ES11——数组降维 flat 与数组遍历降维 flatMap

let arr = [
  [1, 2],
  [3, [4, 5]]
]
console.log(arr.flat());  //[ 1, 2, 3, [ 4, 5 ] ]
console.log(arr.flatMap(item => item));  //[ 1, 2, 3, [ 4, 5 ] ]
console.log(arr.flatMap(item => [ item ]));  //[ [ 1, 2 ], [ 3, [ 4, 5 ] ] ]