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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 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.


Redux Toolkit

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'


For standalone usage, install the reselect package:

npm install reselect

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 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

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

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.


The below example serves as a visual aid:

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