usage of some important javascript functions

Array

1. Array.reduce
 
 有初始值：
 If an initialValue was provided in the call to reduce, then previousValue will be equal to initialValue 
 and currentValue will be equal to the first value in the array. 
 没有初始值：
 If no initialValue was provided, then previousValue will be equal to the first value in the array 
 and currentValue will be equal to the second
 
 一般数值的用法：
         numbers.reduce((acc, item)=>acc+item, init_val) reducer函数(acc, item)=>acc+item返回的值类型也是数字型的
 一般函数的用法：
         funcs.reduce((a, b)=>(...args)=>a(b(...args))  reducer函数(a, b)=>(...args)=>a(b(...args)返回的值类型也是函数类型的