数组扁平化

1.方法一

const flatten = arr => {
      return arr.reduce((pre, cur) => {
        return pre.concat(Array.isArray(cur) ? flatten(cur) : cur)
      }, [])
    }
    const result = flatten(arr)
    console.log(result);

2.方法二

const arr = [1, [2, [3, [4, 5]]], 6]
function flatten(arr) {
  let result = []
  for(let i = 0; i < arr.length; i++) {
    if(Array.isArray(arr[i])) {
      result = result.concat(flatten(arr[i]))
    }else {
      result = result.concat(arr[i])
    }
  }
  return result
}
flatten(arr)