As your application grows, it helps to be more intentional about how your state is organized and how the data flows between your components. Redundant or duplicate state is a common source of bugs. In this chapter, you’ll learn how to structure your state well, how to keep your state update logic maintainable, and how to share state between distant components.
You will learn
- How to think about UI changes as state changes
- How to structure state well
- How “lift state up” to share it between components
- How to control whether the state gets preserved or reset
- How to consolidate complex state logic in a function
- How to pass information without “prop drilling”
- How to scale state management as your app grows
Reacting to input with state
With React, you won’t modify the UI from code directly. For example, you won’t write commands like “disable the button”, “enable the button”, “show the success message”, etc. Instead, you will describe the UI you want to see for the different visual states of your component (“initial state”, “typing state”, “success state”), and then trigger the state changes in response to user input. This is similar to how designers think about UI.
Here is a feedback form built using React. Note how it uses the status state variable to determine whether to enable or disable the submit button, and whether to show the success message instead.
import { useState } from 'react';
export default function FeedbackForm() {
const [message, setMessage] = useState('');
const [error, setError] = useState(null);
const [status, setStatus] = useState('typing');
if (status === 'success') {
return <h1>Thank you!</h1>
}
async function handleSubmit(e) {
e.preventDefault();
setStatus('submitting');
try {
await submitForm();
setStatus('success');
} catch (err) {
setStatus('typing');
setError(err);
}
}
function handleTextareaChange(e) {
setMessage(e.target.value);
}
return (
<form onSubmit={handleSubmit}>
<textarea
value={message}
onChange={handleTextareaChange}
disabled={status === 'submitting'}
/>
<br />
<button disabled={
message.length === 0 ||
status === 'submitting'
}>
Submit
</button>
{error !== null &&
<p className="Error">
{error.message}
</p>
}
</form>
);
}
function submitForm() {
// Pretend it's hitting the network.
return new Promise((resolve, reject) => {
setTimeout(() => {
let shouldError = Math.random() > 0.5;
if (shouldError) {
reject(new Error('Something went wrong'));
} else {
resolve();
}
}, 1500);
});
}
Ready to learn this topic?
Read Reacting to Input with State to learn how to approach interactions with a state-driven mindset.
Read MoreChoosing the state structure
Structuring state well can make a difference between a component that is pleasant to modify and debug, and one that is a constant source of bugs. The most important principle is that state shouldn’t contain redundant or duplicated information. If there’s some unnecessary state, it’s easy to forget to update it, and introduce bugs!
For example, this form has a redundant fullName state variable:
import { useState } from 'react';
export default function Form() {
const [firstName, setFirstName] = useState('');
const [lastName, setLastName] = useState('');
const [fullName, setFullName] = useState('');
function handleFirstNameChange(e) {
setFirstName(e.target.value);
setFullName(e.target.value + ' ' + lastName);
}
function handleLastNameChange(e) {
setLastName(e.target.value);
setFullName(firstName + ' ' + e.target.value);
}
return (
<>
<label>
First name:{' '}
<input
value={firstName}
onChange={handleFirstNameChange}
/>
</label>
<label>
Last name:{' '}
<input
value={lastName}
onChange={handleLastNameChange}
/>
</label>
<h3>
Your full name is: {fullName}
</h3>
</>
);
}
You can remove it and simplify the code by calculating fullName while the component is rendering:
import { useState } from 'react';
export default function Form() {
const [firstName, setFirstName] = useState('');
const [lastName, setLastName] = useState('');
const fullName = firstName + ' ' + lastName;
function handleFirstNameChange(e) {
setFirstName(e.target.value);
}
function handleLastNameChange(e) {
setLastName(e.target.value);
}
return (
<>
<label>
First name:{' '}
<input
value={firstName}
onChange={handleFirstNameChange}
/>
</label>
<label>
Last name:{' '}
<input
value={lastName}
onChange={handleLastNameChange}
/>
</label>
<h3>
Your full name is: {fullName}
</h3>
</>
);
}
Ready to learn this topic?
Read Choosing the State Structure to learn how to make state updates easier and avoid common pitfalls.
Read MoreSharing state between components
Sometimes, you want the state of two components to always change together. To do it, remove state from both of them, move it to their closest common parent, and then pass it down to them via props. This is known as “lifting state up”, and it’s one of the most common things you will do writing React code.
In this example, only one panel should be active at a time. To achieve this, instead of keeping the active state inside each individual panel, the parent component holds the state and specifies the props for its children.
import { useState } from 'react';
export default function Accordion() {
const [activeIndex, setActiveIndex] = useState(0);
return (
<>
<Panel
title="Ingredients"
isActive={activeIndex === 0}
onShow={() => setActiveIndex(0)}
>
Milk, tea bags, and a cinnamon stick.
</Panel>
<Panel
title="Recipe"
isActive={activeIndex === 1}
onShow={() => setActiveIndex(1)}
>
Heat the milk and put tea bags into the pan.
Add the cinnamon stick.
</Panel>
</>
);
}
function Panel({
title,
children,
isActive,
onShow
}) {
return (
<section className="panel">
<h3>{title}</h3>
{isActive ? (
<p>{children}</p>
) : (
<button onClick={onShow}>
Show
</button>
)}
</section>
);
}
Ready to learn this topic?
Read Sharing State Between Components to learn how to lift state up and keep components in sync.
Read MorePreserving and resetting state
When you re-render a component, React needs to decide which parts of the tree to keep (and update), and which parts to discard or re-create from scratch. In most cases, React’s automatic behavior works well enough. By default, React preserves the parts of the tree that “match up” with the previously rendered component tree.
However, sometimes this is not what you want. For example, in this app, typing a message and then switching the recipient does not reset the input. This can cause the user to accidentally send a message to the wrong person:
import { useState } from 'react';
import Chat from './Chat.js';
import ContactList from './ContactList.js';
export default function Messenger() {
const [to, setTo] = useState(contacts[0]);
return (
<div>
<ContactList
contacts={contacts}
selectedContact={to}
onSelect={contact => setTo(contact)}
/>
<Chat contact={to} />
</div>
)
}
const contacts = [
{ name: 'Taylor', email: 'taylor@mail.com' },
{ name: 'Alice', email: 'alice@mail.com' },
{ name: 'Bob', email: 'bob@mail.com' }
];
React lets you override the default behavior, and force a component to reset its state by passing it a different key, like <Chat key={email} />. This tells React that if the recipient is different, it should be considered a different Chat component, and it needs to be re-created from scratch with the new data (and UI like inputs). Now switching between the recipients always resets the input field—even though you render the same component.
import { useState } from 'react';
import Chat from './Chat.js';
import ContactList from './ContactList.js';
export default function Messenger() {
const [to, setTo] = useState(contacts[0]);
return (
<div>
<ContactList
contacts={contacts}
selectedContact={to}
onSelect={contact => setTo(contact)}
/>
<Chat key={to.email} contact={to} />
</div>
)
}
const contacts = [
{ name: 'Taylor', email: 'taylor@mail.com' },
{ name: 'Alice', email: 'alice@mail.com' },
{ name: 'Bob', email: 'bob@mail.com' }
];
Ready to learn this topic?
Read Preserving and Resetting State to learn the lifetime of state and how to control it.
Read MoreExtracting 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.” Your event handlers become concise because they only specify the user “actions.” At the bottom of the file, the reducer function specifies how the state should update in response to each action!
import { useReducer } from 'react';
import AddTask from './AddTask.js';
import TaskList from './TaskList.js';
export default function TaskBoard() {
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 (
<>
<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: 'Buy milk', done: true },
{ id: 1, text: 'Eat tacos', done: false },
{ id: 2, text: 'Brew tea', done: false },
];
Ready to learn this topic?
Read Extracting State Logic into a Reducer to learn how to consolidate logic in the reducer function.
Read MorePassing data deeply with context
Usually, you will pass information from a parent component to a child component via props. But passing props can become cumbersome if you need to pass some prop deeply through the tree, or if many components in the UI tree need the same prop. Context lets the parent component make some information available to any component in the tree below it—no matter how deep it is—without passing it explicitly through props.
Here, the Heading component determines its heading level by “asking” the closest Section for its level. Each Section tracks its own level by asking the parent Section and adding one to it. Every Section provides information to all components below it without passing props—it does that through context.
import Heading from './Heading.js';
import Section from './Section.js';
export default function Page() {
return (
<Section>
<Heading>Title</Heading>
<Section>
<Heading>Heading</Heading>
<Heading>Heading</Heading>
<Heading>Heading</Heading>
<Section>
<Heading>Sub-heading</Heading>
<Heading>Sub-heading</Heading>
<Heading>Sub-heading</Heading>
<Section>
<Heading>Sub-sub-heading</Heading>
<Heading>Sub-sub-heading</Heading>
<Heading>Sub-sub-heading</Heading>
</Section>
</Section>
</Section>
</Section>
);
}
Ready to learn this topic?
Read Passing Data Deeply with Context to learn about using context as an alternative to passing props.
Read MoreScaling up with reducer and context
Reducers let you consolidate a component’s state update logic. Context lets you pass information deep down to other components. You can combine reducers and context together to manage state of a complex screen.
With this approach, a parent component with complex state manages it with a reducer. Other components anywhere deep in the tree can read its state via context. They can also dispatch actions to update that state.
import AddTask from './AddTask.js';
import TaskList from './TaskList.js';
import { TasksProvider } from './TaskBoardContext.js';
export default function TaskBoard() {
return (
<TasksProvider>
<AddTask />
<TaskList />
</TasksProvider>
);
}
Ready to learn this topic?
Read Scaling Up with Reducer and Context to learn how state management scales in a growing app.
Read MoreWhat’s next?
Head over to Reacting to Input with State to start reading this chapter page by page!
Or, if you’re already familiar with these topics, why not read about Escape Hatches?