Example of extending jest matchers in TypeScript

jest_typescript_extend.ts

export {}

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

function myMatcher<T>(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,
})