Extracting State Logic into a Reducer
Components with many state updates spread across many event handlers can get overwhelming. For these cases, you can consolidate all the state update logic outside your component in a single function, called a reducer.
You will learn
- What a reducer function is
- How to refactor
useState
touseReducer
- When to use a reducer
- How to write one well
Consolidate state logic with a reducer
As your components grow in complexity, it can get harder to see at a glance all the different ways in which a component’s state gets updated. For example, the TaskApp
component below holds an array of tasks
in state and uses three different event handlers to add, remove, and edit tasks:
Error
Extra 185 of 186 byte(s) found at buffer[1]
Each of its event handlers calls setTasks
in order to update the state. As this component grows, so does the amount of state logic sprinkled throughout it. To reduce this complexity and keep all your logic in one easy-to-access place, you can move that state logic into a single function outside your component, called a “reducer”.
Reducers are a different way to handle state. You can migrate from useState
to useReducer
in three steps:
- Move from setting state to dispatching actions.
- Write a reducer function.
- Use the reducer from your component.
Step 1: Move from setting state to dispatching actions
Your event handlers currently specify what to do by setting state:
function handleAddTask(text) {
setTasks([
...tasks,
{
id: nextId++,
text: text,
done: false,
},
]);
}
function handleChangeTask(task) {
setTasks(
tasks.map((t) => {
if (t.id === task.id) {
return task;
} else {
return t;
}
})
);
}
function handleDeleteTask(taskId) {
setTasks(tasks.filter((t) => t.id !== taskId));
}
Remove all the state setting logic. What you are left with are three event handlers:
handleAddTask(text)
is called when the user presses “Add”.handleChangeTask(task)
is called when the user toggles a task or presses “Save”.handleDeleteTask(taskId)
is called when the user presses “Delete”.
Managing state with reducers is slightly different from directly setting state. Instead of telling React “what to do” by setting state, you specify “what the user just did” by dispatching “actions” from your event handlers. (The state update logic will live elsewhere!) So instead of “setting tasks
” via an event handler, you’re dispatching an “added/changed/deleted a task” action. This is more descriptive of the user’s intent.
function handleAddTask(text) {
dispatch({
type: 'added',
id: nextId++,
text: text,
});
}
function handleChangeTask(task) {
dispatch({
type: 'changed',
task: task,
});
}
function handleDeleteTask(taskId) {
dispatch({
type: 'deleted',
id: taskId,
});
}
The object you pass to dispatch
is called an “action”:
function handleDeleteTask(taskId) {
dispatch(
// "action" object:
{
type: 'deleted',
id: taskId,
}
);
}
It is a regular JavaScript object. You decide what to put in it, but generally it should contain the minimal information about what happened. (You will add the dispatch
function itself in a later step.)
Step 2: Write a reducer function
A reducer function is where you will put your state logic. It takes two arguments, the current state and the action object, and it returns the next state:
function yourReducer(state, action) {
// return next state for React to set
}
React will set the state to what you return from the reducer.
To move your state setting logic from your event handlers to a reducer function in this example, you will:
- Declare the current state (
tasks
) as the first argument. - Declare the
action
object as the second argument. - Return the next state from the reducer (which React will set the state to).
Here is all the state setting logic migrated to a reducer function:
function tasksReducer(tasks, action) {
if (action.type === 'added') {
return [
...tasks,
{
id: action.id,
text: action.text,
done: false,
},
];
} else if (action.type === 'changed') {
return tasks.map((t) => {
if (t.id === action.task.id) {
return action.task;
} else {
return t;
}
});
} else if (action.type === 'deleted') {
return tasks.filter((t) => t.id !== action.id);
} else {
throw Error('Unknown action: ' + action.type);
}
}
Because the reducer function takes state (tasks
) as an argument, you can declare it outside of your component. This decreases the indentation level and can make your code easier to read.
Deep Dive
Although reducers can “reduce” the amount of code inside your component, they are actually named after the reduce()
operation that you can perform on arrays.
The reduce()
operation lets you take an array and “accumulate” a single value out of many:
const arr = [1, 2, 3, 4, 5];
const sum = arr.reduce(
(result, number) => result + number
); // 1 + 2 + 3 + 4 + 5
The function you pass to reduce
is known as a “reducer”. It takes the result so far and the current item, then it returns the next result. React reducers are an example of the same idea: they take the state so far and the action, and return the next state. In this way, they accumulate actions over time into state.
You could even use the reduce()
method with an initialState
and an array of actions
to calculate the final state by passing your reducer function to it:
import tasksReducer from './tasksReducer.js'; let initialState = []; let actions = [ {type: 'added', id: 1, text: 'Visit Kafka Museum'}, {type: 'added', id: 2, text: 'Watch a puppet show'}, {type: 'deleted', id: 1}, {type: 'added', id: 3, text: 'Lennon Wall pic'}, ]; let finalState = actions.reduce(tasksReducer, initialState); const output = document.getElementById('output'); output.textContent = JSON.stringify(finalState, null, 2);
You probably won’t need to do this yourself, but this is similar to what React does!
Step 3: Use the reducer from your component
Finally, you need to hook up the tasksReducer
to your component. Import the useReducer
Hook from React:
import { useReducer } from 'react';
Then you can replace useState
:
const [tasks, setTasks] = useState(initialTasks);
with useReducer
like so:
const [tasks, dispatch] = useReducer(tasksReducer, initialTasks);
The useReducer
Hook is similar to useState
—you must pass it an initial state and it returns a stateful value and a way to set state (in this case, the dispatch function). But it’s a little different.
The useReducer
Hook takes two arguments:
- A reducer function
- An initial state
And it returns:
- A stateful value
- A dispatch function (to “dispatch” user actions to the reducer)
Now it’s fully wired up! Here, the reducer is declared at the bottom of the component file:
import { useReducer } from 'react'; import AddTask from './AddTask.js'; import TaskList from './TaskList.js'; export default function TaskApp() { const [tasks, dispatch] = useReducer(tasksReducer, initialTasks); function handleAddTask(text) { dispatch({ type: 'added', id: nextId++, text: text, }); } function handleChangeTask(task) { dispatch({ type: 'changed', task: task, }); } function handleDeleteTask(taskId) { dispatch({ type: 'deleted', id: taskId, }); } return ( <> <h1>Prague itinerary</h1> <AddTask onAddTask={handleAddTask} /> <TaskList tasks={tasks} onChangeTask={handleChangeTask} onDeleteTask={handleDeleteTask} /> </> ); } function tasksReducer(tasks, action) { switch (action.type) { case 'added': { return [ ...tasks, { id: action.id, text: action.text, done: false, }, ]; } case 'changed': { return tasks.map((t) => { if (t.id === action.task.id) { return action.task; } else { return t; } }); } case 'deleted': { return tasks.filter((t) => t.id !== action.id); } default: { throw Error('Unknown action: ' + action.type); } } } let nextId = 3; const initialTasks = [ {id: 0, text: 'Visit Kafka Museum', done: true}, {id: 1, text: 'Watch a puppet show', done: false}, {id: 2, text: 'Lennon Wall pic', done: false}, ];
If you want, you can even move the reducer to a different file:
import { useReducer } from 'react'; import AddTask from './AddTask.js'; import TaskList from './TaskList.js'; import tasksReducer from './tasksReducer.js'; export default function TaskApp() { const [tasks, dispatch] = useReducer(tasksReducer, initialTasks); function handleAddTask(text) { dispatch({ type: 'added', id: nextId++, text: text, }); } function handleChangeTask(task) { dispatch({ type: 'changed', task: task, }); } function handleDeleteTask(taskId) { dispatch({ type: 'deleted', id: taskId, }); } return ( <> <h1>Prague itinerary</h1> <AddTask onAddTask={handleAddTask} /> <TaskList tasks={tasks} onChangeTask={handleChangeTask} onDeleteTask={handleDeleteTask} /> </> ); } let nextId = 3; const initialTasks = [ {id: 0, text: 'Visit Kafka Museum', done: true}, {id: 1, text: 'Watch a puppet show', done: false}, {id: 2, text: 'Lennon Wall pic', done: false}, ];
Component logic can be easier to read when you separate concerns like this. Now the event handlers only specify what happened by dispatching actions, and the reducer function determines how the state updates in response to them.
Comparing useState
and useReducer
Reducers are not without downsides! Here’s a few ways you can compare them:
- Code size: Generally, with
useState
you have to write less code upfront. WithuseReducer
, you have to write both a reducer function and dispatch actions. However,useReducer
can help cut down on the code if many event handlers modify state in a similar way. - Readability:
useState
is very easy to read when the state updates are simple. When they get more complex, they can bloat your component’s code and make it difficult to scan. In this case,useReducer
lets you cleanly separate the how of update logic from the what happened of event handlers. - Debugging: When you have a bug with
useState
, it can be difficult to tell where the state was set incorrectly, and why. WithuseReducer
, you can add a console log into your reducer to see every state update, and why it happened (due to whichaction
). If eachaction
is correct, you’ll know that the mistake is in the reducer logic itself. However, you have to step through more code than withuseState
. - Testing: A reducer is a pure function that doesn’t depend on your component. This means that you can export and test it separately in isolation. While generally it’s best to test components in a more realistic environment, for complex state update logic it can be useful to assert that your reducer returns a particular state for a particular initial state and action.
- Personal preference: Some people like reducers, others don’t. That’s okay. It’s a matter of preference. You can always convert between
useState
anduseReducer
back and forth: they are equivalent!
We recommend using a reducer if you often encounter bugs due to incorrect state updates in some component, and want to introduce more structure to its code. You don’t have to use reducers for everything: feel free to mix and match! You can even useState
and useReducer
in the same component.
Writing reducers well
Keep these two tips in mind when writing reducers:
- Reducers must be pure. Similar to state updater functions, reducers run during rendering! (Actions are queued until the next render.) This means that reducers must be pure—same inputs always result in the same output. They should not send requests, schedule timeouts, or perform any side effects (operations that impact things outside the component). They should update objects and arrays without mutations.
- Each action describes a single user interaction, even if that leads to multiple changes in the data. For example, if a user presses “Reset” on a form with five fields managed by a reducer, it makes more sense to dispatch one
reset_form
action rather than five separateset_field
actions. If you log every action in a reducer, that log should be clear enough for you to reconstruct what interactions or responses happened in what order. This helps with debugging!
Writing concise reducers with Immer
Just like with updating objects and arrays in regular state, you can use the Immer library to make reducers more concise. Here, useImmerReducer
lets you mutate the state with push
or arr[i] =
assignment:
{ "dependencies": { "immer": "1.7.3", "react": "latest", "react-dom": "latest", "react-scripts": "latest", "use-immer": "0.5.1" }, "scripts": { "start": "react-scripts start", "build": "react-scripts build", "test": "react-scripts test --env=jsdom", "eject": "react-scripts eject" }, "devDependencies": {} }
Reducers must be pure, so they shouldn’t mutate state. But Immer provides you with a special draft
object which is safe to mutate. Under the hood, Immer will create a copy of your state with the changes you made to the draft
. This is why reducers managed by useImmerReducer
can mutate their first argument and don’t need to return state.
Recap
- To convert from
useState
touseReducer
:- Dispatch actions from event handlers.
- Write a reducer function that returns the next state for a given state and action.
- Replace
useState
withuseReducer
.
- Reducers require you to write a bit more code, but they help with debugging and testing.
- Reducers must be pure.
- Each action describes a single user interaction.
- Use Immer if you want to write reducers in a mutating style.
Challenge 1 of 4: Dispatch actions from event handlers
Currently, the event handlers in ContactList.js
and Chat.js
have // TODO
comments. This is why typing into the input doesn’t work, and clicking on the buttons doesn’t change the selected recipient.
Replace these two // TODO
s with the code to dispatch
the corresponding actions. To see the expected shape and the type of the actions, check the reducer in messengerReducer.js
. The reducer is already written so you won’t need to change it. You only need to dispatch the actions in ContactList.js
and Chat.js
.
import { useReducer } from 'react'; import Chat from './Chat.js'; import ContactList from './ContactList.js'; import { initialState, messengerReducer } from './messengerReducer'; export default function Messenger() { const [state, dispatch] = useReducer(messengerReducer, initialState); const message = state.message; const contact = contacts.find((c) => c.id === state.selectedId); return ( <div> <ContactList contacts={contacts} selectedId={state.selectedId} dispatch={dispatch} /> <Chat key={contact.id} message={message} contact={contact} dispatch={dispatch} /> </div> ); } const contacts = [ {id: 0, name: 'Taylor', email: 'taylor@mail.com'}, {id: 1, name: 'Alice', email: 'alice@mail.com'}, {id: 2, name: 'Bob', email: 'bob@mail.com'}, ];