reselect/website/docs/api/createSelector.mdx

---
id: createSelector
title: createSelector
sidebar_label: createSelector
hide_title: true
description: 'createSelector'
---

import Tabs from '@theme/Tabs'
import TabItem from '@theme/TabItem'

import { InternalLinks } from '@site/src/components/InternalLinks'

# `createSelector`

Accepts one or more "<InternalLinks.InputSelectors />" (either as separate arguments or a single array),
a single "<InternalLinks.ResultFunction />", and an optional options object, and
generates a memoized selector function.

The **Redux docs usage page on [Deriving Data with Selectors](https://redux.js.org/usage/deriving-data-selectors)** covers the purpose and motivation for selectors, why memoized selectors are useful, and typical Reselect usage patterns.

```ts no-emit
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 <InternalLinks.InputSelectors />, can also be passed as separate arguments.          |
| `resultFunc`             | A function that takes the results of the <InternalLinks.InputSelectors /> as separate arguments. |
| `createSelectorOptions?` | An optional options object that allows for further customization per selector.                   |

## Returns

A memoized <InternalLinks.OutputSelector />.

### Output Selector Fields

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

| Name                            | Description                                                                                                                                                                                                                                                                |
| ------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `resultFunc`                    | The final function passed to <InternalLinks.CreateSelector />.                                                                                                                                                                                                             |
| `memoizedResultFunc`            | The memoized version of `resultFunc`.                                                                                                                                                                                                                                      |
| `lastResult`                    | Returns the last result calculated by `memoizedResultFunc`.                                                                                                                                                                                                                |
| `dependencies`                  | The array of the input selectors used by <InternalLinks.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 <InternalLinks.InputSelectors /> (<InternalLinks.Dependencies text={<code>dependencies</code>} />) have been recalculated. This is distinct from `recomputations`, which tracks the recalculations of the <InternalLinks.ResultFunction />. |
| `resetDependencyRecomputations` | Resets the `dependencyRecomputations` count to 0.                                                                                                                                                                                                                          |
| `memoize`                       | Function used to memoize the `resultFunc`.                                                                                                                                                                                                                                 |
| `argsMemoize`                   | Function used to memoize the arguments passed into the <InternalLinks.OutputSelector />.                                                                                                                                                                                   |

## Type Parameters

| Name                          | Description                                                                                                                                                                                                          |
| :---------------------------- | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `InputSelectors`              | The type of the <InternalLinks.InputSelectors /> array.                                                                                                                                                              |
| `Result`                      | The return type of the <InternalLinks.ResultFunction /> as well as the <InternalLinks.OutputSelector />.                                                                                                             |
| `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 <InternalLinks.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 <InternalLinks.CreateSelectorCreator />. |

## Defining a Pre-Typed <InternalLinks.CreateSelector>`createSelector`</InternalLinks.CreateSelector>

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

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

{/* START: createSelector/withTypes.ts */}

<Tabs
  groupId='language'
  defaultValue='ts'
  values={[
    {label: 'TypeScript', value: 'ts'},
    {label: 'JavaScript', value: 'js'},
  ]}>
  <TabItem value='ts'>

```ts title="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
    // highlight-start
    state => state.todos
    // highlight-end
  ],
  todos => todos.map(({ id }) => id)
)
```

  </TabItem>
  <TabItem value='js'>

```js title="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
    // highlight-start
    state => state.todos
    // highlight-end
  ],
  todos => todos.map(({ id }) => id)
)
```

  </TabItem>
</Tabs>

{/* END: createSelector/withTypes.ts */}

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

:::danger 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

1. Wrap your input selectors in a single array
2. You can annotate the parameter types of the result function:

{/* START: createSelector/annotateResultFunction.ts */}

<Tabs
  groupId='language'
  defaultValue='ts'
  values={[
    {label: 'TypeScript', value: 'ts'},
    {label: 'JavaScript', value: 'js'},
  ]}>
  <TabItem value='ts'>

```ts title="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.
  // highlight-start
  (todos: Todo[]) => todos.map(({ id }) => id)
  // highlight-end
)
```

  </TabItem>
  <TabItem value='js'>

```js title="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.
  // highlight-start
  todos => todos.map(({ id }) => id)
  // highlight-end
)
```

  </TabItem>
</Tabs>

{/* END: createSelector/annotateResultFunction.ts */}

:::

:::tip

You can also use this API with <InternalLinks.CreateSelectorCreator /> to create a pre-typed custom selector creator:

{/* START: createSelector/createAppSelector.ts */}

<Tabs
  groupId='language'
  defaultValue='ts'
  values={[
    {label: 'TypeScript', value: 'ts'},
    {label: 'JavaScript', value: 'js'},
  ]}>
  <TabItem value='ts'>

```ts title="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
    // highlight-start
    state => state.alerts
    // highlight-end
  ],
  alerts => alerts.filter(({ read }) => read)
)
```

  </TabItem>
  <TabItem value='js'>

```js title="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
    // highlight-start
    state => state.alerts
    // highlight-end
  ],
  alerts => alerts.filter(({ read }) => read)
)
```

  </TabItem>
</Tabs>

{/* END: createSelector/createAppSelector.ts */}

:::