Harness the power of TypeScript in a single test file with jest's expect.extend() method

This question is similar to Can you limit the scope of a TypeScript global type? but it presents a slightly different scenario that requires clarification (although an answer to this would be greatly appreciated as well).

In my Jest setup, I am attempting to achieve the following:

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

export {};

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

The issue I'm encountering is that I only want to call the `extend` method in a specific file, meaning I don't want other test files to have access to it. The `declare global {}` block seems to be causing challenges in this context.

I've also experimented with a workaround approach:

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

Unfortunately, this alternative method isn't yielding the expected results.

An additional attempt involved directly declaring `namespace jest` outside of the global scope, but due to `jest` being a global namespace, a new namespace was created instead of enhancing the existing one.

Is there a way to confine an augmentation to a specific file and its imports within Jest?

Answer №1

In the same package, it might not be possible to restrict the scope of an augmented type. However, a workaround could be creating a custom expect function with the desired type:

For example:

function customExpectElement(actual: HTMLElement): ElementCustomExpect {
  return expect(actual) as any as ElementCustomExpect;
}

type ElementCustomExpect = JestExpect & CustomElementMatchers;

interface CustomElementMatchers<R = void> extends Matchers<void> {
  toBeCustom(): R;
}

This way, your tests become more straightforward and clear:

it("should be customized", () => {
  customExpectElement(el).toBeCustom();
})

Answer №2

If you are relying on explicit imports from @jest/globals instead of using @types/jest which provides global access to TypeScript, consider adjusting the type of the imported expect.

import { expect as _expect } from '@jest/globals';

const expect = _expect as unknown as {
  <T = unknown>(actual: T): {
    toBeAmazing(): void;
  } & ReturnType<typeof _expect>;
} & typeof _expect;

Although this approach may not fully verify the type of the parameter passed to expect(), it does restrict the matcher's scope to the current file.

Alternatively, utilizing global types (@types/jest), you can redefine expect with a distinct name like

const expectElement = expect as unknown as /* custom type */
.

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