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 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 covers the purpose and motivation for selectors, why memoized selectors are useful, typical Reselect usage patterns, and using selectors with React-Redux.
Installation
While Reselect is not exclusive to Redux, it is already included by default in the official Redux Toolkit package - no further installation needed.
import { createSelector } from '@reduxjs/toolkit'
Standalone
For standalone usage, install the reselect package:
# NPM
npm install reselect
# Yarn
yarn add reselect
# pnpm
pnpm add reselect
Documentation
The Reselect docs are available at https://reselect.js.org, and include usage guides and API references:
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:
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
The below example serves as a visual aid:
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 a new default memoization function:
weakMapMemoize
- 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. The nesting limit has increased from approximately 8 to around 30 output selectors, 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
Click to Expand
Originally inspired by getters in NuclearJS, subscriptions in re-frame and this proposal from speedskater.
一个用来创建带记忆化(memoized)选择器函数的小型库,可以从你的 state 中派生出计算后的数据。它最出名的搭配是 Redux:让 store 只保留尽可能少的状态,不过它同样适用于任何普通的 JS 不可变数据。