useReducer is a Hook that allows us to manage multiple states more efficiently, create complex state logic, and manage states that depend on previous states. The following two use cases are good examples of how we can make use of this hook.

`useReducer` use cases

Manage multiple states: modify an array
Modify complex states, such as arrays or objects: login form

Manage multiple states

useReducer can be used to simplify the way in which multiple states impact a piece of data. In this case, adding, removing, and clearing an array can be achieved by using useReducer instead of three separate states.

import { useReducer } from "react";

const myReducer = (prevState, action) => {
    let array;
    switch (action.type) {
        case 'ADD':
            array = [...prevState];
            array.push(action.payload);
            return array;
        case 'REMOVE':
            array = [...prevState];
            array.pop();
            return array;
        case 'CLEAR':
            return prevState = [];
        default:
            break;
    }
};

const UseCaseMultipleStates = props => {
    const [state, dispatcher] = useReducer(myReducer, ['initial value']);
    console.log(state);

    // Three different state triggers
    const addHandler = () => {
        dispatcher({ type: 'ADD', payload: Math.round((Math.random() * 100 + 100)) });
    };
    const removeHandler = () => {
        dispatcher({ type: 'REMOVE' });
    };
    const clearHandler = () => {
        dispatcher({ type: 'CLEAR' });
    };

    return (
        <>
            <hr />
            <h2>useReducer use case</h2>
            <h3>Manage multiple states: modify an array</h3>
            <button onClick={addHandler}>[+] Add random value to array</button>
            <button style={{ margin: "0 2rem" }} onClick={removeHandler}>[-] Remove last value from array</button>
            <button onClick={clearHandler}>[x] Clear array</button>
            <p>Shopping cart array:</p>
            <p><b>{state.length === 0 && '(empty)'}{state.join(' - ')}</b></p>
        </>
    );
};

export default UseCaseMultipleStates;

Modify complex states, such as arrays or objects: login form

useReducer can be especially handy when dealing with multiple states and a complex state logic.

By handling a login form with this hook instead of multiple useState hooks we can appreciate how powerful this hook is.

Helper function for simulating a login API (thanks to Harry Wolff for this code):

export async function loginHelper({ username, password }) {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            if (username === 'user' && password === 'password') {
                resolve();
            } else {
                reject();
            }
        }, 1000);
    });
}

Main component:

import { useReducer } from "react";
import { loginHelper } from "./loginHelper";

const myReducer = (prevState, action) => {
    switch (action.type) {
        case 'USERNAME':
            return {
                ...prevState,
                username: action.payload,
            };
        case 'PASSWORD':
            return {
                ...prevState,
                password: action.payload,
            };
        case 'LOGGED_IN':
            return {
                ...prevState,
                isLoggedIn: true,
            };
        case 'LOGGED_OUT':
            return {
                ...prevState,
                isLoggedIn: false,
                username: '',
                password: '',
            };
        case 'IS_LOADING':
            return {
                ...prevState,
                isLoading: true,
            };
        case 'IS_NOT_LOADING':
            return {
                ...prevState,
                isLoading: false,
            };
        case 'ERROR':
            return {
                ...prevState,
                isError: true,
                isLoading: false,
            };

        default:
            break;
    }
};

const initialState = {
    username: '',
    password: '',
    isLoggedIn: false,
    isLoading: false,
    isError: false,
};

const UseCaseComplexStates = props => {
    const [state, dispatcher] = useReducer(myReducer, initialState);

    const usernameHandler = e => {
        dispatcher({ type: 'USERNAME', payload: e.target.value });
    };

    const passwordHandler = e => {
        dispatcher({ type: 'PASSWORD', payload: e.target.value });
    };

    const logoutHandler = e => {
        dispatcher({ type: 'LOGGED_OUT' });
    };

    const submitHandler = async e => {
        e.preventDefault();

        // Check credentials (simulated)
        try {
            dispatcher({ type: 'IS_LOADING' });
            await loginHelper({ username: state.username, password: state.password });
            dispatcher({ type: 'IS_NOT_LOADING' });
            dispatcher({ type: 'LOGGED_IN' });
        } catch {
            dispatcher({ type: 'ERROR' });
            alert('🚨 Incorrect username or password');
        }
    };

    return (
        <>
            <hr />
            <h2>useReducer use case</h2>
            <h3>Modify complex states, such as arrays or objects: login form</h3>
            <div style={{ maxWidth: '50%', backgroundColor: '#a8dadc', borderRadius: '1rem', padding: '2rem' }}>
                {state.isLoggedIn
                    ? <><p>Welcome!</p><button onClick={logoutHandler}>Log out!</button></>
                    : <form onSubmit={submitHandler}>
                        <div style={{ margin: '1rem 0' }}>
                            <label htmlFor="username">Username</label>
                            <input type="text" id="username" onChange={usernameHandler} value={state.username} style={{ margin: '0 1rem' }} placeholder='user' />
                        </div>
                        <div style={{ margin: '1rem 0' }}>
                            <label htmlFor="password">Password</label>
                            <input type="password" id="password" onChange={passwordHandler} value={state.password} style={{ margin: '0 1rem' }} placeholder='password' />
                        </div>
                        <div style={{ margin: '1rem 0' }}>
                            <button type="submit" disabled={state.isLoading}>{state.isLoading ? 'Logging you in...' : 'Log in'}</button>
                        </div>

                    </form>
                }
            </div>
        </>
    );
};

export default UseCaseComplexStates;

You can watch all these examples live here.

You can also take a look at the code in this repository.

2 use cases of the useReducer ReactJS hook

useReducer use cases

Manage multiple states

Modify complex states, such as arrays or objects: login form

`useReducer` use cases