SOURCE

function eachFlat (arr=[], depth = 1) {
    const result = [];
    (function flat(arr, depth) {
        for (let item of arr) {
      if (Array.isArray(item) && depth > 0) {
        flat(item, depth - 1)
      } else {
        // 去除空元素，添加非undefined元素
        item !== void 0 && result.push(item);
      }
        }
    })(arr, depth)
    return result
}
console.log(eachFlat([1,undefined,[2,[3,[4,5]]]],3))