@reduxjs/toolkit/src/tests/createReducer.typetest.ts

The official, opinionated, batteries-included toolset for efficient Redux development

import type { Reducer } from 'redux'
import type { ActionReducerMapBuilder } from '@reduxjs/toolkit'
import { createReducer, createAction } from '@reduxjs/toolkit'
import { expectType } from './helpers'

/*
 * 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)
  })

  const numberReducer: Reducer<number> = reducer

  // @ts-expect-error
  const stringReducer: Reducer<string> = reducer
}

/**
 * Test: createReducer() state type can be specified expliclity.
 */
{
  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) => {
    // @ts-expect-error
    builder
      .addCase('increment', incrementHandler)
      .addCase('decrement', 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) =>
    expectType<ActionReducerMapBuilder<number>>(builder)
  )

  expectType<number>(reducer(0, increment(5)))
  // @ts-expect-error
  expectType<string>(reducer(0, increment(5)))
}