Use jest's expect.extend() with TypeScript scoped to a single test file

Question

This question is similar to Can you limit the scope of a TypeScript global type? but a little different (though I would love an answer to it, as well).

In Jest, I want to do something like:

declare global {
  namespace jest {
    interface Matchers<R> {
      toBeAwesome(this: Matchers<void, HTMLElement>): R;
    }
  }
}

export {};

expect.extend({
  toBeAwesome() {
    // ...
  }
});

However, I only call extend in that specific file, and therefore I do not want other test files to be able to access it. The declare global {} bit seems to screw me over.

I've also tried something hacky:

declare const expect: jest.Expect & {
  <T = any>(actual: T): jest.JestMatchers<T> & {
    toBeSelected(this: jest.Matchers<void, HTMLElement>): void;
  };
};

But the method won't even show up.

I also tried directly declaring namespace jest outside of the global scope, but since jest is a global namespace, it created a new namespace instead of augmenting the other.

Is it possible to scope an augmentation to a specific file and any file that imports it?

Answer 1

I don't think you can limit the scope of an augmented type within the same package, but you could define your own expect function with the type you want:

Something like:

function expectElement(actual: HTMLEelement): ElementExpect {
  return expect(actual) as any as ElementExpect
}

type ElementExpect = JestExpect & ElementMatchers

interface ElementMatchers<R = void> extends Matchers<void> {
  toBeSelected(): R
}

Then, arguably your tests are more explicit:

it("should be awesome", () => {
  expectElement(el).toBeAwesome()
})