Incorrect types for lifecycle functions

[Mossop]

🐛 Bug Report

The types for the lifecycle functions (beforeEach etc.) are declared incorrectly in packages/jest-types/src/Global.ts. These functions are documented as being able to return a promise or generator and accepting a timeout parameter but the defined types disallow anything except a void returning function.

To Reproduce

Take the following code:

import { beforeEach } from "@jest/globals";

beforeEach((): Promise<void> => {
  return Promise.resolve();
}, 2000);

This causes TypeScript to complain with Expected 0-1 arguments, but got 2. ts(2554) and causes the typescript-eslint rule no-misused-promises to complain with Promise returned in function argument where a void return was expected..

Expected behavior

The given code should not show any issues.

I think that the type for HookFn should be changed to something like this:

export type LifecycleFn = (done?: DoneFn) => Generator | Promise<void | undefined | unknown> | void | undefined;
export type HookFn = (fn: LifecycleFn, timeout?: number) => void;

envinfo

  System:
    OS: macOS 10.15.4
    CPU: (12) x64 Intel(R) Core(TM) i9-8950HK CPU @ 2.90GHz
  Binaries:
    Node: 14.1.0 - /usr/local/bin/node
    npm: 6.14.4 - /usr/local/bin/npm
  npmPackages:
    jest: ^26.0.1 => 26.0.1

[villasv]

Happening with me too. It looks like the lifecycle hooks got typed as HookFn, but that's probably a mistake. HookFn is probably meant to type the fn parameter for the hooks, like TestFn types the fn parameter for tests.

This would fix it:

export interface TestFrameworkGlobals {
    it: ItConcurrent;
    test: ItConcurrent;
    fit: ItBase & {
        concurrent?: ItConcurrentBase;
    };
    xit: ItBase;
    xtest: ItBase;
    describe: Describe;
    xdescribe: DescribeBase;
    fdescribe: DescribeBase;
    // beforeAll: HookFn;    <--- this is wrong, replace with:
    beforeAll: (fn: HookFn, timeout?: number) => void;
    ...
}

[SimenB]

We still don't support generator functions in the types - anybody up for a PR adding it? 🙂

[villasv]

Oops, forgot about it. Should the typing of it also support generators? Might as well go ahead and update both. (In fact, updating TestFn should already fix for both because HookFn = TestFn).

[villasv]

I failed to find examples of usage of Generators on hooks and tests. It wasn't clear to me if TestFn also has to accept generators. It's also not clear if HookFn should accept a Generator or a GeneratorFunction or a () => Generator. Anyway, all of those will result in compilation errors on packages/jest-circus/src/utils.ts's callAsyncCircusFn that I wasn't sure about how to solve.

Help Wanted indeed.

Wording on the docs is a bit ambiguous.

If the function returns a promise or is a generator, Jest waits for that promise to resolve before running the test.

Generators are not mentioned in the test* methods.

I'll try to use the GeneratorFunction in a way such that the whole project compiles, assuming the implementation of callAsyncCircusFn is correct, and see how it goes as a Pull Request.

[villasv]

Just pinging issue subscribers, the PR is ready.

[SimenB]

Closed by #10836, thanks @villasv!

[github-actions]

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