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createSlice

A function that accepts an initial state, an object of reducer functions, and a "slice name", and automatically generates action creators and action types that correspond to the reducers and state.

This API is the standard approach for writing Redux logic.

Internally, it uses createAction and createReducer, so you may also use Immer to write "mutating" immutable updates:

import { createSlice } from '@reduxjs/toolkit'
import type { PayloadAction } from '@reduxjs/toolkit'

interface CounterState {
  value: number
}

const initialState = { value: 0 } as CounterState

const counterSlice = createSlice({
  name: 'counter',
  initialState,
  reducers: {
    increment(state) {
      state.value++
    },
    decrement(state) {
      state.value--
    },
    incrementByAmount(state, action: PayloadAction<number>) {
      state.value += action.payload
    },
  },
})

export const { increment, decrement, incrementByAmount } = counterSlice.actions
export default counterSlice.reducer

Parameters​

createSlice accepts a single configuration object parameter, with the following options:

function createSlice({
    // A name, used in action types
    name: string,
    // The initial state for the reducer
    initialState: State,
    // An object of "case reducers". Key names will be used to generate actions.
    reducers: Record<string, ReducerFunction | ReducerAndPrepareObject>,
    // A "builder callback" function used to add more reducers
    extraReducers?: (builder: ActionReducerMapBuilder<State>) => void,
    // A preference for the slice reducer's location, used by `combineSlices` and `slice.selectors`. Defaults to `name`.
    reducerPath?: string,
    // An object of selectors, which receive the slice's state as their first parameter.
    selectors?: Record<string, (sliceState: State, ...args: any[]) => any>,
})

initialState​

The initial state value for this slice of state.

This may also be a "lazy initializer" function, which should return an initial state value when called. This will be used whenever the reducer is called with undefined as its state value, and is primarily useful for cases like reading initial state from localStorage.

name​

A string name for this slice of state. Generated action type constants will use this as a prefix.

reducers​

An object containing Redux "case reducer" functions (functions intended to handle a specific action type, equivalent to a single case statement in a switch).

The keys in the object will be used to generate string action type constants, and these will show up in the Redux DevTools Extension when they are dispatched. Also, if any other part of the application happens to dispatch an action with the exact same type string, the corresponding reducer will be run. Therefore, you should give the functions descriptive names.

This object will be passed to createReducer, so the reducers may safely "mutate" the state they are given.

import { createSlice } from '@reduxjs/toolkit'

const counterSlice = createSlice({
  name: 'counter',
  initialState: 0,
  reducers: {
    increment: (state) => state + 1,
  },
})
// Will handle the action type `'counter/increment'`

Customizing Generated Action Creators​

If you need to customize the creation of the payload value of an action creator by means of a prepare callback, the value of the appropriate field of the reducers argument object should be an object instead of a function. This object must contain two properties: reducer and prepare. The value of the reducer field should be the case reducer function while the value of the prepare field should be the prepare callback function:

import { createSlice, nanoid } from '@reduxjs/toolkit'
import type { PayloadAction } from '@reduxjs/toolkit'

interface Item {
  id: string
  text: string
}

const todosSlice = createSlice({
  name: 'todos',
  initialState: [] as Item[],
  reducers: {
    addTodo: {
      reducer: (state, action: PayloadAction<Item>) => {
        state.push(action.payload)
      },
      prepare: (text: string) => {
        const id = nanoid()
        return { payload: { id, text } }
      },
    },
  },
})

The reducers "creator callback" notation​

Alternatively, the reducers field can be a callback which receives a "create" object.

The main benefit of this is that you can create async thunks as part of your slice (though for bundle size reasons, you need a bit of setup for this). Types are also slightly simplified for prepared reducers.

Creator callback for reducers
import { createSlice, nanoid } from '@reduxjs/toolkit'

interface Item {
  id: string
  text: string
}

interface TodoState {
  loading: boolean
  todos: Item[]
}

const todosSlice = createSlice({
  name: 'todos',
  initialState: {
    loading: false,
    todos: [],
  } as TodoState,
  reducers: (create) => ({
    deleteTodo: create.reducer<number>((state, action) => {
      state.todos.splice(action.payload, 1)
    }),
    addTodo: create.preparedReducer(
      (text: string) => {
        const id = nanoid()
        return { payload: { id, text } }
      },
      // action type is inferred from prepare callback
      (state, action) => {
        state.todos.push(action.payload)
      }
    ),
    fetchTodo: create.asyncThunk(
      async (id: string, thunkApi) => {
        const res = await fetch(`myApi/todos?id=${id}`)
        return (await res.json()) as Item
      },
      {
        pending: (state) => {
          state.loading = true
        },
        rejected: (state, action) => {
          state.loading = false
        },
        fulfilled: (state, action) => {
          state.loading = false
          state.todos.push(action.payload)
        },
      }
    ),
  }),
})

export const { addTodo, deleteTodo, fetchTodo } = todosSlice.actions

Create Methods​

create.reducer​

A standard slice case reducer.

Parameters

  • reducer The slice case reducer to use.
create.reducer<Todo>((state, action) => {
  state.todos.push(action.payload)
})

create.preparedReducer​

A prepared reducer, to customize the action creator.

Parameters

The action passed to the case reducer will be inferred from the prepare callback's return.

create.preparedReducer(
  (text: string) => {
    const id = nanoid()
    return { payload: { id, text } }
  },
  (state, action) => {
    state.todos.push(action.payload)
  }
)

create.asyncThunk​

Creates an async thunk instead of an action creator.

Setup

To avoid pulling createAsyncThunk into the bundle size of createSlice by default, some extra setup is required to use create.asyncThunk.

The version of createSlice exported from RTK will throw an error if create.asyncThunk is called.

Instead, import buildCreateSlice and asyncThunkCreator, and create your own version of createSlice:

import { buildCreateSlice, asyncThunkCreator } from '@reduxjs/toolkit'

// name is up to you
export const createSliceWithThunks = buildCreateSlice({
  creators: { asyncThunk: asyncThunkCreator },
})

Then import this createSlice as needed instead of the exported version from RTK.

Parameters

  • payloadCreator The thunk payload creator.
  • config The configuration object. (optional)

The configuration object can contain case reducers for each of the lifecycle actions (pending, fulfilled, and rejected), as well as a settled reducer that will run for both fulfilled and rejected actions (note that this will run after any provided fulfilled/rejected reducers. Conceptually it can be thought of like a finally block.).

Each case reducer will be attached to the slice's caseReducers object, e.g. slice.caseReducers.fetchTodo.fulfilled.

The configuration object can also contain options.

create.asyncThunk(
  async (id: string, thunkApi) => {
    const res = await fetch(`myApi/todos?id=${id}`)
    return (await res.json()) as Item
  },
  {
    pending: (state) => {
      state.loading = true
    },
    rejected: (state, action) => {
      state.error = action.payload ?? action.error
    },
    fulfilled: (state, action) => {
      state.todos.push(action.payload)
    },
    settled: (state, action) => {
      state.loading = false
    }
    options: {
      idGenerator: uuid,
    },
  }
)
note

Typing for the create.asyncThunk works in the same way as createAsyncThunk, with one key difference.

A type for state and/or dispatch cannot be provided as part of the ThunkApiConfig, as this would cause circular types.

Instead, it is necessary to assert the type when needed - getState() as RootState. You may also include an explicit return type for the payload function as well, in order to break the circular type inference cycle.

create.asyncThunk<Todo, string, { rejectValue: { error: string } }>(
  // may need to include an explicit return type
  async (id: string, thunkApi): Promise<Todo> => {
    // Cast types for `getState` and `dispatch` manually
    const state = thunkApi.getState() as RootState
    const dispatch = thunkApi.dispatch as AppDispatch
    try {
      const todo = await fetchTodo()
      return todo
    } catch (e) {
      throw thunkApi.rejectWithValue({
        error: 'Oh no!',
      })
    }
  }
)

For common thunk API configuration options, a withTypes helper is provided:

reducers: (create) => {
  const createAThunk =
    create.asyncThunk.withTypes<{ rejectValue: { error: string } }>()

  return {
    fetchTodo: createAThunk<Todo, string>(async (id, thunkApi) => {
      throw thunkApi.rejectWithValue({
        error: 'Oh no!',
      })
    }),
    fetchTodos: createAThunk<Todo[], string>(async (id, thunkApi) => {
      throw thunkApi.rejectWithValue({
        error: 'Oh no, not again!',
      })
    }),
  }
}

extraReducers​

Conceptually, each slice reducer "owns" its slice of state. There's also a natural correspondance between the update logic defined inside reducers, and the action types that are generated based on those.

However, there are many times that a Redux slice may also need to update its own state in response to action types that were defined elsewhere in the application (such as clearing many different kinds of data when a "user logged out" action is dispatched). This can include action types defined by another createSlice call, actions generated by a createAsyncThunk, RTK Query endpoint matchers, or any other action. In addition, one of the key concepts of Redux is that many slice reducers can independently respond to the same action type.

extraReducers allows createSlice to respond and update its own state in response to other action types besides the types it has generated.

As with the reducers field, each case reducer in extraReducers is wrapped in Immer and may use "mutating" syntax to safely update the state inside.

However, unlike the reducers field, each individual case reducer inside of extraReducers will not generate a new action type or action creator.

If two fields from reducers and extraReducers happen to end up with the same action type string, the function from reducers will be used to handle that action type.

The extraReducers "builder callback" notation​

Similar to createReducer, the extraReducers field uses a "builder callback" notation to define handlers for specific action types, matching against a range of actions, or handling a default case. This is conceptually similar to a switch statement, but with better TS support as it can infer the action type from the provided action creator. It's particularly useful for working with actions produced by createAction and createAsyncThunk.

import { createAction, createSlice, Action } from '@reduxjs/toolkit'
const incrementBy = createAction<number>('incrementBy')
const decrement = createAction('decrement')

interface RejectedAction extends Action {
  error: Error
}

function isRejectedAction(action: Action): action is RejectedAction {
  return action.type.endsWith('rejected')
}

createSlice({
  name: 'counter',
  initialState: 0,
  reducers: {},
  extraReducers: (builder) => {
    builder
      .addCase(incrementBy, (state, action) => {
        // action is inferred correctly here if using TS
      })
      // You can chain calls, or have separate `builder.addCase()` lines each time
      .addCase(decrement, (state, action) => {})
      // You can match a range of action types
      .addMatcher(
        isRejectedAction,
        // `action` will be inferred as a RejectedAction due to isRejectedAction being defined as a type guard
        (state, action) => {}
      )
      // and provide a default case if no other handlers matched
      .addDefaultCase((state, action) => {})
  },
})

See the "Builder Callback Notation" section of the createReducer reference for details on how to use builder.addCase, builder.addMatcher, and builder.addDefault

reducerPath​

Indicates a preference of where the slice should be located. Defaults to name.

This is used by combineSlices and the default generated slice.selectors.

selectors​

A set of selectors that receive the slice state as their first parameter, and any other parameters.

Each selector will have a corresponding key in the resulting selectors object.

Circular types

It's fairly common to have selectors that use other selectors. This is still possible with slice selectors, but defining a selector without a return type can cause a circular type inference problem:

const counterSlice = createSlice({
  name: 'counter',
  initialState: { value: 0 },
  reducers: {},
  selectors: {
    selectValue: (state) => state.value,
    // this creates a cycle, because it's inferring a type from the object we're creating here
    selectTimes: (state, times = 1) =>
      counterSlice.getSelectors().selectValue(state) * times,
  },
})

This cycle can be fixed by providing an explicit return type for the selector:

const counterSlice = createSlice({
  name: 'counter',
  initialState: { value: 0 },
  reducers: {},
  selectors: {
    selectValue: (state) => state.value,
    // explicit return type means cycle is broken
    selectTimes: (state, times = 1): number =>
      counterSlice.getSelectors().selectValue(state) * times,
  },
})

This limitation may be also encountered when using a slice's asyncThunk creator. In the same way, the issue is resolved by explicitly providing a type somewhere in the chain and breaking the cycle.

const counterSlice = createSlice({
  name: 'counter',
  initialState: { value: 0 },
  reducers: (create) => ({
    getCountData: create.asyncThunk(async (_arg, { getState }) => {
      const currentCount = counterSlice.selectors.selectValue(
        getState() as RootState
      )
      // this would cause a circular type, but the type annotation breaks the circle
      const result: Response = await fetch('api/' + currentCount)
      return result.json()
    }),
  }),
  selectors: {
    selectValue: (state) => state.value,
  },
})

Return Value​

createSlice will return an object that looks like:

{
    name: string,
    reducer: ReducerFunction,
    actions: Record<string, ActionCreator>,
    caseReducers: Record<string, CaseReducer>.
    getInitialState: () => State,
    reducerPath: string,
    selectSlice: Selector;
    selectors: Record<string, Selector>,
    getSelectors: (selectState: (rootState: RootState) => State) => Record<string, Selector>
    injectInto: (injectable: Injectable, config?: InjectConfig & { reducerPath?: string }) => InjectedSlice
}

Each function defined in the reducers argument will have a corresponding action creator generated using createAction and included in the result's actions field using the same function name.

The generated reducer function is suitable for passing to the Redux combineReducers function as a "slice reducer".

You may want to consider destructuring the action creators and exporting them individually, for ease of searching for references in a larger codebase.

The functions passed to the reducers parameter can be accessed through the caseReducers return field. This can be particularly useful for testing or direct access to reducers created inline.

Result's function getInitialState provides access to the initial state value given to the slice. If a lazy state initializer was provided, it will be called and a fresh value returned.

injectInto creates an instance of the slice that is aware it's been injected - see combineSlices.

note

The result object is conceptually similar to a "Redux duck" code structure. The actual code structure you use is up to you, but it's worth keeping in mind that actions are not exclusively limited to a single slice. Any part of the reducer logic can (and should!) respond to any dispatched action.

Selectors​

Slice selectors are written to expect the slice's state as their first parameter, but the slice may be located anywhere inside the store's root state.

As a result, there are two ways of getting final selectors:

selectors​

Most commonly, the slice is reliably mounted under its reducerPath.

Following this, the slice has a selectSlice selector attached, which assumes that the slice is located under rootState[slice.reducerPath].

slice.selectors then uses this selector to wrap each of the selectors provided.

import { createSlice } from '@reduxjs/toolkit'

interface CounterState {
  value: number
}

const counterSlice = createSlice({
  name: 'counter',
  initialState: { value: 0 } as CounterState,
  reducers: {
    // omitted
  },
  selectors: {
    selectValue: (sliceState) => sliceState.value,
  },
})

console.log(counterSlice.selectSlice({ counter: { value: 2 } })) // { value: 2 }

const { selectValue } = counterSlice.selectors

console.log(selectValue({ counter: { value: 2 } })) // 2
note

The original selector passed is attached to the wrapped selector as .unwrapped. For example:

import { createSlice, createSelector } from '@reduxjs/toolkit'

interface CounterState {
  value: number
}

const counterSlice = createSlice({
  name: 'counter',
  initialState: { value: 0 } as CounterState,
  reducers: {
    // omitted
  },
  selectors: {
    selectDouble: createSelector(
      (sliceState: CounterState) => sliceState.value,
      (value) => value * 2
    ),
  },
})

const { selectDouble } = counterSlice.selectors

console.log(selectDouble({ counter: { value: 2 } })) // 4
console.log(selectDouble({ counter: { value: 3 } })) // 6
console.log(selectDouble.unwrapped.recomputations) // 2

getSelectors​

slice.getSelectors is called with a single parameter, a selectState callback. This function should receive the store root state (or whatever you expect to call the resulting selectors with) and return the slice state.

const { selectValue } = counterSlice.getSelectors(
  (rootState: RootState) => rootState.aCounter
)

console.log(selectValue({ aCounter: { value: 2 } })) // 2

If no selectState callback is passed, selectors will be returned as is - expecting the slice state as their first parameter (the same as calling slice.getSelectors(state => state)).

const { selectValue } = counterSlice.getSelectors()

console.log(selectValue({ value: 2 })) // 2
note

The slice.selectors object is the equivalent of calling

const { selectValue } = counterSlice.getSelectors(counterSlice.selectSlice)
// or
const { selectValue } = counterSlice.getSelectors(
  (state: RootState) => state[counterSlice.reducerPath]
)

Examples​

import { createSlice, createAction, configureStore } from '@reduxjs/toolkit'
import type { PayloadAction } from '@reduxjs/toolkit'
import { combineReducers } from 'redux'

const incrementBy = createAction<number>('incrementBy')
const decrementBy = createAction<number>('decrementBy')

const counter = createSlice({
  name: 'counter',
  initialState: 0 as number,
  reducers: {
    increment: (state) => state + 1,
    decrement: (state) => state - 1,
    multiply: {
      reducer: (state, action: PayloadAction<number>) => state * action.payload,
      prepare: (value?: number) => ({ payload: value || 2 }), // fallback if the payload is a falsy value
    },
  },
  extraReducers: (builder) => {
    builder.addCase(incrementBy, (state, action) => {
      return state + action.payload
    })
    builder.addCase(decrementBy, (state, action) => {
      return state - action.payload
    })
  },
})

const user = createSlice({
  name: 'user',
  initialState: { name: '', age: 20 },
  reducers: {
    setUserName: (state, action) => {
      state.name = action.payload // mutate the state all you want with immer
    },
  },
  extraReducers: (builder) => {
    builder.addCase(counter.actions.increment, (state, action) => {
      state.age += 1
    })
  },
})

const store = configureStore({
  reducer: {
    counter: counter.reducer,
    user: user.reducer,
  },
})

store.dispatch(counter.actions.increment())
// -> { counter: 1, user: {name : '', age: 21} }
store.dispatch(counter.actions.increment())
// -> { counter: 2, user: {name: '', age: 22} }
store.dispatch(counter.actions.multiply(3))
// -> { counter: 6, user: {name: '', age: 22} }
store.dispatch(counter.actions.multiply())
// -> { counter: 12, user: {name: '', age: 22} }
console.log(counter.actions.decrement.type)
// -> "counter/decrement"
store.dispatch(user.actions.setUserName('eric'))
// -> { counter: 12, user: { name: 'eric', age: 22} }
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