Add support for TypeScript's Assertion Functions

[swashcap]

🚀 Feature Proposal

Support TypeScript 3.7's new assertion functions.

Motivation

Jest should support this TypeScript language feature to make authoring tests simpler.

Example

Let's say I have a function under test that returns a nullable value:

export interface Data {
  anotherCoolProp: any
  somethingNeat: any
}

export function gimmeData(): Data | null {
  // Implementation details
  // ...
}

Currently, testing the results requires null checks for every assertion:

test('gimme that data', () => {
  const data = gimmeData()

  expect(data).toBeTruthy()

  expect(data!.anotherCoolProp).toEqual('coolio')
  expect(data!.somethingNeat).toEqual('neato')
})

// or:
test('gimme that data', () => {
  const data = gimmeData()

  if (!data) {
    throw new Error('Expected data to be non-null')
  }

  expect(data.anotherCoolProp).toEqual('coolio')
  expect(data.somethingNeat).toEqual('neato')
})

The first uses the ! non-null operator, which opts out of type safety and can lead to confusing error reports. The second seems non-idiomatic: why not write expect(data).toBeTruthy()?

With TypeScript 3.7 you can now define Jest's global expect to behave this way:

declare function <T>expect(value: T): asserts value is NonNullable<T>

(Although typing w/ the Jest's chaining will be a bit more involved.)

Pitch

Why does this feature belong in the Jest core platform?

This feature request relates to assertions, which are at the core of Jest.

[SimenB]

The types are not maintained in this repo (yet, at least) - this issue belongs in DefinitelyTyped.

I do agree it'd be nice, though! Please add a link in this issue when you create one over there

[hjr3]

I made an issue here: https://github.com/DefinitelyTyped/DefinitelyTyped/issues/41179

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