`createSelector`

Accepts one or more "input selectors" (either as separate arguments or a single array), a single "result function", and an optional options object, and generates a memoized selector function.

The Redux docs usage page on Deriving Data with Selectors covers the purpose and motivation for selectors, why memoized selectors are useful, and typical Reselect usage patterns.

const selectTodosByCategory = createSelector(
  [
    // Pass input selectors with typed arguments
    (state: RootState) => state.todos,
    (state: RootState, category: string) => category,
  ],
  // Extracted values are passed to the result function for recalculation
  (todos, category) => {
    return todos.filter(t => t.category === category)
  },
)

Parameters

Name	Description
`inputSelectors`	An array of input selectors, can also be passed as separate arguments.
`resultFunc`	A function that takes the results of the input selectors as separate arguments.
`createSelectorOptions?`	An optional options object that allows for further customization per selector.

Returns

A memoized output selector.

Output Selector Fields

The output selectors created by createSelector have several additional properties attached to them:

Name	Description
`resultFunc`	The final function passed to `createSelector`.
`memoizedResultFunc`	The memoized version of `resultFunc`.
`lastResult`	Returns the last result calculated by `memoizedResultFunc`.
`dependencies`	The array of the input selectors used by `createSelector` to compose `resultFunc`.
`recomputations`	Counts the number of times `memoizedResultFunc` has been recalculated.
`resetRecomputations`	Resets the count of `recomputations` count to 0.
`dependencyRecomputations`	Counts the number of times the input selectors (`dependencies`) have been recalculated. This is distinct from `recomputations`, which tracks the recalculations of the result function.
`resetDependencyRecomputations`	Resets the `dependencyRecomputations` count to 0.
`memoize`	Function used to memoize the `resultFunc`.
`argsMemoize`	Function used to memoize the arguments passed into the output selector.

Type Parameters

Name	Description
`InputSelectors`	The type of the input selectors array.
`Result`	The return type of the result function as well as the output selector.
`OverrideMemoizeFunction`	The type of the optional `memoize` function that could be passed into the options object to override the original `memoize` function that was initially passed into `createSelectorCreator`.
`OverrideArgsMemoizeFunction`	The type of the optional `argsMemoize` function that could be passed into the options object to override the original `argsMemoize` function that was initially passed into `createSelectorCreator`.

Defining a Pre-Typed `createSelector`

As of Reselect 5.1.0, you can create a "pre-typed" version of createSelector where the state type is predefined. This allows you to set the state type once, eliminating the need to specify it with every createSelector call.

To do this, you can call createSelector.withTypes<StateType>():

TypeScript
JavaScript

createSelector/withTypes.ts
import { createSelector } from 'reselect'

export interface RootState {
  todos: { id: number; completed: boolean }[]
  alerts: { id: number; read: boolean }[]
}

export const createAppSelector = createSelector.withTypes<RootState>()

const selectTodoIds = createAppSelector(
  [
    // Type of `state` is set to `RootState`, no need to manually set the type
    state => state.todos,
  ],
  todos => todos.map(({ id }) => id),
)

createSelector/withTypes.js
import { createSelector } from 'reselect'

export const createAppSelector = createSelector.withTypes()

const selectTodoIds = createAppSelector(
  [
    // Type of `state` is set to `RootState`, no need to manually set the type
    state => state.todos,
  ],
  todos => todos.map(({ id }) => id),
)

Import and use the pre-typed createAppSelector instead of the original, and the type for state will be used automatically.

Known Limitations

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:

TypeScript
JavaScript

createSelector/annotateResultFunction.ts
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 createAppSelector = createSelector.withTypes<RootState>()

const selectTodoIds = createAppSelector(
  // 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),
)

createSelector/annotateResultFunction.js
import { createSelector } from 'reselect'

export const createAppSelector = createSelector.withTypes()

const selectTodoIds = createAppSelector(
  // 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 => todos.map(({ id }) => id),
)

tip

You can also use this API with createSelectorCreator to create a pre-typed custom selector creator:

TypeScript
JavaScript

createSelector/createAppSelector.ts
import microMemoize from 'micro-memoize'
import { shallowEqual } from 'react-redux'
import { createSelectorCreator, lruMemoize } from 'reselect'

export interface RootState {
  todos: { id: number; completed: boolean }[]
  alerts: { id: number; read: boolean }[]
}

export const createAppSelector = createSelectorCreator({
  memoize: lruMemoize,
  argsMemoize: microMemoize,
  memoizeOptions: {
    maxSize: 10,
    equalityCheck: shallowEqual,
    resultEqualityCheck: shallowEqual,
  },
  argsMemoizeOptions: {
    isEqual: shallowEqual,
    maxSize: 10,
  },
  devModeChecks: {
    identityFunctionCheck: 'never',
    inputStabilityCheck: 'always',
  },
}).withTypes<RootState>()

const selectReadAlerts = createAppSelector(
  [
    // Type of `state` is set to `RootState`, no need to manually set the type
    state => state.alerts,
  ],
  alerts => alerts.filter(({ read }) => read),
)

createSelector/createAppSelector.js
import microMemoize from 'micro-memoize'
import { shallowEqual } from 'react-redux'
import { createSelectorCreator, lruMemoize } from 'reselect'

export const createAppSelector = createSelectorCreator({
  memoize: lruMemoize,
  argsMemoize: microMemoize,
  memoizeOptions: {
    maxSize: 10,
    equalityCheck: shallowEqual,
    resultEqualityCheck: shallowEqual,
  },
  argsMemoizeOptions: {
    isEqual: shallowEqual,
    maxSize: 10,
  },
  devModeChecks: {
    identityFunctionCheck: 'never',
    inputStabilityCheck: 'always',
  },
}).withTypes()

const selectReadAlerts = createAppSelector(
  [
    // Type of `state` is set to `RootState`, no need to manually set the type
    state => state.alerts,
  ],
  alerts => alerts.filter(({ read }) => read),
)

createSelector

Parameters​

Returns​

Output Selector Fields​

Type Parameters​

Defining a Pre-Typed createSelector​