js数组内置方法

let arr = [];
1.push，在数组末尾添加新的元素

  arr.push(1) //arr = [1]

2.pop,在数组的末尾删除元素

  arr.pop() //arr = []

3.unshfit,在数组前添加元素

  arr.push(2)
  arr.unshfit(1) //arr = [2,1]

4.shfit,在数组前删除元素

  arr.shfit() //arr = [1]

5.flat,多维数组扁平化

  arr = [1,2,3,[4,5,6,[7,8,9,[10]]]]
  arr.flat(Infinity)//Infinity无穷的 arr = [1,2,3,4,5,6,7,8,9,10]

6.sort,数组排序

 arr = [2,3,1];
arr.sort(function(){
  return 1
})//1,2,3
arr.sort(function(){
  return -1
})//3,2,1

7.reduce,遍历数组汇总为一个结果

  arr = [1,2,3,4]
  arr.reduce((preVal,currentVal)=>{
    return preVal+currentVal
  })//10
  arr.reduce((preVal,currentVal)=>{
    return preVal-currentVal
  }),5)//-5

8.find,查询数组中满足查询条件的第一个值,否则返回undefined

  arr = [1,2,3]
  arr.find((e)=>{return e>0})//1
  arr.find((e)=>{return e<0})//undefined

9.findIndex,查询数组中某个值的下标，找到了返回下标，否则返回-1

  arr = [1,2,3]
  arr.findIndex((e)=>e==1)//0
  arr.findIndex((e)=>e==0)//-1

10.includes,查询数组中是否含有这个元素，有则返回true,否则返回false

  arr = [1,2,3]
  arr.includes(1)//true
  arr.includes(0)//false

11.Array.from(),将一个类数组转化成数组

  let str = 'arr';
  Array.from(str)//['a','r','r']

12.Array.isArray(),判断是否是数组

  let str = 'arr'
 Array.isArray(str ) //false

13.some,判断数组中是否有一项满足添加，有则返回true,否则返回false

  arr = [1,2,3]
  arr.some((e)=>{
    return e%2==0
  })//true

13.every,判断数组中所有项都满足添加，是则返回true,否则返回false

  arr = [1,2,3];
  arr.every((e)=>{
    return e>0
  })//true
  arr.every((e)=>{
    return e>1
  })//false

14.splice,可使数组添加、替换、删除元素(改变原数组)

  arr = [1,2,3]
  arr.splice(1)//删除元素 arr = [1]
  arr.splce(1,2,4)//替换元素 arr = [1,4]
  arr.splece(2,3,5)//添加元素 arr = [1,4,5]

15.slice,获取数组元素，不改变原数组

  arr = [1,2,3]
  let newArr = []
  newArr = arr.slice(1)//newArr  = [2,3]

16.concat,使两个数组合并为一个数组

  arr = [1,2,3]
  newArr = [4,5,6]
  arr.concat(newArr)//arr = [1,2,3,4,5,6]

17join,将数组转成字符串

  arr = [1,2,3]
  let str = ''
  str = arr.join()//1,2,3
  str = arr.join('')//123
  str = arr.join('@')//1@2@3