手写reduce实现

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1const arr = [1, 2, 3, 4];
2
3Array.prototype.MyReduce = function (callback, initialArray) {
4  let instance = this;
5  if (!callback) {
6    return initialArray;
7  }
8  const fn = function (res, index) {
9    if (index >= instance.length) {
10      return res;
11    }
12    const currentValue = instance[index];
13    return fn(callback(res, currentValue, index, instance), index + 1);
14  };
15  return fn(initialArray, 0);
16};
17
18const res = arr.MyReduce((result, currentValue, index, array) => {
19  return result + currentValue;
20}, 0);
21
22console.log("res", res);