Discussion on: Types vs. Interfaces in Typescript

[peerreynders]

An interface merge can be used to "add" new functionality to an existing class (as long as only public features are accessed):

class MyClass {
  readonly #value: number;

  constructor(value: number) {
    this.#value = value;
  }

  get value():number {
    return this.#value;
  }
}

interface MyClass {
  square():number;
}

function square(this:MyClass): number {
  const value = this.value;
  return value * value; 
}

MyClass.prototype.square = square;

const container = new MyClass(42);
console.assert(container.square() === 1764, "Square isn't 1764");