数组映射、遍历与截取

映射与截取不会改变原数组

let arr=[0,1,2,3,4,5,6,7,8,9]

let flatArr=[[1,2],[3,4],[5,1]]

1.let arr1=arr.slice(0,3) //返回新数组[0,1,2]
let arr2=arr.slice(1) // >[1,2,3,4,5,6,7,8,9]
let arr3=arr.slice(-1) // >[9]
let arr4=arr.slice(1,-1) // >[1,2,3,4,5,6,7,8]

2.let arr5=arr.map(e=>e*2) //返回新数组[0,2,4,6,8,10,12,14,16,18]
3.arr.forEach(e=>e*2) //无返回值只是遍历计算，一般情况下不可停止
4.let arr6=flatArr.flat() // >arr6=[1,2,3,4,5,1]
5.let arr7=arr.concat([-1,-2]) // >arr7=[0,1,2,3,4,5,6,7,8,9,-1,-2]