Managing State

Intermediate

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.

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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 quiz 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.

Fork

import { useState } from 'react';

export default function Form() {
  const [answer, setAnswer] = useState('');
  const [error, setError] = useState(null);
  const [status, setStatus] = useState('typing');

  if (status === 'success') {
    return <h1>That's right!</h1>
  }

  async function handleSubmit(e) {
    e.preventDefault();
    setStatus('submitting');
    try {
      await submitForm(answer);
      setStatus('success');
    } catch (err) {
      setStatus('typing');
      setError(err);
    }
  }

  function handleTextareaChange(e) {
    setAnswer(e.target.value);
  }

  return (
    <>
      <h2>City quiz</h2>
      <p>
        In which city is there a billboard that turns air into drinkable water?
      </p>
      <form onSubmit={handleSubmit}>
        <textarea
          value={answer}
          onChange={handleTextareaChange}
          disabled={status === 'submitting'}
        />
        <br />
        <button disabled={
          answer.length === 0 ||
          status === 'submitting'
        }>
          Submit
        </button>
        {error !== null &&
          <p className="Error">
            {error.message}
          </p>
        }
      </form>
    </>
  );
}

function submitForm(answer) {
  // Pretend it's hitting the network.
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      let shouldError = answer.toLowerCase() !== 'lima'
      if (shouldError) {
        reject(new Error('Good guess but a wrong answer. Try again!'));
      } 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.

Choosing 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 unnecessary state, it’s easy to forget to update it, and introduce bugs!

For example, this form has a redundant fullName state variable:

Fork

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 (
    <>
      <h2>Let’s check you in</h2>
      <label>
        First name:{' '}
        <input
          value={firstName}
          onChange={handleFirstNameChange}
        />
      </label>
      <label>
        Last name:{' '}
        <input
          value={lastName}
          onChange={handleLastNameChange}
        />
      </label>
      <p>
        Your ticket will be issued to: <b>{fullName}</b>
      </p>
    </>
  );
}

You can remove it and simplify the code by calculating fullName while the component is rendering:

Fork

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 (
    <>
      <h2>Let’s check you in</h2>
      <label>
        First name:{' '}
        <input
          value={firstName}
          onChange={handleFirstNameChange}
        />
      </label>
      <label>
        Last name:{' '}
        <input
          value={lastName}
          onChange={handleLastNameChange}
        />
      </label>
      <p>
        Your ticket will be issued to: <b>{fullName}</b>
      </p>
    </>
  );
}

This might seem like a small change, but many bugs in React apps are fixed this way.

Ready to learn this topic?

Read Choosing the State Structure to learn how to design the state shape to avoid bugs.

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.

Fork

import { useState } from 'react';

export default function Accordion() {
  const [activeIndex, setActiveIndex] = useState(0);
  return (
    <>
      <h2>Almaty, Kazakhstan</h2>
      <Panel
        title="About"
        isActive={activeIndex === 0}
        onShow={() => setActiveIndex(0)}
      >
        With a population of about 2 million, Almaty is Kazakhstan's largest city. From 1929 to 1997, it was its capital city.
      </Panel>
      <Panel
        title="Etymology"
        isActive={activeIndex === 1}
        onShow={() => setActiveIndex(1)}
      >
        The name comes from <span lang="kk-KZ">алма</span>, the Kazakh word for "apple" and is often translated as "full of apples". In fact, the region surrounding Almaty is thought to be the ancestral home of the apple, and the wild <i lang="la">Malus sieversii</i> is considered a likely candidate for the ancestor of the modern domestic apple.
      </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.

Preserving 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. In this chat app, typing a message and then switching the recipient does not reset the input. This can make the user accidentally send a message to the wrong person:

Fork

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 that needs to be re-created from scratch with the new data (and UI like inputs). Now switching between the recipients resets the input field—even though you render the same component.

Fork

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.

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 “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!

Fork

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 }
];

Ready to learn this topic?

Read Extracting State Logic into a Reducer to learn how to consolidate logic in the reducer function.

Passing data deeply with context

Usually, you will pass information from a parent component to a child component via props. But passing props can become inconvenient if you need to pass some prop through many components, or if many components need the same information. 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.

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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.

Scaling 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.

Fork

import AddTask from './AddTask.js';
import TaskList from './TaskList.js';
import { TasksProvider } from './TasksContext.js';

export default function TaskApp() {
  return (
    <TasksProvider>
      <h1>Day off in Kyoto</h1>
      <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.

What’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?

Previousstate 内の配列の更新 Nextstate を使って入力に反応する

Managing State

この章で学ぶこと

Reacting to input with state

Ready to learn this topic?

Choosing the state structure

Ready to learn this topic?

Sharing state between components

Ready to learn this topic?

Preserving and resetting state

Ready to learn this topic?

Extracting state logic into a reducer

Ready to learn this topic?

Passing data deeply with context

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Scaling up with reducer and context

Ready to learn this topic?

What’s next?