What is a Redux Reducer?

This tutorial will teach you "how Redux works", as well as why these patterns exist. Fair warning though - learning the concepts is different from putting them into practice in actual apps.

The initial code will be less concise than the way we suggest writing real app code, but writing it out long-hand is the best way to learn. Once you understand how everything fits together, we'll look at using Redux Toolkit to simplify things. Redux Toolkit is the recommended way to build production apps with Redux, and is built on all of the concepts that we will look at throughout this tutorial. Once you understand the core concepts covered here, you'll understand how to use Redux Toolkit more efficiently.

:::info

If you're looking to learn more about how Redux is used to write real-world applications, please see:

• The "Modern Redux" page in this tutorial, which shows how to convert the low-level examples from earlier sections into the modern patterns we do recommend for real-world usage
• The "Redux Essentials" tutorial, which teaches "how to use Redux, the right way" for real-world apps, using our latest recommended patterns and practices.

:::

We've tried to keep these explanations beginner-friendly, but we do need to make some assumptions about what you know already so that we can focus on explaining Redux itself. This tutorial assumes that you know:

:::important Prerequisites

• Familiarity with HTML & CSS.
• Familiarity with ES6 syntax and features
• Understanding of the array and object spread operators
• Knowledge of React terminology: JSX, State, Function Components, Props, and Hooks
• Knowledge of asynchronous JavaScript and making AJAX requests

:::

If you're not already comfortable with those topics, we encourage you to take some time to become comfortable with them first, and then come back to learn about Redux. We'll be here when you're ready!

Finally, you should make sure that you have the React and Redux DevTools extensions installed in your browser:

• React DevTools Extension:

  • React DevTools Extension for Chrome
  • React DevTools Extension for Firefox

• Redux DevTools Extension:

  • Redux DevTools Extension for Chrome
  • Redux DevTools Extension for Firefox

What is Redux?

It helps to understand what this "Redux" thing is in the first place. What does it do? What problems does it help me solve? Why would I want to use it?

Redux is a pattern and library for managing and updating application state, using events called "actions". It serves as a centralized store for state that needs to be used across your entire application, with rules ensuring that the state can only be updated in a predictable fashion.

Why Should I Use Redux?

Redux helps you manage "global" state - state that is needed across many parts of your application.

The patterns and tools provided by Redux make it easier to understand when, where, why, and how the state in your application is being updated, and how your application logic will behave when those changes occur. Redux guides you towards writing code that is predictable and testable, which helps give you confidence that your application will work as expected.

When Should I Use Redux?

Redux helps you deal with shared state management, but like any tool, it has tradeoffs. There are more concepts to learn, and more code to write. It also adds some indirection to your code, and asks you to follow certain restrictions. It's a trade-off between short term and long term productivity.

Redux is more useful when:

• You have large amounts of application state that are needed in many places in the app
• The app state is updated frequently over time
• The logic to update that state may be complex
• The app has a medium or large-sized codebase, and might be worked on by many people

Not all apps need Redux. Take some time to think about the kind of app you're building, and decide what tools would be best to help solve the problems you're working on.

:::info Want to Know More?

If you're not sure whether Redux is a good choice for your app, these resources give some more guidance:

• When (and when not) to reach for Redux
• The Tao of Redux, Part 1 - Implementation and Intent
• Redux FAQ: When should I use Redux?
• You Might Not Need Redux

:::

Redux Libraries and Tools

Redux is a small standalone JS library. However, it is commonly used with several other packages:

React-Redux

Redux can integrate with any UI framework, and is most frequently used with React. React-Redux is our official package that lets your React components interact with a Redux store by reading pieces of state and dispatching actions to update the store.

Redux Toolkit

Redux Toolkit is our recommended approach for writing Redux logic. It contains packages and functions that we think are essential for building a Redux app. Redux Toolkit builds in our suggested best practices, simplifies most Redux tasks, prevents common mistakes, and makes it easier to write Redux applications.

Redux DevTools Extension

The Redux DevTools Extension shows a history of the changes to the state in your Redux store over time. This allows you to debug your applications effectively, including using powerful techniques like "time-travel debugging".

Redux Basics

Now that you know what Redux is, let's briefly look at the pieces that make up a Redux app and how it works.

:::info

The rest of the description on this page focuses solely on the Redux core library (the redux package). We'll talk about the other Redux-related packages as we go through the rest of the tutorial.

:::

The Redux Store

The center of every Redux application is the store. A "store" is a container that holds your application's global state.

A store is a JavaScript object with a few special functions and abilities that make it different than a plain global object:

• You must never directly modify or change the state that is kept inside the Redux store
• Instead, the only way to cause an update to the state is to create a plain action object that describes "something that happened in the application", and then dispatch the action to the store to tell it what happened.
• When an action is dispatched, the store runs the root reducer function, and lets it calculate the new state based on the old state and the action
• Finally, the store notifies subscribers that the state has been updated so the UI can be updated with the new data.

Redux Core Example App

Let's look at a minimal working example of a Redux app - a small counter application:

Because Redux is a standalone JS library with no dependencies, this example is written by only loading a single script tag for the Redux library, and uses basic JS and HTML for the UI. In practice, Redux is normally used by installing the Redux packages from NPM, and the UI is created using a library like React.

:::info

Part 5: UI and React shows how to use Redux and React together.

:::

Let's break this example down into its separate parts to see what's happening.

State, Actions, and Reducers

We start by defining an initial state value to describe the application:

//// Define an initial state value for the appconst initialState = {value: 0};

For this app, we're going to track a single number with the current value of our counter.

Redux apps normally have a JS object as the root piece of the state, with other values inside that object.

Then, we define a reducer function. The reducer receives two arguments, the current state and an action object describing what happened. When the Redux app starts up, we don't have any state yet, so we provide the initialState as the default value for this reducer:

//// Create a "reducer" function that determines what the new state// should be when something happens in the appfunction counterReducer(state = initialState, action) {// Reducers usually look at the type of action that happened// to decide how to update the stateswitch (action.type) {case 'counter/incremented':return { ...state, value: state.value + 1 };case 'counter/decremented':return { ...state, value: state.value - 1 };default:// If the reducer doesn't care about this action type,// return the existing state unchangedreturn state;}}

Action objects always have a type field, which is a string you provide that acts as a unique name for the action. The type should be a readable name so that anyone who looks at this code understands what it means. In this case, we use the word 'counter' as the first half of our action type, and the second half is a description of "what happened". In this case, our 'counter' was 'incremented', so we write the action type as 'counter/incremented'.

Based on the type of the action, we either need to return a brand-new object to be the new state result, or return the existing state object if nothing should change. Note that we update the state immutably by copying the existing state and updating the copy, instead of modifying the original object directly.

Store

Now that we have a reducer function, we can create a store instance by calling the Redux library createStore API.

//// Create a new Redux store with the `createStore` function,// and use the `counterReducer` for the update logicconst store = Redux.createStore(counterReducer);

We pass the reducer function to createStore, which uses the reducer function to generate the initial state, and to calculate any future updates.

UI

In any application, the user interface will show existing state on screen. When a user does something, the app will update its data and then redraw the UI with those values.

//// Our "user interface" is some text in a single HTML elementconst valueEl = document.getElementById('value');// Whenever the store state changes, update the UI by// reading the latest store state and showing new datafunction render() {const state = store.getState();
    valueEl.innerHTML = state.value.toString();}// Update the UI with the initial datarender();// And subscribe to redraw whenever the data changes in the future
store.subscribe(render);

In this small example, we're only using some basic HTML elements as our UI, with a single <div> showing the current value.

So, we write a function that knows how to get the latest state from the Redux store using the store.getState() method, then takes that value and updates the UI to show it.

The Redux store lets us call store.subscribe() and pass a subscriber callback function that will be called every time the store is updated. So, we can pass our render function as the subscriber, and know that each time the store updates, we can update the UI with the latest value.

Redux itself is a standalone library that can be used anywhere. This also means that it can be used with any UI layer.

Dispatching Actions

Finally, we need to respond to user input by creating action objects that describe what happened, and dispatching them to the store. When we call store.dispatch(action), the store runs the reducer, calculates the updated state, and runs the subscribers to update the UI.

//// Handle user inputs by "dispatching" action objects,// which should describe "what happened" in the app
document.getElementById('increment').addEventListener('click', function () {
    store.dispatch({ type: 'counter/incremented' });});

document.getElementById('decrement').addEventListener('click', function () {
    store.dispatch({ type: 'counter/decremented' });});

document.getElementById('incrementIfOdd').addEventListener('click', function () {// We can write logic to decide what to do based on the stateif (store.getState().value % 2 !== 0) {
        store.dispatch({ type: 'counter/incremented' });}});

document.getElementById('incrementAsync').addEventListener('click', function () {// We can also write async logic that interacts with the storesetTimeout(function () {
        store.dispatch({ type: 'counter/incremented' });}, 1000);});

Here, we'll dispatch the actions that will make the reducer add 1 or subtract 1 from the current counter value.

We can also write code that only dispatches an action if a certain condition is true, or write some async code that dispatches an action after a delay.

Data Flow

We can summarize the flow of data through a Redux app with this diagram. It represents how:

• actions are dispatched in response to a user interaction like a click
• the store runs the reducer function to calculate a new state
• the UI reads the new state to display the new values

(Don't worry if these pieces aren't quite clear yet! Keep this picture in your mind as you go through the rest of this tutorial, and you'll see how the pieces fit together.)

What You've Learned

That counter example was small, but it does show all the working pieces of a real Redux app. Everything we'll talk about in the following sections expands on those basic pieces.

With that in mind, let's review what we've learned so far:

:::tip Summary

• Redux is a library for managing global application state

  • Redux is typically used with the React-Redux library for integrating Redux and React together
  • Redux Toolkit is the recommended way to write Redux logic

• Redux uses several types of code

  • Actions are plain objects with a type field, and describe "what happened" in the app
  • Reducers are functions that calculate a new state value based on previous state + an action
  • A Redux store runs the root reducer whenever an action is dispatched

State Management

Let's start by looking at a small React counter component. It tracks a number in component state, and increments the number when a button is clicked:

//xfunction Counter() {// State: a counter valueconst [counter, setCounter] = useState(0);// Action: code that causes an update to the state when something happensconst increment = () => {setCounter((prevCounter) => prevCounter + 1);};// View: the UI definitionreturn (<div>Value: {counter} <button onClick={increment}>Increment</button></div>);}

It is a self-contained app with the following parts:

• The state, the source of truth that drives our app;
• The view, a declarative description of the UI based on the current state
• The actions, the events that occur in the app based on user input, and trigger updates in the state

This is a small example of "one-way data flow":

• State describes the condition of the app at a specific point in time
• The UI is rendered based on that state
• When something happens (such as a user clicking a button), the state is updated based on what occurred
• The UI re-renders based on the new state

However, the simplicity can break down when we have multiple components that need to share and use the same state, especially if those components are located in different parts of the application. Sometimes this can be solved by "lifting state up" to parent components, but that doesn't always help.

One way to solve this is to extract the shared state from the components, and put it into a centralized location outside the component tree. With this, our component tree becomes a big "view", and any component can access the state or trigger actions, no matter where they are in the tree!

By defining and separating the concepts involved in state management and enforcing rules that maintain independence between views and states, we give our code more structure and maintainability.

This is the basic idea behind Redux: a single centralized place to contain the global state in your application, and specific patterns to follow when updating that state to make the code predictable.

Immutability

"Mutable" means "changeable". If something is "immutable", it can never be changed.

JavaScript objects and arrays are all mutable by default. If I create an object, I can change the contents of its fields. If I create an array, I can change the contents as well:

//const obj = { a: 1, b: 2 };// still the same object outside, but the contents have changed
obj.b = 3;const arr = ['a', 'b'];// In the same way, we can change the contents of this array
arr.push('c');
arr[1] = 'd';

This is called mutating the object or array. It's the same object or array reference in memory, but now the contents inside the object have changed.

In order to update values immutably, your code must make copies of existing objects/arrays, and then modify the copies.

We can do this by hand using JavaScript's array / object spread operators, as well as array methods that return new copies of the array instead of mutating the original array:

//const obj = {a: {// To safely update obj.a.c, we have to copy each piecec: 3},b: 2};const obj2 = {// copy obj...obj,// overwrite aa: {// copy obj.a...obj.a,// overwrite cc: 42}};const arr = ['a', 'b'];// Create a new copy of arr, with "c" appended to the endconst arr2 = arr.concat('c');// or, we can make a copy of the original array:const arr3 = arr.slice();// and mutate the copy:
arr3.push('c');

Redux expects that all state updates are done immutably. We'll look at where and how this is important a bit later, as well as some easier ways to write immutable update logic.

:::info Want to Know More?

For more info on how immutability works in JavaScript, see:

• A Visual Guide to References in JavaScript
• Immutability in React and Redux: The Complete Guide

:::

Redux Terminology

There's some important Redux terms that you'll need to be familiar with before we continue:

Actions

An action is a plain JavaScript object that has a type field. You can think of an action as an event that describes something that happened in the application.

The type field should be a string that gives this action a descriptive name, like "todos/todoAdded". We usually write that type string like "domain/eventName", where the first part is the feature or category that this action belongs to, and the second part is the specific thing that happened.

An action object can have other fields with additional information about what happened. By convention, we put that information in a field called payload.

A typical action object might look like this:

//const addTodoAction = {type: 'todos/todoAdded',payload: 'Buy milk'};

Reducers

A reducer is a function that receives the current state and an action object, decides how to update the state if necessary, and returns the new state: (state, action) => newState. You can think of a reducer as an event listener which handles events based on the received action (event) type.

:::info

"Reducer" functions get their name because they're similar to the kind of callback function you pass to the Array.reduce() method.

:::

Reducers must always follow some specific rules:

• They should only calculate the new state value based on the state and action arguments
• They are not allowed to modify the existing state. Instead, they must make immutable updates, by copying the existing state and making changes to the copied values.
• They must not do any asynchronous logic, calculate random values, or cause other "side effects"

We'll talk more about the rules of reducers later, including why they're important and how to follow them correctly.

The logic inside reducer functions typically follows the same series of steps:

• Check to see if the reducer cares about this action

  • If so, make a copy of the state, update the copy with new values, and return it
• Otherwise, return the existing state unchanged

Here's a small example of a reducer, showing the steps that each reducer should follow:

//const initialState = { value: 0 };function counterReducer(state = initialState, action) {// Check to see if the reducer cares about this actionif (action.type === 'counter/incremented') {// If so, make a copy of `state`return {...state,// and update the copy with the new valuevalue: state.value + 1};}// otherwise return the existing state unchangedreturn state;}

Reducers can use any kind of logic inside to decide what the new state should be: if/else, switch, loops, and so on.

//const numbers = [2, 5, 8];const addNumbers = (previousResult, currentItem) => {
    console.log({ previousResult, currentItem });return previousResult + currentItem;};const initialValue = 0;const total = numbers.reduce(addNumbers, initialValue);// {previousResult: 0, currentItem: 2}// {previousResult: 2, currentItem: 5}// {previousResult: 7, currentItem: 8}

console.log(total);// 15

//const actions = [{ type: 'counter/incremented' }, { type: 'counter/incremented' }, { type: 'counter/incremented' }];const initialState = { value: 0 };const finalResult = actions.reduce(counterReducer, initialState);
console.log(finalResult);// {value: 3}

Store

The current Redux application state lives in an object called the store .

The store is created by passing in a reducer, and has a method called getState that returns the current state value:

//import { configureStore } from '@reduxjs/toolkit';const store = configureStore({ reducer: counterReducer });

console.log(store.getState());// {value: 0}

Dispatch

The Redux store has a method called dispatch. The only way to update the state is to call store.dispatch() and pass in an action object. The store will run its reducer function and save the new state value inside, and we can call getState() to retrieve the updated value:

//
store.dispatch({ type: 'counter/incremented' });

console.log(store.getState());// {value: 1}

You can think of dispatching actions as "triggering an event" in the application. Something happened, and we want the store to know about it. Reducers act like event listeners, and when they hear an action they are interested in, they update the state in response.

Selectors

Selectors are functions that know how to extract specific pieces of information from a store state value. As an application grows bigger, this can help avoid repeating logic as different parts of the app need to read the same data:

//const selectCounterValue = (state) => state.value;const currentValue = selectCounterValue(store.getState());
console.log(currentValue);// 2

Core Concepts and Principles

Overall, we can summarize the intent behind Redux's design in three core concepts:

Single Source of Truth

The global state of your application is stored as an object inside a single store. Any given piece of data should only exist in one location, rather than being duplicated in many places.

This makes it easier to debug and inspect your app's state as things change, as well as centralizing logic that needs to interact with the entire application.

:::tip

This does not mean that every piece of state in your app must go into the Redux store! You should decide whether a piece of state belongs in Redux or your UI components, based on where it's needed.

:::

State is Read-Only

The only way to change the state is to dispatch an action, an object that describes what happened.

This way, the UI won't accidentally overwrite data, and it's easier to trace why a state update happened. Since actions are plain JS objects, they can be logged, serialized, stored, and later replayed for debugging or testing purposes.

Changes are Made with Pure Reducer Functions

To specify how the state tree is updated based on actions, you write reducer functions. Reducers are pure functions that take the previous state and an action, and return the next state. Like any other functions, you can split reducers into smaller functions to help do the work, or write reusable reducers for common tasks.

Redux Application Data Flow

Earlier, we talked about "one-way data flow", which describes this sequence of steps to update the app:

• State describes the condition of the app at a specific point in time
• The UI is rendered based on that state
• When something happens (such as a user clicking a button), the state is updated based on what occurred
• The UI re-renders based on the new state

For Redux specifically, we can break these steps into more detail:

• Initial setup:

  • A Redux store is created using a root reducer function
  • The store calls the root reducer once, and saves the return value as its initial state
  • When the UI is first rendered, UI components access the current state of the Redux store, and use that data to decide what to render. They also subscribe to any future store updates so they can know if the state has changed.

• Updates:

  • Something happens in the app, such as a user clicking a button
  • The app code dispatches an action to the Redux store, like dispatch({type: 'counter/incremented'})
  • The store runs the reducer function again with the previous state and the current action, and saves the return value as the new state
  • The store notifies all parts of the UI that are subscribed that the store has been updated
  • Each UI component that needs data from the store checks to see if the parts of the state they need have changed.
  • Each component that sees its data has changed forces a re-render with the new data, so it can update what's shown on the screen

Here's what that data flow looks like visually:

What You've Learned

:::tip Summary

• Redux's intent can be summarized in three principles

  • Global app state is kept in a single store
  • The store state is read-only to the rest of the app
  • Reducer functions are used to update the state in response to actions

• Redux uses a "one-way data flow" app structure

  • State describes the condition of the app at a point in time, and UI renders based on that state

  • When something happens in the app:

    • The UI dispatches an action
    • The store runs the reducers, and the state is updated based on what occurred
    • The store notifies the UI that the state has changed
  • The UI re-renders based on the new state

Project Setup

For this tutorial, we've created a pre-configured starter project that already has React set up, includes some default styling, and has a fake REST API that will allow us to write actual API requests in our app. You'll use this as the basis for writing the actual application code.

To get started, you can open and fork this CodeSandbox:

You can also clone the same project from this Github repo. After cloning the repo, you can install the tools for the project with npm install, and start it with npm start.

If you'd like to see the final version of what we're going to build, you can check out the tutorial-steps branch, or look at the final version in this CodeSandbox.

Creating a New Redux + React Project

Once you've finished this tutorial, you'll probably want to try working on your own projects. We recommend using the Redux templates for Create-React-App as the fastest way to create a new Redux + React project. It comes with Redux Toolkit and React-Redux already configured, using a modernized version of the "counter" app example you saw in Part 1. This lets you jump right into writing your actual application code without having to add the Redux packages and set up the store.

If you want to know specific details on how to add Redux to a project, see this explanation:

//x
ReactDOM.render(<Provider store={store}><App /></Provider>,
    document.getElementById('root'));

Exploring the Initial Project

This initial project is based on the standard Create-React-App project template, with some modifications.

Let's take a quick look at what the initial project contains:

• /src

  • index.js: the entry point file for the application. It renders the main <App> component.
  • App.js: the main application component.
  • index.css: styles for the complete application

  • /api

    • client.js: a small AJAX request client that allows us to make GET and POST requests
    • server.js: provides a fake REST API for our data. Our app will fetch data from these fake endpoints later.
  • /exampleAddons: contains some additional Redux addons that we'll use later in the tutorial to show how things work

If you load the app now, you should see a welcome message, but the rest of the app is otherwise empty.

With that, let's get started!

Starting the Todo Example App

Our example application will be a small "todo" application. You've probably seen todo app examples before - they make good examples because they let us show how to do things like tracking a list of items, handling user input, and updating the UI when that data changes, which are all things that happen in a normal application.

Defining Requirements

Let's start by figuring out the initial business requirements for this application:

• The UI should consist of three main sections:

  • An input box to let the user type in the text of a new todo item
  • A list of all the existing todo items
  • A footer section that shows the number of non-completed todos, and shows filtering options
• Todo list items should have a checkbox that toggles their "completed" status. We should also be able to add a color-coded category tag for a predefined list of colors, and delete todo items.
• The counter should pluralize the number of active todos: "0 items", "1 item", "3 items", etc
• There should be buttons to mark all todos as completed, and to clear all completed todos by removing them

• There should be two ways to filter the displayed todos in the list:

  • Filtering based on showing "All", "Active", and "Completed" todos
  • Filtering based on selecting one or more colors, and showing any todos whose tag that match those colors

We'll add some more requirements later on, but this is enough to get us started.

The end goal is an app that should look like this:

Designing the State Values

One of the core principles of React and Redux is that your UI should be based on your state. So, one approach to designing an application is to first think of all the state needed to describe how the application works. It's also a good idea to try to describe your UI with as few values in the state as possible, so there's less data you need to keep track of and update.

Conceptually, there are two main aspects of this application:

• The actual list of current todo items
• The current filtering options

We'll also need to keep track of the data the user is typing into the "Add Todo" input box, but that's less important and we'll handle that later.

For each todo item, we need to store a few pieces of information:

• The text the user entered
• The boolean flag saying if it's completed or not
• A unique ID value
• A color category, if selected

Our filtering behavior can probably be described with some enumerated values:

• Completed status: "All", "Active", and "Completed"
• Colors: "Red", "Yellow", "Green", "Blue", "Orange", "Purple"

Looking at these values, we can also say that the todos are "app state" (the core data that the application works with), while the filtering values are "UI state" (state that describes what the app is doing right now). It can be helpful to think about these different kinds of categories to help understand how the different pieces of state are being used.

Designing the State Structure

With Redux, our application state is always kept in plain JavaScript objects and arrays. That means you may not put other things into the Redux state - no class instances, built-in JS types like Map / Set Promise / Date, functions, or anything else that is not plain JS data.

The root Redux state value is almost always a plain JS object, with other data nested inside of it.

Based on this information, we should now be able to describe the kinds of values we need to have inside our Redux state:

• First, we need an array of todo item objects. Each item should have these fields:

  • id: a unique number
  • text: the text the user typed in
  • completed: a boolean flag
  • color: An optional color category

• Then, we need to describe our filtering options. We need to have:

  • The current "completed" filter value
  • An array of the currently selected color categories

So, here's what an example of our app's state might look like:

//const todoAppState = {todos: [{ id: 0, text: 'Learn React', completed: true },{ id: 1, text: 'Learn Redux', completed: false, color: 'purple' },{ id: 2, text: 'Build something fun!', completed: false, color: 'blue' }],filters: {status: 'Active',colors: ['red', 'blue']}};

It's important to note that it's okay to have other state values outside of Redux!. This example is small enough so far that we actually do have all our state in the Redux store, but as we'll see later, some data really doesn't need to be kept in Redux (like "is this dropdown open?" or "current value of a form input").

Designing Actions

Actions are plain JavaScript objects that have a type field. As mentioned earlier, you can think of an action as an event that describes something that happened in the application.

In the same way that we designed the state structure based on the app's requirements, we should also be able to come up with a list of some of the actions that describe what's happening:

• Add a new todo entry based on the text the user entered
• Toggle the completed status of a todo
• Select a color category for a todo
• Delete a todo
• Mark all todos as completed
• Clear all completed todos
• Choose a different "completed" filter value
• Add a new color filter
• Remove a color filter

We normally put any extra data needed to describe what's happening into the action.payload field. This could be a number, a string, or an object with multiple fields inside.

The Redux store doesn't care what the actual text of the action.type field is. However, your own code will look at action.type to see if an update is needed. Also, you will frequently look at action type strings in the Redux DevTools Extension while debugging to see what's going on in your app. So, try to choose action types that are readable and clearly describe what's happening - it'll be much easier to understand things when you look at them later!

Based on that list of things that can happen, we can create a list of actions that our application will use:

• {type: 'todos/todoAdded', payload: todoText}
• {type: 'todos/todoToggled', payload: todoId}
• {type: 'todos/colorSelected, payload: {todoId, color}}
• {type: 'todos/todoDeleted', payload: todoId}
• {type: 'todos/allCompleted'}
• {type: 'todos/completedCleared'}
• {type: 'filters/statusFilterChanged', payload: filterValue}
• {type: 'filters/colorFilterChanged', payload: {color, changeType}}

In this case, the actions primarily have a single extra piece of data, so we can put that directly in the action.payload field. We could have split the color filter behavior into two actions, one for "added" and one for "removed", but in this case we'll do it as one action with an extra field inside specifically to show that we can have objects as an action payload.

Like the state data, actions should contain the smallest amount of information needed to describe what happened.

Writing Reducers

Now that we know what our state structure and our actions look like, it's time to write our first reducer.

Reducers are functions that take the current state and an action as arguments, and return a new state result. In other words, (state, action) => newState.

Creating the Root Reducer

A Redux app really only has one reducer function: the "root reducer" function that you will pass to createStore later on. That one root reducer function is responsible for handling all of the actions that are dispatched, and calculating what the entire new state result should be every time.

Let's start by creating a reducer.js file in the src folder, alongside index.js and App.js.

Every reducer needs some initial state, so we'll add some fake todo entries to get us started. Then, we can write an outline for the logic inside the reducer function:

// title="src/reducer.js"const initialState = {todos: [{ id: 0, text: 'Learn React', completed: true },{ id: 1, text: 'Learn Redux', completed: false, color: 'purple' },{ id: 2, text: 'Build something fun!', completed: false, color: 'blue' }],filters: {status: 'All',colors: []}};// Use the initialState as a default valueexport default function appReducer(state = initialState, action) {// The reducer normally looks at the action type field to decide what happensswitch (action.type) {// Do something here based on the different types of actionsdefault:// If this reducer doesn't recognize the action type, or doesn't// care about this specific action, return the existing state unchangedreturn state;}}

A reducer may be called with undefined as the state value when the application is being initialized. If that happens, we need to provide an initial state value so the rest of the reducer code has something to work with. Reducers normally use ES6 default argument syntax to provide initial state: (state = initialState, action).

Next, let's add the logic to handle the 'todos/todoAdded' action.

We first need to check if the current action's type matches that specific string. Then, we need to return a new object containing all of the state, even for the fields that didn't change.

// title="src/reducer.js"function nextTodoId(todos) {const maxId = todos.reduce((maxId, todo) => Math.max(todo.id, maxId), -1);return maxId + 1;}// Use the initialState as a default valueexport default function appReducer(state = initialState, action) {// The reducer normally looks at the action type field to decide what happensswitch (action.type) {// Do something here based on the different types of actions        case 'todos/todoAdded': {            // We need to return a new state object            return {                // that has all the existing state data                ...state,                // but has a new array for the `todos` field                todos: [                    // with all of the old todos                    ...state.todos,                    // and the new todo object                    {                        // Use an auto-incrementing numeric ID for this example                        id: nextTodoId(state.todos),                        text: action.payload,                        completed: false                    }                ]            };        }        default:// If this reducer doesn't recognize the action type, or doesn't// care about this specific action, return the existing state unchangedreturn state;}}

That's... an awful lot of work to add one todo item to the state. Why is all this extra work necessary?

Rules of Reducers

We said earlier that reducers must always follow some special rules:

• They should only calculate the new state value based on the state and action arguments
• They are not allowed to modify the existing state. Instead, they must make immutable updates, by copying the existing state and making changes to the copied values.
• They must not do any asynchronous logic or other "side effects"

:::tip

A "side effect" is any change to state or behavior that can be seen outside of returning a value from a function. Some common kinds of side effects are things like:

• Logging a value to the console
• Saving a file
• Setting an async timer
• Making an AJAX HTTP request
• Modifying some state that exists outside of a function, or mutating arguments to a function
• Generating random numbers or unique random IDs (such as Math.random() or Date.now())

:::

Any function that follows these rules is also known as a "pure" function, even if it's not specifically written as a reducer function.

But why are these rules important? There's a few different reasons:

• One of the goals of Redux is to make your code predictable. When a function's output is only calculated from the input arguments, it's easier to understand how that code works, and to test it.
• On the other hand, if a function depends on variables outside itself, or behaves randomly, you never know what will happen when you run it.
• If a function modifies other values, including its arguments, that can change the way the application works unexpectedly. This can be a common source of bugs, such as "I updated my state, but now my UI isn't updating when it should!"
• Some of the Redux DevTools capabilities depend on having your reducers follow these rules correctly

The rule about "immutable updates" is particularly important, and worth talking about further.

Reducers and Immutable Updates

Earlier, we talked about "mutation" (modifying existing object/array values) and "immutability" (treating values as something that cannot be changed).

:::warning

In Redux, our reducers are never allowed to mutate the original / current state values!

//// ❌ Illegal - by default, this will mutate the state!
state.value = 123;

:::

There are several reasons why you must not mutate state in Redux:

• It causes bugs, such as the UI not updating properly to show the latest values
• It makes it harder to understand why and how the state has been updated
• It makes it harder to write tests
• It breaks the ability to use "time-travel debugging" correctly
• It goes against the intended spirit and usage patterns for Redux

So if we can't change the originals, how do we return an updated state?

:::tip

Reducers can only make copies of the original values, and then they can mutate the copies.

//// ✅ This is safe, because we made a copyreturn {...state,value: 123};

:::

We already saw that we can write immutable updates by hand, by using JavaScript's array / object spread operators and other functions that return copies of the original values.

This becomes harder when the data is nested. A critical rule of immutable updates is that you must make a copy of every level of nesting that needs to be updated.

However, if you're thinking that "writing immutable updates by hand this way looks hard to remember and do correctly"... yeah, you're right! :)

Writing immutable update logic by hand is hard, and accidentally mutating state in reducers is the single most common mistake Redux users make.

:::tip

In real-world applications, you won't have to write these complex nested immutable updates by hand. In Part 8: Modern Redux with Redux Toolkit, you'll learn how to use Redux Toolkit to simplify writing immutable update logic in reducers.

:::

Handling Additional Actions

With that in mind, let's add the reducer logic for a couple more cases. First, toggling a todo's completed field based on its ID:

// title="src/reducer.js"export default function appReducer(state = initialState, action) {switch (action.type) {case 'todos/todoAdded': {return {...state,todos: [...state.todos,{id: nextTodoId(state.todos),text: action.payload,completed: false}]};}        case 'todos/todoToggled': {            return {                // Again copy the entire state object                ...state,                // This time, we need to make a copy of the old todos array                todos: state.todos.map((todo) => {                    // If this isn't the todo item we're looking for, leave it alone                    if (todo.id !== action.payload) {                        return todo;                    }                    // We've found the todo that has to change. Return a copy:                    return {                        ...todo,                        // Flip the completed flag                        completed: !todo.completed                    };                })            };        }        default:return state;}}

And since we've been focusing on the todos state, let's add a case to handle the "visibility selection changed" action as well:

// title="src/reducer.js"export default function appReducer(state = initialState, action) {switch (action.type) {case 'todos/todoAdded': {return {...state,todos: [...state.todos,{id: nextTodoId(state.todos),text: action.payload,completed: false}]};}case 'todos/todoToggled': {return {...state,todos: state.todos.map((todo) => {if (todo.id !== action.payload) {return todo;}return {...todo,completed: !todo.completed
                    };})};}        case 'filters/statusFilterChanged': {            return {                // Copy the whole state                ...state,                // Overwrite the filters value                filters: {                    // copy the other filter fields                    ...state.filters,                    // And replace the status field with the new value                    status: action.payload                }            };        }        default:return state;}}

We've only handled 3 actions, but this is already getting a bit long. If we try to handle every action in this one reducer function, it's going to be hard to read it all.

That's why reducers are typically split into multiple smaller reducer functions - to make it easier to understand and maintain the reducer logic.

Splitting Reducers

As part of this, Redux reducers are typically split apart based on the section of the Redux state that they update. Our todo app state currently has two top-level sections: state.todos and state.filters. So, we can split the large root reducer function into two smaller reducers - a todosReducer and a filtersReducer.

So, where should these split-up reducer functions live?

We recommend organizing your Redux app folders and files based on "features" - code that relates to a specific concept or area of your application. The Redux code for a particular feature is usually written as a single file, known as a "slice" file, which contains all the reducer logic and all of the action-related code for that part of your app state.

Because of that, the reducer for a specific section of the Redux app state is called a "slice reducer". Typically, some of the action objects will be closely related to a specific slice reducer, and so the action type strings should start with the name of that feature (like 'todos') and describe the event that happened (like 'todoAdded'), joined together into one string ('todos/todoAdded').

In our project, create a new features folder, and then a todos folder inside that. Create a new file named todosSlice.js, and let's cut and paste the todo-related initial state over into this file:

// title="src/features/todos/todosSlice.js"const initialState = [{ id: 0, text: 'Learn React', completed: true },{ id: 1, text: 'Learn Redux', completed: false, color: 'purple' },{ id: 2, text: 'Build something fun!', completed: false, color: 'blue' }];function nextTodoId(todos) {const maxId = todos.reduce((maxId, todo) => Math.max(todo.id, maxId), -1);return maxId + 1;}export default function todosReducer(state = initialState, action) {switch (action.type) {default:return state;}}

Now we can copy over the logic for updating the todos. However, there's an important difference here. This file only has to update the todos-related state - it's not nested any more! This is another reason why we split up reducers. Since the todos state is an array by itself, we don't have to copy the outer root state object in here. That makes this reducer easier to read.

This is called reducer composition, and it's the fundamental pattern of building Redux apps.

Here's what the updated reducer looks like after we handle those actions:

// title="src/features/todos/todosSlice.js"export default function todosReducer(state = initialState, action) {switch (action.type) {        case 'todos/todoAdded': {            // Can return just the new todos array - no extra object around it            return [                ...state,                {                    id: nextTodoId(state),                    text: action.payload,                    completed: false                }            ];        }        case 'todos/todoToggled': {            return state.map((todo) => {                if (todo.id !== action.payload) {                    return todo;                }                return {                    ...todo,                    completed: !todo.completed                };            });        }        default:return state;}}

That's a bit shorter and easier to read.

Now we can do the same thing for the visibility logic. Create src/features/filters/filtersSlice.js, and let's move all the filter-related code over there:

// title="src/features/filters/filtersSlice.js"const initialState = {status: 'All',colors: []};export default function filtersReducer(state = initialState, action) {switch (action.type) {        case 'filters/statusFilterChanged': {            return {                // Again, one less level of nesting to copy                ...state,                status: action.payload            };        }        default:return state;}}

We still have to copy the object containing the filters state, but since there's less nesting, it's easier to read what's happening.

:::info

To keep this page shorter, we'll skip showing how to write the reducer update logic for the other actions.

Try writing the updates for those yourself, based on the requirements described above.

If you get stuck, see the CodeSandbox at the end of this page for the complete implementation of these reducers.

:::

Combining Reducers

We now have two separate slice files, each with its own slice reducer function. But, we said earlier that the Redux store needs one root reducer function when we create it. So, how can we go back to having a root reducer without putting all the code in one big function?

Since reducers are normal JS functions, we can import the slice reducers back into reducer.js, and write a new root reducer whose only job is to call the other two functions.

// title="src/reducer.js"import todosReducer from './features/todos/todosSlice';import filtersReducer from './features/filters/filtersSlice';export default function rootReducer(state = {}, action) {// always return a new object for the root statereturn {// the value of `state.todos` is whatever the todos reducer returnstodos: todosReducer(state.todos, action),// For both reducers, we only pass in their slice of the statefilters: filtersReducer(state.filters, action)};}

Note that each of these reducers is managing its own part of the global state. The state parameter is different for every reducer, and corresponds to the part of the state it manages.

This allows us to split up our logic based on features and slices of state, to keep things maintainable.

combineReducers

We can see that the new root reducer is doing the same thing for each slice: calling the slice reducer, passing in the slice of the state owned by that reducer, and assigning the result back to the root state object. If we were to add more slices, the pattern would repeat.

The Redux core library includes a utility called combineReducers, which does this same boilerplate step for us. We can replace our hand-written rootReducer with a shorter one generated by combineReducers.

Now that we need combineReducers, it's time to actually install the Redux core library:

//
npm install redux

Once that's done, we can import combineReducers and use it:

// title="src/reducer.js"import { combineReducers } from 'redux';import todosReducer from './features/todos/todosSlice';import filtersReducer from './features/filters/filtersSlice';const rootReducer = combineReducers({// Define a top-level state field named `todos`, handled by `todosReducer`todos: todosReducer,filters: filtersReducer
});export default rootReducer;

combineReducers accepts an object where the key names will become the keys in your root state object, and the values are the slice reducer functions that know how to update those slices of the Redux state.

Remember, the key names you give to combineReducers decides what the key names of your state object will be!

What You've Learned

State, Actions, and Reducers are the building blocks of Redux. Every Redux app has state values, creates actions to describe what happened, and uses reducer functions to calculate new state values based on the previous state and an action.

Here's the contents of our app so far:

:::tip Summary

• Redux apps use plain JS objects, arrays, and primitives as the state values

  • The root state value should be a plain JS object
  • The state should contain the smallest amount of data needed to make the app work
  • Classes, Promises, functions, and other non-plain values should not go in the Redux state
  • Reducers must not create random values like Math.random() or Date.now()
  • It's okay to have other state values that are not in the Redux store (like local component state) side-by side with Redux

• Actions are plain objects with a type field that describe what happened

  • The type field should be a readable string, and is usually written as 'feature/eventName'
  • Actions may contain other values, which are typically stored in the action.payload field
  • Actions should have the smallest amount of data needed to describe what happened

• Reducers are functions that look like (state, action) => newState

  • Reducers must always follow special rules:

    • Only calculate the new state based on the state and action arguments
    • Never mutate the existing state - always return a copy
    • No "side effects" like AJAX calls or async logic

• Reducers should be split up to make them easier to read

  • Reducers are usually split based on top-level state keys or "slices" of state
  • Reducers are usually written in "slice" files, organized into "feature" folders
  • Reducers can be combined together with the Redux combineReducers function
  • The key names given to combineReducers define the top-level state object keys