How To Extend A Primitive Type In Typescript?

I want to create an interface like this: interface Show { show(): string; } function doit(s: Show) { return 'Showed: ' + s.show(); } Then we can use it with a new class: clas

Solution 1:

Just tell TypeScript about it by adding to Number :

interfaceNumber{
    show():string;
}

Number.prototype.show = function() { 
  return'' + this; 
}

var foo = 123;
foo.show();

Please note that even tough it is supported it is considered bad practice to do so even in JavaScript land : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Inheritance_and_the_prototype_chain#Bad_practice.3A_Extension_of_native_prototypes