数组方法重写

forEach

Array.prototype.myEach = function (fn) {
  var arr = this;
  var obj = arguments[1] || window;
  for (let i = 0; i < arr.length; i++) {
    const item = arr[i];
    fn.apply(obj, [item, i, arr])
  }
}

map

Array.prototype.myMap = function (fn) {
  var arr = this;
  var obj = arguments[1] || window;
  var newArr = [];
  for (let i = 0; i < arr.length; i++) {
    const item = deepClone(arr[i]);
    newArr.push(
      fn.apply(obj, [item, i, arr])
    ) 
  }
  return newArr;
}

reduce

Array.prototype.myReduce = function (fn, initialValue) {
  let arr = this;
  let prev;
  if (initialValue) {
    prev = initialValue;
    for (let i = 0; i < arr.length; i++) {
      const item = arr[i];
      prev = fn(prev, item, i, arr);
    }
  } else {
    prev = arr[0];
    for (let i = 1; i < arr.length; i++) {
      const item = arr[i];
      prev = fn(prev, item, i - 1, arr);
    }
  }
  return prev;
}

数组扁平化：

const arr = [1, [2, [3, 4]]];

function flatten (arr) {
  let res = [];
  for (const v of arr) {
    if(Array.isArray(v)) {
      res.push(...flatten(v));
    } else {
      res.push(v);
    }
  }
  return res;
}