手写数组方法

手写 reduce/map/filter/forEach/flat 等是高频题，重点在 回调参数 (item, index, array)、this 指向、空数组/无初始值等边界。

map 的实现见 数组相关。

reduce

最容易踩坑的是不传初始值的情况：用第一个元素作为初始值，并从下标 1 开始；空数组无初始值要抛错。

Array.prototype.myReduce = function (fn, initialValue) {
  if (typeof fn !== "function") {
    throw new TypeError(`${fn} is not a function`);
  }
  const arr = this;
  let acc;
  let startIndex = 0;

  if (arguments.length >= 2) {
    acc = initialValue;
  } else {
    if (arr.length === 0) {
      throw new TypeError("Reduce of empty array with no initial value");
    }
    acc = arr[0];
    startIndex = 1;
  }

  for (let i = startIndex; i < arr.length; i++) {
    acc = fn(acc, arr[i], i, arr);
  }
  return acc;
};

filter

Array.prototype.myFilter = function (fn, thisArg) {
  if (typeof fn !== "function") {
    throw new TypeError(`${fn} is not a function`);
  }
  const res = [];
  for (let i = 0; i < this.length; i++) {
    if (fn.call(thisArg, this[i], i, this)) {
      res.push(this[i]);
    }
  }
  return res;
};

forEach

Array.prototype.myForEach = function (fn, thisArg) {
  if (typeof fn !== "function") {
    throw new TypeError(`${fn} is not a function`);
  }
  for (let i = 0; i < this.length; i++) {
    fn.call(thisArg, this[i], i, this);
  }
};

flat（支持深度）

function flat(arr, depth = 1) {
  if (depth <= 0) return arr.slice();
  return arr.reduce(
    (acc, cur) =>
      acc.concat(Array.isArray(cur) ? flat(cur, depth - 1) : cur),
    []
  );
}

// 完全拉平：depth 传 Infinity
flat([1, [2, [3, [4]]]], Infinity); // [1, 2, 3, 4]

数组扁平化的更多写法见 数组扁平化。