how can i extend the String object by adding a function?

Question

I am trying to add a function to the global String object by doing the following:

function repetirCaracteres() {

  String.prototype.repeatCharacters = function(palabra){
  const letras = palabra.split('')
  const repetir = letras.map(letras => letras.repeat(2))
  let letrasRepetidas = repetir.join('');
  return letrasRepetidas;
  }
  
 }

repetirCaracteres()

I want to do a function that receives a string and each character of the string is repeated twice.

Logic seems ok to me but for some reason i have the following error:

String prototype is read only, properties should not be added.

@Unmitigated i edited my question with the complete block of code. I am trying this in codesandbox. — tukitooo1998, Feb 25 '23 at 19:05
This isn't actually an error, but likely an ESLint warning. Your code will still work, but it's recommended to not modify built-in prototypes. — Unmitigated, Feb 25 '23 at 19:11
I cannot reproduce this. The ECMA Script specification allows the `String.prototype` object to get user defined properties. — trincot, Feb 25 '23 at 19:12
https://stackoverflow.com/questions/14034180/why-is-extending-native-objects-a-bad-practice — Đinh Carabus, Feb 25 '23 at 19:13
But taking a step back, ... *why* do you want to define a *method* on a prototype when that method doesn't use `this`? It seems you don't need a method then. — trincot, Feb 25 '23 at 19:14
@tukitooo1998 please add an example how you intend to use/call this method. — Thomas, Feb 25 '23 at 19:24

score 0 · Accepted Answer · answered Feb 25 '23 at 19:53

Don't know, why it did not work for you. It works here without problems:

String.prototype.rr=function(){
 return this.split("").map(c=>c+c).join("")}
console.log("ABC".rr())

In my snippet I used this as the input string. Without this there would be no reason for defining a String.prototype method.

how can i extend the String object by adding a function?

1 Answers1