createSelector
Overview
The createSelector
utility from the Reselect library, re-exported for ease of use.
For more details on using createSelector
, see:
- The Reselect API documentation
- React-Redux docs: Hooks API - Using memoizing selectors
- Idiomatic Redux: Using Reselect Selectors for Encapsulation and Performance
- React/Redux Links: Reducers and Selectors
Prior to v0.7, RTK re-exported createSelector
from selectorator
, which
allowed using string keypaths as input selectors. This was removed, as it ultimately did not provide enough benefits, and
the string keypaths made static typing for selectors difficult.
createDraftSafeSelector
In general, we recommend against using selectors inside of reducers:
- Selectors typically expect the entire Redux state object as an argument, while slice reducers only have access to a specific subset of the entire Redux state
- Reselect's
createSelector
relies on reference comparisons to determine if inputs have changed, and if an Immer Proxy-wrapped draft value is passed in to a selector, the selector may see the same reference and think nothing has changed.
However, some users have requested the ability to create selectors that will work correctly inside of Immer-powered reducers. One use case for this might be collecting an ordered set of items when using createEntityAdapter
, such as const orderedTodos = todosSelectors.selectAll(todosState)
, and then using orderedTodos
in the rest of the reducer logic.
Besides re-exporting createSelector
, RTK also exports a wrapped version of createSelector
named createDraftSafeSelector
that allows you to create selectors that can safely be used inside of createReducer
and createSlice
reducers with Immer-powered mutable logic. When used with plain state values, the selector will still memoize normally based on the inputs. But, when used with Immer draft values, the selector will err on the side of recalculating the results, just to be safe.
All selectors created by entityAdapter.getSelectors
are "draft safe" selectors by default.
Example:
const selectSelf = (state: State) => state
const unsafeSelector = createSelector(selectSelf, (state) => state.value)
const draftSafeSelector = createDraftSafeSelector(
selectSelf,
(state) => state.value,
)
// in your reducer:
state.value = 1
const unsafe1 = unsafeSelector(state)
const safe1 = draftSafeSelector(state)
state.value = 2
const unsafe2 = unsafeSelector(state)
const safe2 = draftSafeSelector(state)
After executing that, unsafe1
and unsafe2
will be of the same value, because the memoized selector was
executed on the same object - but safe2
will actually be different from safe1
(with the updated value of 2
),
because the safe selector detected that it was executed on a Immer draft object and recalculated using the current
value instead of returning a cached value.
createDraftSafeSelectorCreator
RTK also exports a createDraftSafeSelectorCreator
function, the "draft safe" equivalent of createSelectorCreator
.
import {
createDraftSafeSelectorCreator,
weakMapMemoize,
} from '@reduxjs/toolkit'
const createWeakMapDraftSafeSelector =
createDraftSafeSelectorCreator(weakMapMemoize)
const selectSelf = (state: State) => state
const draftSafeSelector = createWeakMapDraftSafeSelector(
selectSelf,
(state) => state.value,
)
Defining a Pre-Typed createDraftSelector
As of RTK 2.1, you can define a "pre-typed" version of createDraftSafeSelector
that can have the type for state
built in. This lets you set up those types once, so you don't have to repeat them each time you call createDraftSafeSelector
.
const createTypedDraftSafeSelector =
createDraftSafeSelector.withTypes<RootState>()
Import and use the pre-typed createTypedDraftSafeSelector
function, and it will automatically know that the state
argument is of type RootState
.
Currently this approach only works if input selectors are provided as a single array.
If you pass the input selectors as separate inline arguments, the parameter types of the result function will not be inferred. As a workaround you can either
- Wrap your input selectors in a single array
- You can annotate the parameter types of the result function:
import { createSelector } from 'reselect'
interface Todo {
id: number
completed: boolean
}
interface Alert {
id: number
read: boolean
}
export interface RootState {
todos: Todo[]
alerts: Alert[]
}
export const createTypedDraftSafeSelector =
createDraftSafeSelector.withTypes<RootState>()
const selectTodoIds = createTypedDraftSafeSelector(
// Type of `state` is set to `RootState`, no need to manually set the type
(state) => state.todos,
// ❌ Known limitation: Parameter types are not inferred in this scenario
// so you will have to manually annotate them.
(todos: Todo[]) => todos.map(({ id }) => id),
)