Nodejs Utililty Methods String Swap

String.prototype.swap = function(a, b) {
    var array = this.split('');
    var tmp = array[a];
    array[a] = array[b];
    array[b] = tmp;
    return array.join('');
function permutation(str, start) {
    if (start == str.length - 1) {
...

var pers=[];
function permute(s,l,r){
  if(l==r) pers.push(s);
  else {
    for(var i=l;i<=r;i++) {
      s=s.swap(l,i);
      permute(s,l+1,r);
      s=s.swap(l,i);
String.prototype.swap=function(l,r) {
  return this.split('').map((a,i)=>i==l?this[r]:i==r?this[l]:a).join('');
permute('abc',0,2);
console.log(pers);

String.prototype.swapLetters = function(index) {
    var words = this.split("");
    var firstLetter = words[index];
    var lastLetter = words[words.length - 1 - index];
    var temp = firstLetter;
    words[index] = lastLetter;
    words[words.length -1 - index] = temp;
    return words.join("");
};
...

String.prototype.swap_chars = function(i,j) {
    var array_string = this.split("");
    tmp = array_string[i];
    array_string[i] = array_string[j];
    array_string[j] = tmp;
    return array_string.join("");
};

String.prototype.swap_substr = function(i) {
    return this.substr(0, i) + this.substr(i).split("").reverse().join("");
};