无涯教程-Javascript - reduce()函数

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JavaScript array.reduce()方法对数组的两个值(从左到右)计算函数。

reduce() - 语法

array.reduce(callback[, initialValue]);

  • callback       - 要对数组中的每个值执行的回调函数。

  • initialValue - 用作回调的第一个调用的第一个参数的对象。

reduce() - 返回值

返回数组的缩减单值。

reduce() - 相容性

此方法是ECMA-262标准的JavaScript扩展；因此，它可能不存在于该标准的其他实现中。要使其工作，您需要在脚本顶部添加以下代码。

if (!Array.prototype.reduce) {
   Array.prototype.reduce=function(fun /*, initial*/) {
      var len=this.length;

  </span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(</span><span class="kwd">typeof</span><span class="pln"> fun </span><span class="pun">!=</span><span class="pln"> </span><span class="str">"function"</span><span class="pun">)</span><span class="pln">
  </span><span class="kwd">throw</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">TypeError</span><span class="pun">();</span><span class="pln">
  
  </span><span class="com">//no value to return if no initial value and an empty array</span><span class="pln">
  </span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(</span><span class="pln">len </span><span class="pun">==</span><span class="pln"> </span><span class="lit">0</span><span class="pln"> </span><span class="pun">&amp;&amp;</span><span class="pln"> arguments</span><span class="pun">.</span><span class="pln">length </span><span class="pun">==</span><span class="pln"> </span><span class="lit">1</span><span class="pun">)</span><span class="pln">
  </span><span class="kwd">throw</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">TypeError</span><span class="pun">();</span><span class="pln">
  
  </span><span class="kwd">var</span><span class="pln"> i</span><span class="pun">=</span><span class="lit">0</span><span class="pun">;</span><span class="pln">
  </span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(</span><span class="pln">arguments</span><span class="pun">.</span><span class="pln">length </span><span class="pun">&gt;=</span><span class="pln"> </span><span class="lit">2</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
     </span><span class="kwd">var</span><span class="pln"> rv</span><span class="pun">=</span><span class="pln">arguments</span><span class="pun">[</span><span class="lit">1</span><span class="pun">];</span><span class="pln">
  </span><span class="pun">}</span><span class="pln"> </span><span class="kwd">else</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
     </span><span class="kwd">do</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
        </span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(</span><span class="pln">i </span><span class="kwd">in</span><span class="pln"> </span><span class="kwd">this</span><span class="pun">)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
           rv</span><span class="pun">=</span><span class="kwd">this</span><span class="pun">[</span><span class="pln">i</span><span class="pun">++];</span><span class="pln">
           </span><span class="kwd">break</span><span class="pun">;</span><span class="pln">
        </span><span class="pun">}</span><span class="pln">
        
        </span><span class="com">//if array contains no values, no initial value to return</span><span class="pln">
        </span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(++</span><span class="pln">i </span><span class="pun">&gt;=</span><span class="pln"> len</span><span class="pun">)</span><span class="pln">
        </span><span class="kwd">throw</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="typ">TypeError</span><span class="pun">();</span><span class="pln">
     </span><span class="pun">}</span><span class="pln">
     </span><span class="kwd">while</span><span class="pln"> </span><span class="pun">(</span><span class="kwd">true</span><span class="pun">);</span><span class="pln">
  </span><span class="pun">}</span><span class="pln">
  </span><span class="kwd">for</span><span class="pln"> </span><span class="pun">(;</span><span class="pln"> i </span><span class="pun">&lt;</span><span class="pln"> len</span><span class="pun">;</span><span class="pln"> i</span><span class="pun">++)</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
     </span><span class="kwd">if</span><span class="pln"> </span><span class="pun">(</span><span class="pln">i </span><span class="kwd">in</span><span class="pln"> </span><span class="kwd">this</span><span class="pun">)</span><span class="pln">
     rv</span><span class="pun">=</span><span class="pln">fun</span><span class="pun">.</span><span class="pln">call</span><span class="pun">(</span><span class="kwd">null</span><span class="pun">,</span><span class="pln"> rv</span><span class="pun">,</span><span class="pln"> </span><span class="kwd">this</span><span class="pun">[</span><span class="pln">i</span><span class="pun">],</span><span class="pln"> i</span><span class="pun">,</span><span class="pln"> </span><span class="kwd">this</span><span class="pun">);</span><span class="pln">
  </span><span class="pun">}</span><span class="pln">
  </span><span class="kwd">return</span><span class="pln"> rv</span><span class="pun">;</span><span class="pln">



};
}

reduce() - 示例

<html>
   <head>
      <title>JavaScript Array reduce Method</title>
   </head>
   
   <body>   
      <script type="text/javascript">
         if (!Array.prototype.reduce) {
            Array.prototype.reduce=function(fun /*, initial*/) {
               var len=this.length;
               
               if (typeof fun != "function")
               throw new TypeError();
               
               //no value to return if no initial value and an empty array
               if (len == 0 && arguments.length == 1)
               throw new TypeError();
               
               var i=0;
               if (arguments.length >= 2) {
                  var rv=arguments[1];
               } else {
                  do {
                     if (i in this) {
                        rv=this[i++];
                        break;
                     }
                     
                     //if array contains no values, no initial value to return
                     if (++i >= len)
                     throw new TypeError();
                  }
                  while (true);
               }
               for (; i < len; i++) {
                  if (i in this)
                  rv=fun.call(null, rv, this[i], i, this);
               }
               return rv;
            };
         }
         var total=[0, 1, 2, 3].reduce(function(a, b){ return a + b; });
         document.write("total is : " + total ); 
      </script>      
   </body>
</html>

运行上面代码输出

total is : 6

参考链接

www.learnfk.com/javascript/…

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