String prototype changing the string value

Question

I'm trying to change the string value inside its protoype function:

String.prototype.myfunction=function(){
    this += "a";
    return this;
};

But as it seems I just can't change the value. If I try to run that on the console I just get the error: Uncaught ReferenceError: Invalid left-hand side in assignment

Is it possible to change the strings value? Thanks in advance

See also [how do i get “this = this” in prototype working](http://stackoverflow.com/q/28148196/1048572) or [Assign object to “this”](https://stackoverflow.com/questions/15598549/assign-object-to-this) — Bergi, Nov 20 '16 at 22:32

score 3 · Answer 1 · answered Nov 20 '16 at 22:19

3

I think it has to do with the immutability of strings and the fact that this is a constant.

It works just fine if your example is trivially changed to:

String.prototype.myfunction=function(){
    return this + 'a';
};

answered Nov 20 '16 at 22:19

Corey Ogburn

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What I actually wanted was to change its value, I know returning this+"a" works fine. Anyway, thanks for the help – Marcelo43 Nov 20 '16 at 22:23
Treat strings in JavaScript as immutable. Work with the object returned from the function instead. – TimoStaudinger Nov 20 '16 at 22:25

score 0 · Answer 2 · answered Nov 20 '16 at 22:24

Strings are immutable. You may want to use an array of characters instead:

class MyString extends Array {
  constructor(str) {
    super(...String(str));
  }
  myfunction() {
    this.push("a");
    return this;
  }
  toString() {
    return this.join('');
  }
}
var s = new MyString("hello!");
s.myfunction();
console.log(s + '');

String prototype changing the string value

2 Answers2

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