How to extend String Prototype and use it next, in Typescript?

Question

I am extending String prototype chain with a new method but when I try to use it it throws me an error: property 'padZero' does not exist on type 'string'. Could anyone solve this for me?

The code is below. You can also see the same error in Typescript Playground.

interface NumberConstructor {
    padZero(length: number);
}
interface StringConstructor {
    padZero(length: number): string;
}
String.padZero = (length: number) => {
    var s = this;
    while (s.length < length) {
      s = '0' + s;
    }
    return s;
};
Number.padZero = function (length) {
    return String(this).padZero(length);
}

Answer 1

If you want to augment the class, and not the instance, augment the constructor:

declare global {
  interface StringConstructor {
    padZero(s: string, length: number): string;
  }
}

String.padZero = (s: string, length: number) => {
  while (s.length < length) {
    s = '0' + s;
  }
  return s;
};

console.log(String.padZero('hi', 5))

export {}

*The empty export on the bottom is required if you declare global in .ts and do not export anything. This forces the file to be a module. *

If you want the function on the instance (aka prototype),

declare global {
  interface String {
    padZero(length: number): string;
  }
}

String.prototype.padZero = function (length: number) {
  let d = String(this)
  while (d.length < length) {
    d = '0' + d;
  }
  return d;
};

console.log('hi'.padZero(5))

export {}