JS 实现reduce

Array.prototype.reduceArr = function (fn, initValue) {
    if (Object.prototype.toString.call(fn) !== "[object Function]") {
        throw new Error('current params is not correct')
    }
    let arr = this;
    let initIndex;
    let acc;
    initIndex = arguments.length === 1 ? 1 : 0;
    acc = arguments.length === 1 ? arr[0] : initValue;
    for (let i = initIndex; i < arr.length; i++) {
        acc = fn(acc, arr[i], i, arr);

    }
    return acc;
}
let arr = [1, 2, 3, 4, 5]
let res = arr.reduceArr((pre, cur) => { return pre + cur; }, 10)

console.log(res)

附录:参考来源