[Feature]: typesafe assertions

🚀 Feature Proposal

this will always fail, but since typescript infers the type as unknown, there’s no compile error:

expect('foo').toBe(1) // no error
expect<string>('foo').toBe(1) // no error

Motivation

it will allow you to identify errors in your tests much earlier, speeding up development 🚀🚀🚀

Example

// the new and improved expect 🚀:
declare const expect: <T>(actual: T) => { toBe: (expected: T) => void}

// usage:
expect('foo').toBe(1) // TS2345: Argument of type 'number' is not assignable to parameter of type 'string'.

Pitch

the current expect types make using typescript with jest pretty much useless. i think if jest is going to support typescript, it should support basic functionality like this

[SimenB]

Would love toBe to work as type predicate 👍 Not sure how feasible it is, tho

[mrazauskas]

Actually, no need to have any option or to wrestle with the Compiler API and no need to involve ts-jest too:

import {expect} from 'jest-strictly-typed-expect';

Could work like this too (I did not try):

import type {StrictlyTypedMatchers} from 'jest-strictly-typed-matchers';

declare module 'expect' {
    interface Matchers<R> extends StrictlyTypedMatchers<R> {}
}