selfReduce.js

const selfReduce = function (fn, initialValue) {
    let arr = Array.prototype.slice.call(this)
    let res
    let startIndex
    if (initialValue === undefined) {
        for (let i = 0; i < arr.length; i++) {
            if (!arr.hasOwnProperty(i)) continue
            startIndex = i
            res = arr[i]
            break
        }
    } else {
        res = initialValue
    }

    for (let i = ++startIndex || 0; i < arr.length; i++) {
        if (!arr.hasOwnProperty(i)) continue
        res = fn.call(null, res, arr[i], i, this)
    }
    return res
}

Array.prototype.selfReduce = selfReduce;