展运算符（…数组）替代数组的apply方法</h2><p>在es5中，需要用apply将数组转化为函数参数，在es6中就不需要这种方式了，可以使用…来代替
<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">f</span>(<span class="params">x, y, z</span>) </span>{</span><br><span class="line"> <span class="built_in">console</span>.log(x + <span class="string">' '</span> + y + <span class="string">' '</span> + z);</span><br><span class="line">}</span><br><span class="line"><span class="keyword">var</span> args = [<span class="number">1</span>, <span class="number">2</span>, <span class="number">3</span>];</span><br><span class="line"><span class="comment">// f.apply(null, args)</span></span><br><span class="line">f(...args)</span><br></pre></td></tr></table></figure></p>
<p>数组push的例子：
<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> arr1 = [<span class="number">0</span>, <span class="number">1</span>, <span class="number">2</span>]</span><br><span class="line"><span class="keyword">var</span> arr2 = [<span class="number">3</span>, <span class="number">4</span>, <span class="number">5</span>]</span><br><span class="line"><span class="keyword">var</span> arr3 = arr1.concat(arr2)</span><br><span class="line"><span class="built_in">console</span>.log(arr3) <span class="comment">//[ 0, 1, 2, 3, 4, 5 ]</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> arr4 = arr1.push(arr2)</span><br><span class="line"><span class="built_in">console</span>.log(arr1) <span class="comment">//[ 0, 1, 2, [ 3, 4, 5 ] ]</span></span><br></pre></td></tr></table></figure></p>
<p>如果想使用arr1.push方法的话，就需要用apply传arr2了
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> arr4 = <span class="built_in">Array</span>.prototype.push.apply(arr1, arr2) <span class="comment">//[ 0, 1, 2, 3, 4, 5 ]</span></span><br></pre></td></tr></table></figure></p>
<p>如果使用扩展运算符的话，就简单一些了
<figure class="highlight javascript"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> arr4 = arr1.push(...arr2)</span><br></pre></td></tr></table></figure></p>
<h2 id="name属性"><a href="#name属性" class="headerlink" title="name属性"></a>name属性</h2><p>函数的name属性可以返回函数名
<figure class="highlight plain"><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">function abc(){</span><br><span class="line"> console.log(abc.name)</span><br><span class="line">}</span><br><span class="line">abc() // 'abc'</span><br></pre></td></tr></table></figure></p>
<h2 id="函数的静态变量和函数"><a href="#函数的静态变量和函数" class="headerlink" title="函数的静态变量和函数"></a>函数的静态变量和函数</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><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">Box</span>(<span class="params"></span>)</span>{}</span><br><span class="line"></span><br><span class="line">Box.num = <span class="number">12</span>; <span class="comment">//静态变量</span></span><br><span class="line">Box.fn = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{}; <span class="comment">//静态函数</span></span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.log(Box.num); <span class="comment">//12</span></span><br><span class="line"><span class="built_in">console</span>.log(Box.fn); <span class="comment">//function(){}</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> Box.fn); <span class="comment">//function</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> t = <span class="keyword">new</span> Box();</span><br><span class="line"><span class="built_in">console</span>.log(t.num); <span class="comment">//undefined</span></span><br><span class="line"><span class="built_in">console</span>.log(t.fn); <span class="comment">//undefined</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> t.fn);<span class="comment">//undefined</span></span><br></pre></td></tr></table></figure>
<p>静态变量和静态函数是Box对象的属性和方法，不属于实例。</p>
<h2 id="函数的实例函数和变量"><a href="#函数的实例函数和变量" class="headerlink" title="函数的实例函数和变量"></a>函数的实例函数和变量</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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">Box</span>(<span class="params"></span>)</span>{</span><br><span class="line"> <span class="keyword">this</span>.a = []; <span class="comment">//实例变量</span></span><br><span class="line"> <span class="keyword">this</span>.fn = <span class="function"><span class="keyword">function</span>(<span class="params"></span>)</span>{}; <span class="comment">//实例方法</span></span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.log(Box.a); <span class="comment">//undefined</span></span><br><span class="line"><span class="built_in">console</span>.log(Box.fn); <span class="comment">//undefined</span></span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> Box.fn); <span class="comment">//undefined</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> t = <span class="keyword">new</span> Box();</span><br><span class="line"><span class="keyword">var</span> t2 = <span class="keyword">new</span> Box();</span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.log(t.a); <span class="comment">//[]</span></span><br><span class="line"><span class="built_in">console</span>.log(t2.a); <span class="comment">//[]</span></span><br><span class="line">t.a.push(<span class="number">1</span>); <span class="comment">// t.a [1]</span></span><br><span class="line"><span class="built_in">console</span>.log(t2.a); <span class="comment">//[]</span></span><br><span class="line"></span><br><span class="line"><span class="built_in">console</span>.log(<span class="keyword">typeof</span> t.fn);</span><br></pre></td></tr></table></figure>
<p>每个实例都有一套实例属性和实例方法，互不影响。</p>
<p>原型上的属性和方法，是实例共用的。</p>
<h2 id="函数传参"><a href="#函数传参" class="headerlink" title="函数传参"></a>函数传参</h2><h3 id="基本类型-基本类型的变量复制"><a href="#基本类型-基本类型的变量复制" class="headerlink" title="基本类型 (基本类型的变量复制)"></a>基本类型 (基本类型的变量复制)</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><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> count = <span class="number">10</span>;</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">num</span>(<span class="params">num1</span>)</span>{</span><br><span class="line"> num1 = <span class="number">1</span>;</span><br><span class="line"> <span class="keyword">return</span> num1;</span><br><span class="line">}</span><br><span class="line"><span class="keyword">var</span> result = num(count);</span><br><span class="line"><span class="built_in">console</span>.log(result);<span class="comment">//1</span></span><br><span class="line"><span class="built_in">console</span>.log(count);<span class="comment">//10，并未变成1</span></span><br></pre></td></tr></table></figure>
<h3 id="引用类型"><a href="#引用类型" class="headerlink" title="引用类型"></a>引用类型</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><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> person = {</span><br><span class="line"> name : <span class="string">"Tom"</span></span><br><span class="line">};</span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">obj</span>(<span class="params">peo</span>)</span>{</span><br><span class="line"> peo.name = <span class="string">"Jerry"</span>;</span><br><span class="line"> <span class="keyword">return</span> peo;</span><br><span class="line">}</span><br><span class="line"><span class="keyword">var</span> result = obj(person);</span><br><span class="line"><span class="built_in">console</span>.log(result.name);<span class="comment">// Jerry</span></span><br><span class="line"><span class="built_in">console</span>.log(person.name);<span class="comment">// Jerry</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><span class="line">11</span><br><span class="line">12</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">var</span> person = {</span><br><span class="line"> name : <span class="string">"Tom"</span></span><br><span class="line">}; </span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">obj</span>(<span class="params">peo</span>)</span>{</span><br><span class="line"> peo = {</span><br><span class="line"> name : <span class="string">"Jerry"</span></span><br><span class="line"> };</span><br><span class="line"> <span class="keyword">return</span> peo;</span><br><span class="line">}</span><br><span class="line"><span class="keyword">var</span> result = obj(person);</span><br><span class="line"><span class="built_in">console</span>.log(result.name);<span class="comment">// Jerry</span></span><br><span class="line"><span class="built_in">console</span>.log(person.name);<span class="comment">// Tom</span></span><br></pre></td></tr></table></figure>
<p>person传递给函数中的peo，但在函数内部peo又指向了一个新对象，所以result.name是新对象的值，person还是指向原对象，所以并没有改变。</p>
<p>ECMAScript中所有函数的参数都是按值传递的。 ——《JS高程》</p>
<p>我们可以把ECMAScript函数的参数想象成局部变量，在向参数传递基本类型的值时，被传递的值被复制给一个局部变量。</p>
<p>在向函数传递引用类型时，会把这个值在内存中的地址复制给一个局部变量，因此这个局部变量的变化会反映在函数的外部。</p>
<p>即使在函数内部修改了参数的值，但原始的引用仍然保持未变。</p>
<h2 id="尾调用优化"><a href="#尾调用优化" class="headerlink" title="尾调用优化"></a>尾调用优化</h2><h3 id="介绍"><a href="#介绍" class="headerlink" title="介绍"></a>介绍</h3><p>尾调用指某个函数的最后一步是调用另一个函数。</p>
<h3 id="demo"><a href="#demo" class="headerlink" title="demo"></a>demo</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><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></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">f</span>(<span class="params">x</span>) </span>{</span><br><span class="line"> <span class="keyword">let</span> y = g(x);</span><br><span class="line"> <span class="keyword">return</span> y</span><br><span class="line">}<span class="comment">//不是尾调用，因为最后一步返回了y，不是调用函数</span></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">f</span>(<span class="params">x</span>) </span>{</span><br><span class="line"> <span class="keyword">return</span> g(x) + <span class="number">1</span></span><br><span class="line">}<span class="comment">//不是尾调用，因为最后一步是加一，不是调用函数</span></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">f</span>(<span class="params">x</span>) </span>{</span><br><span class="line"> g(x)</span><br><span class="line">}<span class="comment">//不是尾调用，因为函数的最后一步是一个默认的return undefined；</span></span><br><span class="line"></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">f</span>(<span class="params">x</span>) </span>{</span><br><span class="line"> <span class="keyword">return</span> g(x)</span><br><span class="line">}<span class="comment">//是尾调用</span></span><br></pre></td></tr></table></figure>
<h3 id="为什么尾调用"><a href="#为什么尾调用" class="headerlink" title="为什么尾调用"></a>为什么尾调用</h3><p>函数调用会在内存形成一个“调用记录”，又称“调用帧”。每形成一个调用帧就会占用一定的内存，假如有一个函数A里面调用了函数B，函数B里面又调用了函数C，以此类推。</p>
<p>在不是尾调用的情况下，因为js是单线程同步的，所以只有当函数B执行完毕才会执行函数A的return语句（函数没写return会有一个默认的return undefiend；），此时函数B的调用帧才会消失。但事实上，只有当函数B执行到return语句时才会执行完毕，而在函数B执行return之前会先调用函数C，所以当这种函数调用越来越多的时候，形成的调用帧也会越来越多占用的内存就会越来越大。俗话说精满自溢，内存也会有溢出的时候是吧，这就不行了！所以要用尾调用！</p>
<p>尾调用由于是函数的最后一步操作，所以不需要保留外层函数的调用帧，只要直接用内层函数的调用帧，取代外层函数的调用帧就可以了。</p>
<p>比如在执行递归的时候就可以利用尾调用达到优化内存的目的。函数调用自身，称为递归。如果尾调用自身，就称为尾递归。</p>
<p>递归非常耗费内存，因为需要同时保存成千上百个调用帧，很容易发生“栈溢出”。但对于尾递归来说，由于只存在一个调用帧，所以永远不会发生“栈溢出”错误。</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></pre></td><td class="code"><pre><span class="line"><span class="comment">// 递归 计算n的阶乘，最多需要保存n个调用记录，复杂度 O(n) </span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">factorial</span>(<span class="params">n</span>) </span>{</span><br><span class="line"> <span class="keyword">if</span> (n === <span class="number">1</span>) <span class="keyword">return</span> <span class="number">1</span>;</span><br><span class="line"> <span class="keyword">return</span> n * factorial(n - <span class="number">1</span>)</span><br><span class="line">}</span><br><span class="line">factorial(<span class="number">5</span>) <span class="comment">//120</span></span><br><span class="line"></span><br><span class="line"><span class="comment">// 改成尾递归，只保留一个调用记录，复杂度 O(1) </span></span><br><span class="line"><span class="function"><span class="keyword">function</span> <span class="title">factorial</span>(<span class="params">n, total</span>) </span>{</span><br><span class="line"> <span class="keyword">if</span> (n === <span class="number">1</span>) <span class="keyword">return</span> total;</span><br><span class="line"> <span class="keyword">return</span> factorial(n - <span class="number">1</span>, n * total)</span><br><span class="line">}</span><br><span class="line">factorial(<span class="number">5</span>, <span class="number">1</span>) <span class="comment">//120</span></span><br></pre></td></tr></table></figure>
<h2 id="函数声明覆盖"><a href="#函数声明覆盖" class="headerlink" title="函数声明覆盖"></a>函数声明覆盖</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></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="built_in">console</span>.log(<span class="string">"foo"</span>);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="keyword">var</span> foo = <span class="number">1</span>;</span><br><span class="line"><span class="comment">// 1 如果foo不赋值 则打印函数</span></span><br></pre></td></tr></table></figure>
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<p><span>本文标题:</span><a href="/es6function.html">es6函数新特性</a></p>
<p><span>文章作者:</span><a href="/" title="访问 陈宇（cosyer） 的个人博客">陈宇（cosyer）</a></p>
<p><span>发布时间:</span>2018年06月08日 - 19:06</p>
<p><span>最后更新:</span>2019年01月20日 - 17:01</p>
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