reduce.js

Array.prototype.customReduce = function (fn, initialValue) {
  for (let i = 0; i < this.length; i++) {
    // 当未传初始值时取数组中的第一项作为初始值
    if (typeof initialValue === "undefined") {
      initialValue = fn(this[i], this[i + 1], i, this);
      i++;
    } else {
      initialValue = fn(initialValue, this[i], i, this);
    }
  }

  return initialValue;
};

let arr = [
  1,
  [1, 2, 2],
  [3, 4, 5, 5],
  [6, 7, 8, 9, [11, 12, [12, 13, [14]]]],
  10
];

const flatten2 = (arr) =>
  arr.customReduce((acc, cur, index, array) => {
    if (Array.isArray(cur)) {
      return acc.concat(flatten2(cur));
    } else {
      return acc.concat(cur);
    }
  }, []);

console.log(flatten2(arr));