How to add to the prototype of types from other modules in TypeScript?

Question

Let's say I would like to add this method onto String:

String.prototype.frenchQuote = function() {
  return '« ' + this + ' »';
}

TypeScript will complain that there is no frenchQuote method on String.

Online, many suggestions can be found to then declare it like so:

interface String {
  frenchQuote(): string
}

However, TypeScript flags this interface as unused, and still complains about the non-existent method.

Is there a new way to augment the signature of imported types or built-ins?

Using TypeScript version 3.7

you might just need to specify a namespace (global, window?) so it picks up the interface — pushkin, Feb 06 '20 at 22:35
@VladGincher none of the answers are accepted there, and the most upvoted one doesn't seem to work in TypeScript 3.7 — P Varga, Feb 06 '20 at 22:41

score 0 · Answer 1 · answered Feb 06 '20 at 22:46

0

Thank you @pushkin for the hint. The solution was to signal to TS that the interface is global:

declare global {
  interface String {
    frenchQuote(): string
  }
}

answered Feb 06 '20 at 22:46

P Varga

1 Answers1