js数组扁平化 ​

let arr = [1,[2,3,[4,5,6]],[7,8,9,[10,11]]]
  function reduceArr (arr) {
    return newArr1 = arr.reduce((res, item) => {
      return res.concat(Array.isArray(item) ? reduceArr(item) : item)
    }, [])
  } 
  console.log(reduceArr(arr))

  console.log(arr.toString().split(','))
  function mapArr (arr) {
    return arr.toString().split(',').map(item => {
      return item * 1
    })
  }
  console.log(mapArr(arr))

  console.log([].concat(...arr))
  function someArr(arr) {
    while(arr.some(item=>Array.isArray(item))) {
        arr = [].concat(...arr);
    }
    return arr;
  }
  console.log(flatten(arr))