push,unshift(当前数组改变，返回值为添加完数组的长度)

        var arr = [1,2,3]
        var arr1 = arr.push(4)
        console.log('arr',arr)//[1,2,3,4]
        console.log('arr1',arr1) // 4

        var arr = [1,2,3]
        var arr1 = arr.unshift(0)
        console.log('arr',arr)//[0,1,2,3]
        console.log('arr1',arr1) // 4

pop,shift(当前数组改变，返回值为被删除元素)

        var arr = [1,2,3]
        var arr1 = arr.pop()
        console.log('arr',arr)//[1,2]
        console.log('arr1',arr1) // 3

        var arr = [1,2,3]
        var arr1 = arr.shift()
        console.log('arr',arr)//[2,3]
        console.log('arr1',arr1) // 1

splice(i,n)(当前数组改变，返回值为被删除元素)

删除从下标i开始的n个元素

        var arr = [1,2,3,4,5]
        var arr1 = arr.splice(1,2)
        console.log('arr',arr)//[1,4,5]
        console.log('arr1',arr1) // [2，3]

concat(原数组不变，返回新数组)

    var arr = [1,2,3,4,5]
    var arr1  = [9,0]
    arr2 = arr.concat(arr1)
    console.log('arr',arr)// [1,2,3,4,5]
    console.log('arr1',arr1) //[9,0]
    console.log('arr2',arr2)//[1,2,3,4,5,9,0]

sort(原数组和新数组都改变，且按照字符编码，第一个数字排序)

    var arr = [1,20,30,104,105]
    var arr1 = arr.sort()
    console.log('arr',arr) // [1, 104, 105, 20, 30]
    console.log('arr1',arr1)//[1, 104, 105, 20, 30]

reverse(原数组和新数组都改变)

   var arr = [1,20,30,104,105]
    var arr1 = arr.reverse()
    console.log('arr',arr) // [105, 104, 30, 20, 1]
    console.log('arr1',arr1) // [105, 104, 30, 20, 1]

slice(start,end) (原数组不变，返回截取出来的数组)

(删除从下标start到end的元素)
var arr = [1,2,3,4,5]
var arr1 = arr.slice(2,3)
console.log('arr',arr)// [1,2,3,4,5]
console.log('arr1',arr1)//[3]

map(原数组不变，新数组为计算后的数组)

      var arr = [1,2,3,4,5]
        var arr1 = arr.map(a => {
           return a+1
        }
      )
        console.log('arr',arr) //[1,2,3,4,5]
        console.log('arr1',arr1)//[2,3,4,5,6]

filter(原数组不变，新数组为计算后的数组)

        var arr = [1,2,3,4,5]
        var arr1 = arr.filter(a => {
           return a>2
        })
        console.log('arr',arr) //[1,2,3,4,5]
        console.log('arr1',arr1) //[3,4,5]

forEach(原数组不变，没有返回值)

        var arr = [1,2,3,4,5]
        var arr1 = arr.forEach((a,i) => {
           a = 0
        })
        console.log('arr',arr) //[1,2,3,4,5]
        console.log('arr1',arr1) //undefined

forEach(item, index, arr)，三个参数，如果直接用item=xxx是无法改变原数组的，但是如果用arr[index]就可以改变原数组。

var s = [1,2,3,4];
s.forEach(item=>{
   item = 'a'
});
console.log(s);// ["1", "2", "3", "4"] 未改变原数组


s.forEach((item, index, arr)=>{
   arr[index] = 'b'
});
console.log(s);// ["b", "b", "b", "b"] 改变了原数组

数组里面的子元素是对象，那么是可以改变对应属性的

var s = [{a:1}, {a:1}];
s.forEach(item=>{
    item = null;
});
console.log(s);//[{a: 1} ,{a: 1}] 未改变原数组
 
s.forEach(item=>{
    item.a = 666;
});
console.log(s); // [{a: 666}, {a: 666}] //改变的原数组里面的对象属性