fix(types/store): Unexpectedly narrowed return type of function Store['getState']

[exuanbo]

---

name: 🐛 Fix the return type of function Store['getState']
about: Unexpectedly narrowed type

PR Type

Does this PR add a new feature, or fix a bug?

This PR fixes a bug in the type definitions.

Why should this PR be included?

What is the current behavior, and the steps to reproduce the issue?

The return type of function Store['getState'] is unexpectedly narrowed to non-nullable.

import { type Reducer, createStore } from 'redux'

type NonNullableState = { count: number }
const nullableReducer: Reducer<NonNullableState | null> = () => null

const storeWithNullableState = createStore(nullableReducer)
const nullableState = storeWithNullableState.getState()
//    ^? const nullableState: NonNullableState

TS Playground: https://www.typescriptlang.org/play?#code/JYWwDg9gTgLgBAbzjAnmApnASugJgVwGN0oAaOQqdAQxnQGUZpMBfOAMyghDgHIqCAD14AoEagxwAchAB2U-ABtF1AEaKGMWpgC8iChHyyYALjiz8IVSTgsRhOQGd4F5Wo04CxKGc9ESADwy8koq6pracAA+5qEAfHB6ABQAlIkJropiDrLOcM7MAOrAMAAWCm7hjJF6lDR0jMxJme7oft4p9k4uoa3VdIn5TFTFZRVhGv3oAHQA5ugwU6kiAPQrcBtwAHoA-GJAA

What is the expected behavior?

If StateExt type parameter is not provided, Store['getState'] should return the untouched type S.

How does this PR fix the problem?

The problem is in this line:

redux/src/types/store.ts

Lines 114 to 119 in 7876f8e

	/**
	* Reads the state tree managed by the store.
	*
	* @returns The current state tree of your application.
	*/
	getState(): S & StateExt

If StateExt is not given a specific value, the type will be S & {} which narrows the type to non-nullable.

The behavior of intersection with {} is changed in TypeScript 4.8, "Improved Intersection Reduction, Union Compatibility, and Narrowing".

In this PR:

• The default value of the type parameter StateExt in the Store interface is widened to unknown. This can ensure that S & unknown is still S.
• In other types for creating store, the type arguments StateExt passed to Store is widened using a utility type UnknownIfNonSpecific.

type UnknownIfNonSpecific<T> = {} extends T ? unknown : T

Checklist

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Is there an existing issue for this PR?
  • No.
• Have the files been linted and formatted?
• Have the docs been updated to match the changes in the PR?
• Have the tests been updated to match the changes in the PR?
• Have you run the tests locally to confirm they pass?

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[EskiMojo14]

good catch, thanks! admittedly i don't think stores with a nullable state are very common, but you're correct that this is an issue.

any chance you could check whether the same issue needs fixing on the RTK side of things too?

[EskiMojo14]

thanks!