createSelector

createSelector sits at the heart of memoized selectors in Redux. This guide walks through the concept step by step, with examples, a cheatsheet, and common mistakes to avoid.

createSelector Overview

createSelector lets you work with Redux in a way that stays clear, repeatable, and easy to scale. Instead of ad-hoc steps, you follow a pattern that other developers recognise immediately.

The key is to keep createselector focused and predictable. Start from the minimal example here, then add only the complexity your situation actually needs.

import { createSelector } from '@reduxjs/toolkit';

const selectItems = (state) => state.cart.items;

export const selectCartTotal = createSelector(
  [selectItems],
  (items) => items.reduce((sum, i) => sum + i.price * i.qty, 0),
);

createSelector memoizes derived data so it only recomputes when its inputs change.

createSelector Example

const slice = createSlice({ name, initialState, reducers });
const store = configureStore({ reducer: { key: slice.reducer } });
// dispatch(slice.actions.something())

Start from a minimal createSelector example and grow it only as needed.
Keep things explicit so createSelector behaves the same for everyone on the team.
Name things clearly so teammates understand your createSelector at a glance.
Verify createSelector works as expected before relying on it in important work.

Redux Cheatsheet

Quick Redux Toolkit reference related to createselector.

Concept	Example	Purpose
Store	`configureStore({ reducer })`	Hold app state
Slice	`createSlice({ name, reducers })`	State + actions together
Read state	`useSelector((s) => s.x)`	Get data in components
Dispatch	`useDispatch()`	Send actions
Async	`createAsyncThunk(...)`	Handle side effects
Data fetching	`createApi(...)`	RTK Query endpoints
Derived data	`createSelector(...)`	Memoized computations

How createSelector Works in Redux

createSelector follows Redux's predictable data flow: components dispatch actions, reducers compute the next state from the previous state and the action, and subscribed components re-render.

createSelector memoizes derived data so it only recomputes when its inputs change.

State lives in a single, read-only store.
Actions are the only way to describe a change.
Reducers are pure functions that return the next state.
Redux Toolkit removes most boilerplate with slices and thunks.

Practical Guidance for createSelector

In modern apps, createselector should use Redux Toolkit rather than hand-written Redux. Keep state minimal, colocate logic in slices, and use RTK Query for server data.

Concern	Recommendation
Boilerplate	Use Redux Toolkit (createSlice)
Server data	Prefer RTK Query over manual thunks
Performance	Memoize selectors with createSelector
Types	Use typed useAppSelector/useAppDispatch hooks

Common Mistakes

Skipping edge cases and error handling when using createselector.
Not verifying the result of createselector before moving on.
Over-complicating createselector before you actually need the extra flexibility.
Ignoring documentation, which makes createselector hard for the next person to follow.

Key Takeaways

createSelector is a core part of working effectively with Redux.
Start small and keep createselector focused on a single goal.
Apply consistent patterns so createselector scales across your project.
Practise and document createselector to keep your workflow maintainable.

Pro Tip

When you get stuck on createselector, reduce it to the smallest example first — most Redux problems become obvious once the noise is gone.