# Reselect [![npm package][npm-badge]][npm][![Coveralls][coveralls-badge]][coveralls][![GitHub Workflow Status][build-badge]][build]![TypeScript][typescript-badge] 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 [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 [Deriving Data with Selectors](https://redux.js.org/usage/deriving-data-selectors)** covers the purpose and motivation for selectors, why memoized selectors are useful, typical Reselect usage patterns, and using selectors with [React-Redux]. ## Installation ### Redux Toolkit While Reselect is not exclusive to [Redux], it is already included by default in [the official Redux Toolkit package](https://redux-toolkit.js.org) - no further installation needed. ```ts import { createSelector } from '@reduxjs/toolkit' ``` ### Standalone For standalone usage, install the `reselect` package: ```bash # NPM npm install reselect # Yarn yarn add reselect ``` --- ## Documentation The Reselect docs are available at **https://reselect.js.org**, and include usage guides and API references: - [**Introduction**](#https://reselect.js.org/introduction/getting-started) - [**How Does Reselect Work?**](#https://reselect.js.org/introduction/how-does-reselect-work) - **API Reference**: - **[`createSelector`]** - **[`createSelectorCreator`]** - **[`createStructuredSelector`]** - [**Development-Only Stability Checks**](#https://reselect.js.org/api/development-only-stability-checks) - **[`lruMemoize`]** - **[`weakMapMemoize`]** - [**FAQ**](#https://reselect.js.org/FAQ) ## 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] 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 [this CodeSandbox](https://codesandbox.io/s/reselect-example-g3k9gf?file=/src/index.js): ```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 ``` 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 - [**Selector Function**](#selector-function): A function that accepts one or more JavaScript values as arguments, and derives a result. When used with [Redux], the first argument is typically the entire Redux store state. - [**input selectors**](#input-selectors): Basic selector functions used as building blocks for creating a memoized selector. They are passed as the first argument(s) to [`createSelector`], and are called with all selector arguments. They are responsible for extracting and providing necessary values to the [result function]. - [**Output Selector**](#output-selector): The actual memoized selectors created by [`createSelector`]. - [**Result Function**](#result-function): The function that comes after the [input selectors]. It takes the [input selectors]' return values as arguments and returns a result. - [**`Dependencies`**](#dependencies): Same as [input selectors]. They are what the [output selector] "depends" on. The below example serves as a visual aid: ```ts const outputSelector = createSelector( [inputSelector1, inputSelector2, inputSelector3], // synonymous with `dependencies`. resultFunc // Result function ) ``` --- ## What's New in 5.0.0? Version 5.0.0 introduces several new features and improvements: - **Customization Enhancements**: - Added the ability to pass an options object to [`createSelectorCreator`], allowing for customized `memoize` and `argsMemoize` functions, alongside their respective options (`memoizeOptions` and `argsMemoizeOptions`). - The [`createSelector`] function now supports direct customization of `memoize` and `argsMemoize` within its options object. - **Memoization Functions**: - Introduced new experimental memoization functions: `weakMapMemoize` and `unstable_autotrackMemoize`. - Incorporated `memoize` and `argsMemoize` into the [output selector fields] for debugging purposes. - **TypeScript Support and Performance**: - Discontinued support for TypeScript versions below 4.7, aligning with modern TypeScript features. - Significantly improved TypeScript performance for nesting [output selectors][output selector]. The nesting limit has increased from approximately 8 to around 30 [output selectors][output selector], greatly reducing the occurrence of the infamous `Type instantiation is excessively deep and possibly infinite` error. - **Selector API Enhancements**: - Removed the second overload of `createStructuredSelector` due to its susceptibility to runtime errors. - **Additional Functionalities**: - Added `dependencyRecomputations` and `resetDependencyRecomputations` to the [output selector fields]. These additions provide greater control and insight over [input selectors], complementing the new `argsMemoize` API. - Introduced `inputStabilityCheck`, a development tool that runs the [input selectors] twice using the same arguments and triggers a warning If they return differing results for the same call. - Introduced `identityFunctionCheck`, a development tool that checks to see if the [result function] returns its own input. These updates aim to enhance flexibility, performance, and developer experience. For detailed usage and examples, refer to the updated documentation sections for each feature. - **Breaking Changes**: - Removed `ParametricSelector` and `OutputParametricSelector` types. Their functionalities are now integrated into `Selector` and `OutputSelector` respectively, which inherently support additional parameters.

--- ## License MIT ## References

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Originally inspired by getters in [NuclearJS](https://github.com/optimizely/nuclear-js.git), [subscriptions](https://github.com/Day8/re-frame#just-a-read-only-cursor) in [re-frame](https://github.com/Day8/re-frame) and this [proposal](https://github.com/reduxjs/redux/pull/169) from [speedskater](https://github.com/speedskater). [typescript-badge]: https://img.shields.io/badge/TypeScript-v4%2E7%2B-007ACC?style=for-the-badge&logo=TypeScript&logoColor=black&labelColor=blue&color=gray [build-badge]: https://img.shields.io/github/actions/workflow/status/reduxjs/reselect/build-and-test-types.yml?branch=master&style=for-the-badge [build]: https://github.com/reduxjs/reselect/actions/workflows/build-and-test-types.yml [npm-badge]: https://img.shields.io/npm/v/reselect.svg?style=for-the-badge [npm]: https://www.npmjs.org/package/reselect [coveralls-badge]: https://img.shields.io/coveralls/reduxjs/reselect/master.svg?style=for-the-badge [coveralls]: https://coveralls.io/github/reduxjs/reselect [Redux]: https://redux.js.org 'Redux' [React]: https://react.dev 'React' [React-Redux]: https://react-redux.js.org 'React-Redux' [selector]: #selector-function 'Selector Function' [input selectors]: #input-selectors 'Input Selectors' [output selector]: #output-selector 'Output Selector' [result function]: #result-function 'Result Function' [output selector fields]: https://reselect.js.org/api/createSelector#output-selector-fields 'Output Selector Fields' [`createSelector`]: https://reselect.js.org/api/createSelector 'createSelector' [`createSelectorCreator`]: https://reselect.js.org/api/createSelectorCreator 'createSelectorCreator' [`lruMemoize`]: https://reselect.js.org/api/lruMemoize 'lruMemoize' [`weakMapMemoize`]: https://reselect.js.org/api/weakMapMemoize 'weakMapMemoize' [`createStructuredSelector`]: https://reselect.js.org/api/createStructuredSelector 'createStructuredSelector'

	Stars	Issues	Version	Updated	Created
lodash.memoize	58,888	70	4.1.2	8 years ago	11 years ago
redux-observable	7,850	70	3.0.0-rc.2	4 months ago	8 years ago
redux-saga			1.3.0	4 months ago	8 years ago
redux-thunk	17,733	2	3.1.0	5 months ago	9 years ago
reselect	18,997	40	5.1.0	4 months ago	9 years ago

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