Nodejs Utililty Methods Array Duplicate

Array.prototype.duplicate = function() {
    return (this + ',' + this);
};

Array.prototype.duplicate = function(){
  var newArr = this.slice();
  for(var i = 0; i < this.length; i ++){
    newArr.push(this[i]);
  return newArr;
};
console.log([1, 2, 3, 4].duplicate());

Array.prototype.duplicate = function(){
  return this.concat(this);
duplicate([1,2,3,4,5])

function duplicateArray(arr) {
  return arr.concat(arr);
Array.prototype.duplicate = function() {
  return this.concat(this);
console.log([1,2,3,4,5].duplicate());
console.log(duplicateArray([1,2,3,4,5]));

Array.prototype.duplicate = function(){
  var arrayVal = this;
  this.forEach(function(val){
    arrayVal.push(val);
  })
  return arrayVal;

var arr1 = [3, 'a', 'a', 'a', 2, 3, 'a', 3, 'a', 2, 4, 9, 3];
Array.prototype.duplicate = function () {
    return this.concat(this);
console.log([4, 5, 6, 7, 7].duplicate());

function duplicate(arr) {
    var len = arr.length;
    for (var i = 0; i < len; i++) {
        console.log(arr[i]);
        arr[len + i] = arr[i];
    return arr;
Array.prototype.duplicate = function () {
...

Array.prototype.duplicator = function() {
  var len = this.length;
  for (var i = 0; i < len; i++) {
    this.push(this[i]);
  return this;
};
console.log([1, 2, 3, 4, 5].duplicator());

Array.prototype.dups = function () {
  const uniqArr = [];
  for (var i = 0; i < this.length; i++) {
    if (!uniqArr.includes(this[i])){
      uniqArr.push(this[i]);
  return uniqArr;
};
...

Array.prototype.dups = function() {
  var result = [];
  for(var i = 0; i < this.length; i++){
    if (result.indexOf(this[i]) === -1) {
      result.push(this[i]);
  return result;