Nodejs Utililty Methods Array Reduce

Array.prototype.reduce = function(testFunction,initialValue){
  var accumulatedValue,counter;
  if(this.length == 0){
    return this;
  }else{
    if(arguments.length == 1){
      counter = 1;
      accumulatedValue = this[0];
    }else if(arguments.length >= 2){
...

Array.prototype.reduce = function(testFunction,initialValue){
  var accumulatedValue,counter;
  if(this.length == 0){
    return this;
  }else{
    if(arguments.length == 1){
      counter = 1;
      accumulatedValue = this[0];
    }else if(arguments.length >= 2){
...

Array.prototype.reduce = function(testFunction,initialValue){
  var accumulatedValue,counter;
  if(this.length == 0){
    return this;
  }else{
    if(arguments.length == 1){
      counter = 1;
      accumulatedValue = this[0];
    }else if(arguments.length >= 2){
...

'use strict';
class PrefixTree {
    constructor() {
        this.root = {};
    insert(str) {
        Array.prototype.reduce.call(str, (trav, char) => {
            if (!trav[char]) {
                trav[char] = {};
...

Array.prototype.reduceRight = function(aggregate, initial) {
  const len = this.length;
  let startIndex = len - 1;
  if(typeof initial === 'undefined') {
    initial = this[0];
    startIndex = len - 2;
  for(let i = startIndex; i >= 0; i -= 1) {
    initial = aggregate(initial, this[i], i, this);
...

Array.prototype.reduceRight = function(fun ) {
    var len = this.length >>> 0;
    if (typeof fun != "function") {
        throw new TypeError();
    if (len == 0 && arguments.length == 1) {
        throw new TypeError();
    var i = len - 1;
...

Array.prototype.reduceRight2 = function(pasteback, initial){
  for(var i = this.length
      , value = arguments.length>1 ? initial : this[--i]
  ; i--;){
    value = pasteback(value, this[i], i, this);
  return value;
};

Array.prototype._reduce = function(f){
    let acc = this[0];
    for(let i = 1; i < this.length; i++){
        acc = f(acc,this[i])
    return acc;