selfMap.js

const selfMap = function (fn, context) {
    let arr = Array.prototype.slice.call(this)
    let mappedArr = Array()
    for (let i = 0; i < arr.length; i++) {
        if (!arr.hasOwnProperty(i)) continue;
        mappedArr[i] = fn.call(context, arr[i], i, this)
    }
    return mappedArr
}

Array.prototype.selfMap = selfMap;