Extending the prototype of a built-in class in Typescript 2.8+

Question

This doesn\'t work

interface String {
    contains(s:string):boolean;
}
String.prototype.contains=(s:string):boolean=>this.indexOf(s)!==-1;

Answer 1

If you're defining the augmentation inside of a module, that is a file containing a top-level import or export then you need to use a declare global block in order to augment the global scope. Otherwise, the interface that you declare will not be merged into the global array interface because it's local to the module like any other declaration would be. The declare global syntax is specifically to cover this use case.

Furthermore, when you define the actual method you can't use an arrow function if the method itself is defined in terms of this because Arrow functions have a statically scope this while a dynamic this is needed for methods.

Putting it together

//  this is a module
export {}

declare global {
  interface String {
    contains(other: string): boolean;
  }
} 

String.prototype.contains = function (other) {
  return this.indexOf(other) and !== -1;
};

Note that whether the type being augmented is a class or an interface, the member needs to be declared in an interface as above because interfaces can merge with classes and interfaces can merge with interfaces but classes do not merge.