[Feature]: Ergonomic way to jest.fn over a function type in TypeScript

🚀 Feature Proposal

TypeScript users need a good way to represent that a jest.fn() should adhere to a particular function type. I propose its generic type parameters be switched to a single type parameter representing a function type.

jest.fn<MyFunctionType>();
jest.fn<(input: string) => number>();

Motivation

Right now jest.fn is generic on a T for the return type and Y for the args. That means there’s no easy way to go from some function type to a jest.fn of that type.

The closest is to use ReturnType and Parameters:

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

type MyFunctionType = (input: string) => number;

const spy = jest.fn<ReturnType<MyFunctionType>, Parameters<MyFunctionType>>();

https://www.typescriptlang.org/play?#code/JYWwDg9gTgLgBAbzgKwKYGd4F84DMoQhwDkAAmpgPQDmANhAEYCGt6xA3AFCcwCeYqOAFleAMQCuAOwDGMYBEkAVfoIC8cABTBJYcTABccTFG3UAlHFUA+OJPEgGqKF07SFmI2F6WUGGADpcSQAeACVUGHEoJRVgkQkZOQVlASsAGjgABSYoJhAIp3Q4sSlZeRjUqw0zLiA

Edit: or a wrapper like…

export const fn = <Reference extends (...args: any) => any>(reference?: Reference) =>
    jest.fn(reference);

Example

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

type MyFunctionType = (input: string) => number;

const spy = jest.fn<MyFunctionType>>();

Pitch

Proper function mock types is going to be an increasingly relevant pain point as ES Modules become more popular. @types/jest defaults the generics for jest.fn() mocks to any instead of unknown, so many users haven’t needed to have proper test types till now.

[SimenB]

https://github.com/facebook/jest/releases/tag/v28.0.0-alpha.6

[mrazauskas]

Working on it. Almost there. (Shape it up, just have to figure out a couple of details.)