reselect/website/docs/introduction/getting-started.mdx

---
id: getting-started
title: Getting Started
sidebar_label: Getting Started
sidebar_position: 1
hide_title: true
description: 'Getting Started'
---

import { InternalLinks } from '@site/src/components/InternalLinks'
import { ExternalLinks } from '@site/src/components/ExternalLinks'
import PackageTabs from '@site/src/components/PackageManagerTabs'
import Tabs from '@theme/Tabs'
import TabItem from '@theme/TabItem'
import Link from '@docusaurus/Link'

# Getting Started with Reselect

A library for creating memoized "selector" functions. Commonly used with Redux, but usable with any plain JS immutable data as well.

- Selectors can compute derived data, allowing <ExternalLinks.Redux /> to store the minimal possible state.
- Selectors are efficient. A selector is not recomputed unless one of its arguments changes.
- Selectors are composable. They can be used as input to other selectors.

The **Redux docs usage page on <Link to="https://redux.js.org/usage/deriving-data-selectors">Deriving Data with Selectors</Link>** covers the purpose and motivation for selectors, why memoized selectors are useful, typical Reselect usage patterns, and using selectors with <ExternalLinks.ReactRedux />.

## Installation

### Redux Toolkit

While Reselect is not exclusive to <ExternalLinks.Redux />, it is already included by default in <ExternalLinks.ReduxToolkit text='the official Redux Toolkit package' /> - no further installation needed.

```ts
import { createSelector } from '@reduxjs/toolkit'
```

### Standalone

For standalone usage, install the `reselect` package:

<PackageTabs />

---

## Basic Usage

Reselect exports a `createSelector` API, which generates memoized selector functions. `createSelector` accepts one or more input selectors, which extract values from arguments, and a result function that receives the extracted values and should return a derived value. If the generated output selector is called multiple times, the output will only be recalculated when the extracted values have changed.

You can play around with the following **example** in <Link to='https://codesandbox.io/s/reselect-example-g3k9gf?file=/src/index.js'>this CodeSandbox</Link>:

{/* START: basicUsage.ts */}

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

```ts title="basicUsage.ts"
import { createSelector } from 'reselect'

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

const state: RootState = {
  todos: [
    { id: 0, completed: false },
    { id: 1, completed: true }
  ],
  alerts: [
    { id: 0, read: false },
    { id: 1, read: true }
  ]
}

const selectCompletedTodos = (state: RootState) => {
  console.log('selector ran')
  return state.todos.filter(todo => todo.completed === true)
}

selectCompletedTodos(state) // selector ran
selectCompletedTodos(state) // selector ran
selectCompletedTodos(state) // selector ran

const memoizedSelectCompletedTodos = createSelector(
  [(state: RootState) => state.todos],
  todos => {
    console.log('memoized selector ran')
    return todos.filter(todo => todo.completed === true)
  }
)

memoizedSelectCompletedTodos(state) // memoized selector ran
memoizedSelectCompletedTodos(state)
memoizedSelectCompletedTodos(state)

console.log(selectCompletedTodos(state) === selectCompletedTodos(state)) //=> false

console.log(
  memoizedSelectCompletedTodos(state) === memoizedSelectCompletedTodos(state)
) //=> true
```

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

```js title="basicUsage.js"
import { createSelector } from 'reselect'

const state = {
  todos: [
    { id: 0, completed: false },
    { id: 1, completed: true }
  ],
  alerts: [
    { id: 0, read: false },
    { id: 1, read: true }
  ]
}

const selectCompletedTodos = state => {
  console.log('selector ran')
  return state.todos.filter(todo => todo.completed === true)
}

selectCompletedTodos(state) // selector ran
selectCompletedTodos(state) // selector ran
selectCompletedTodos(state) // selector ran

const memoizedSelectCompletedTodos = createSelector(
  [state => state.todos],
  todos => {
    console.log('memoized selector ran')
    return todos.filter(todo => todo.completed === true)
  }
)

memoizedSelectCompletedTodos(state) // memoized selector ran
memoizedSelectCompletedTodos(state)
memoizedSelectCompletedTodos(state)

console.log(selectCompletedTodos(state) === selectCompletedTodos(state)) //=> false

console.log(
  memoizedSelectCompletedTodos(state) === memoizedSelectCompletedTodos(state)
) //=> true
```

  </TabItem>
</Tabs>

{/* END: basicUsage.ts */}

As you can see from the example above, `memoizedSelectCompletedTodos` does not run the second or third time, but we still get the same return value as last time.

In addition to skipping unnecessary recalculations, `memoizedSelectCompletedTodos` returns the existing result reference if there is no recalculation. This is important for libraries like React-Redux or React that often rely on reference equality checks to optimize UI updates.

---

## Terminology

- <InternalLinks.Selector text={<b>Selector Function</b>} />
  : A function that accepts one or more JavaScript values as arguments, and derives
  a result. When used with <ExternalLinks.Redux />, the first argument is typically
  the entire Redux store state.
- <InternalLinks.InputSelectors text={<b>Input Selectors</b>} />
  : Basic selector functions used as building blocks for creating a memoized selector.
  They are passed as the first argument(s) to <InternalLinks.CreateSelector />, and
  are called with all selector arguments. They are responsible for extracting and
  providing necessary values to the <InternalLinks.ResultFunction />.
- <InternalLinks.OutputSelector text={<b>Output Selector</b>} />
  : The actual memoized selectors created by <InternalLinks.CreateSelector />.
- <InternalLinks.ResultFunction text={<b>Result Function</b>} />
  : The function that comes after the <InternalLinks.InputSelectors />
  . It takes the <InternalLinks.InputSelectors />' return values as arguments
  and returns a result.
- <InternalLinks.Dependencies text={<b>Dependencies</b>} />
  : Same as <InternalLinks.InputSelectors />
  . They are what the <InternalLinks.OutputSelector /> "depends" on.

The below example serves as a visual aid:

```ts
const outputSelector = createSelector(
  [inputSelector1, inputSelector2, inputSelector3], // synonymous with `dependencies`.
  resultFunc // Result function
)
```