source: node_modules/@reduxjs/toolkit/src/tests/createReducer.test-d.ts

import type { ActionReducerMapBuilder, Reducer } from '@reduxjs/toolkit'
import { createAction, createReducer } from '@reduxjs/toolkit'

describe('type tests', () => {
  test('createReducer() infers type of returned reducer.', () => {
    const incrementHandler = (
      state: number,
      action: { type: 'increment'; payload: number },
    ) => state + 1

    const decrementHandler = (
      state: number,
      action: { type: 'decrement'; payload: number },
    ) => state - 1

    const reducer = createReducer(0 as number, (builder) => {
      builder
        .addCase('increment', incrementHandler)
        .addCase('decrement', decrementHandler)
    })

    expectTypeOf(reducer).toMatchTypeOf<Reducer<number>>()

    expectTypeOf(reducer).not.toMatchTypeOf<Reducer<string>>()
  })

  test('createReducer() state type can be specified explicitly.', () => {
    const incrementHandler = (
      state: number,
      action: { type: 'increment'; payload: number },
    ) => state + action.payload

    const decrementHandler = (
      state: number,
      action: { type: 'decrement'; payload: number },
    ) => state - action.payload

    createReducer(0 as number, (builder) => {
      builder
        .addCase('increment', incrementHandler)
        .addCase('decrement', decrementHandler)
    })

    // @ts-expect-error
    createReducer<string>(0 as number, (builder) => {
      expectTypeOf(builder.addCase)
        .parameter(1)
        .not.toMatchTypeOf(incrementHandler)

      expectTypeOf(builder.addCase)
        .parameter(1)
        .not.toMatchTypeOf(decrementHandler)
    })
  })

  test('createReducer() ensures state type is mutable within a case reducer.', () => {
    const initialState: { readonly counter: number } = { counter: 0 }

    createReducer(initialState, (builder) => {
      builder.addCase('increment', (state) => {
        state.counter += 1
      })
    })
  })

  test('builder callback for actionMap', () => {
    const increment = createAction<number, 'increment'>('increment')

    const reducer = createReducer(0, (builder) =>
      expectTypeOf(builder).toEqualTypeOf<ActionReducerMapBuilder<number>>(),
    )

    expectTypeOf(reducer(0, increment(5))).toBeNumber()

    expectTypeOf(reducer(0, increment(5))).not.toBeString()
  })
})