Extending third party module that is globally exposed

Question

I am trying to add a custom matcher to Jest in Typescript. This works fine, but I can\'t get Typescript to recognize the extended Matchers.

myMatcher.ts

Answer 1

Answer by @AluanHaddad is almost correct, sans a few types. This one works:

export {};

declare global {
  namespace jest {
    interface Matchers {
      myMatcher: (received: string) => R;
    }
  }
}

function myMatcher(this: jest.MatcherUtils, received: string, expected: string): jest.CustomMatcherResult {
  const pass = received === expected;
  return {
    pass,
    message: (): string => `expected ${received} to be ${expected}`,
  }
}

expect.extend({
  myMatcher,
});

For a real-world example, see https://github.com/Quantum-Game/quantum-tensors/blob/master/tests/customMatchers.ts (and that the tests actually pass: https://travis-ci.com/Quantum-Game/quantum-tensors).