数组扁平化

只扁平一层

[1, 2, [3, [4, [5, 6]]]] ===> [ 1, 2, 3, [ 4, [ 5, 6 ] ] ]

const flattenDeep = (arr = []) => {
  return arr.reduce((pre, cur) => pre.concat(cur), [])
}

全部扁平

[1, 2, [3, [4, [5, 6]]]] ===> [ 1, 2, 3, 4, 5, 6 ]

const flattenDeep = (arr = []) => {
  return arr.reduce((pre, cur) => {
    return pre.concat(Array.isArray(cur) ? flattenDeep(cur) : cur)
  }, [])
}