数组的五大迭代方法

forEach();
some();
every();
map();
filter();

第一个参数是遍历的数组内容，第二个参数是对应的数组索引，
第三个参数是数组本身
var arr = [1,2,3,4];
var sum = 0;
arr.forEach(function(value,index,array){
    array[index] == value;    //结果为true
    sum+=value;  
    });
console.log(sum);    //结果为 10

 "use strict";
 var a = [1,2,3,4];
 var b = [1,2,3,4,5,6];
 console.log(a.every(item_a =>{
     return b.some(item_b => {
         return item_a===item_b;});
 }));

var oldArray = [1,2,3,4,5,6,7,8,9,10];
var newArray = oldArray.filter(val=>{
    return val < 6;
});
console.log(newArray);

var oldArray = [1,2,3,4,5];
var newArray = oldArray.map(val=>{
    return val+3;
});
console.log(newArray);