《JS权威指南学习总结--8.8 函数式编程和8.8.1使用函数处理数组》

内容要点：

   和Lisp、Haskell不同，JS并非函数式编程语言，但在JS中可以像操控对象一样操控函数，

  也就是说可以在JS中应用函数式编程技术。ES5中的数组方法（诸如map()和reduce()）就可以非常适用于函数式编程风格。

一.使用函数处理数组

   假设有一个数组，数组元素都是数字，我们想要计算这些元素的平均值和标准差。

   若用非函数式编程风格的话，代码是这样：

        var data = [1,1,3,5,5]; //这里是待处理的数组

       //平均数是所有元素的累加和值除以元素个数

        var total = 0;

        for(var i=0;i<data.length;i++) total +=data[i];

        var mean = total/data.length; //平均数是3

      //计算标准差，首先计算每个数据减去平均数之后偏差的平方然后求和

        total = 0;

        for(var i=0;i<data.length;i++){

             var deviation = data[i] - mean;

            total +=deviation*deviation;

         }

        var stddev = Math.sqrt(total/(data.length-1)); //标准差的值是2

      可以使用数组方法map()和reduce()来实现同样的计算，

          //首先定义两个简单的函数

           var sum = function(x,y){ return x+y; };

           var square = function(x){ return x*x; };

         //然后将这些函数和数组方法配合使用计算平均数和标准差

           var data = [1,1,3,5,5];

           var mean = data.reduce(sum)/data.length;

           var deviations = data.map(function(x){ return x-mean; });

           var stddev = Math.sqrt(deviations.map(square).reduce(sum)/(data.length-1));

           console.log(mean); //3
           console.log(deviations); //[-2,-2,0,2,2]
           console.log(deviations.map(square));//[4, 4, 0, 4, 4]
           console.log(deviations.map(square).reduce(sum)); //16
           console.log(stddev); //2

         ES3并不包含这些数组方法，如果不存在内置方法的话我们可以自定义map()和reduce()函数：

            //对于每个数组元素调用函数f()，并返回一个结果数组。如果Array.prototype.map定义了的话，就使用这个方法。

              var map = Array.prototype.map

                             ?  function(a,f){ return a.map(f); }

                             :   function(a,f){

                                     var result = [];

                                      for(var i =0,len=a.length;i<len;i++){

                                          if(i in a) results[i] = f.call(null,a[i],i,a);

                                 }

                                   return result;

                              };

               //使用函数f()和可选的初始值将数组a减至一个值，如果Array.prototype.reduce存在的话，就使用这个方法

                 var reduce = Array.prototype.reduce

                                    ? function(a,f,initial){   //如果reduce()方法存在的话

                                    if(arguments.length>2)

                                           return a.reduce(f,initial); //如果传入了一个初始值

                                           else return a.reduce(f); //否则没有初始值 

                                         }

                                      ：function(a,f,initial){   //这个算法来自ES5规范

                                          var i =0,len = a.length,accumulator;

                                          //以特定的初始值开始，否则第一个值取自a

                                           if(arguments.length>2) accumulator = initial;

                                           else{

                                                  if(len ==0) throw TypeError();

                                                   while(i<len){

                                                          if(i in a){

                                                                accumulator = a[i++];

                                                                 break;

                                                             }

                                                            else i++;

                                                    }

                                                   if(i==len)  throw TypeError();

                                             }

                                        //对于数组中剩下的元素依次调用f()

                                        while(i<len){

                                             if(i in a)  accumulator = f.call(undefined,accumulator,a[i],i,a);

                                             i++;                        

                                          }

                                          return accumulator;

                                 };

      使用定义的map()和reduce()函数，计算平均值和标准差的代码：

            var data = [1,1,3,5,5];

            var sum = function(x,y){ return x+y; };

            var square = function(x,y){ return x*y; };       

            var mean = reduce(data,sum)/data.length;

            var deviations = map(data,function(x){ return x-mean; });      

            var  stddev = Math.sqrt(reduce(map(deviations,square),sum)/(data.length-1));