[Feature]: Extensible, type-friendly and explicit test/expect API

[marionebl]

🚀 Feature Proposal

Take a leaf out of playwright's playbook and expose an explicit, extensible test and expect API. Include the fixture concept to help factor setup and cleanup code out of tests files in a browsable, type safe and explicit way. This follows the prior art established by playwright: https://playwright.dev/docs/api/class-fixtures

Motivation

Doubling down on being type system friendly is a good investment in the JavaScript ecosystem of 2022. Providing test authors with tools to keep tests focused will lead to better test suites. Thi

Example

In its simplest form this shifts jest usage from relying on globals to explicitly imported functions

import { test } from '@jest/test';

test('1 + 1 = 2', () =>  {
  expect(1 + 1).toBe(2);
});

It also affords a way for test suite authors to provide fixtures in a standardised and easily reusable way

// browser-tests.ts
import { test as base } from '@jest/test';
export { expect } from '@jest/test';

export const test = base.extend({
   browser: (use) => {
     const browser = setup(/* ... */);
     await use(browser)
     await teardown(browser);
   }
});

// tests.ts
import { test, expect } from '../browser-tests.ts';

test('1 + 1 = 2 in a browser', async ({  browser }) =>  {
   const result = await browser.evaluate(() => 1 + 1);
   expect(result).toBe(2);
});

Pitch

Belongs into core as it touches on a low level how jest test suites are structured.

[SimenB]

In its simplest form this shifts jest usage from relying on globals to explicitly imported functions

That's already possible via @jest/globals: https://jestjs.io/blog/2020/05/05/jest-26#a-new-way-to-consume-jest---jestglobals (the typescript caveat is no longer true: https://jestjs.io/docs/next/expect#expectextendmatchers)

---

However, the part about teardown and stuff looks interesting!

[Parth0105]

Hi @SimenB would love working on the feature request.

[marionebl]

That's already possible via @jest/globals: https://jestjs.io/blog/2020/05/05/jest-26#a-new-way-to-consume-jest---jestglobals (the typescript caveat is no longer true: https://jestjs.io/docs/next/expect#expectextendmatchers)

Thanks for the pointer at @jest/globals - I was behind the times! :)

[SimenB]

I suggested something like this way back when: https://github.com/facebook/jest/pull/8654#issuecomment-510385442 (see bottom of that comment) - that looks pretty much like what base.extend in the OP is.

---

Hey @Parth0105, thanks for offering! While I'm keen on getting something like what is suggested here, I think we'll need to go through a longer design phase before we start writing any code. And I don't know when I'll have the time and/or energy for that.

However, if you're up for it, I'd love to review some code. But I cannot provide any guarantees it'll land, so it might be a bit of a waste of time for you 🙂

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