Adding Interactivity
Some things on the screen update in response to user input. For example, clicking an image gallery switches the active image. In React, data that changes over time is called state. You can add state to any component, and update it as needed. In this chapter, you'll learn how to write components that handle interactions, update their state, and display different output over time.
- How to handle user-initiated events
- How to make components "remember" information with state
- How React updates the UI in two phases
- Why state doesn't update right after you change it
- How to queue multiple state updates
- How to update an object in state
- How to update an array in state
Responding to events
React lets you add event handlers to your JSX. Event handlers are your own functions that will be triggered in response to user interactions like clicking, hovering, focusing on form inputs, and so on.
Built-in components like <button> only support built-in browser events like onClick. However, you can also create your own components, and give their event handler props any application-specific names that you like.
export default function App() {
return (
<Toolbar
onPlayMovie={() => alert('Playing!')}
onUploadImage={() => alert('Uploading!')}
/>
);
}
function Toolbar({ onPlayMovie, onUploadImage }) {
return (
<div>
<Button onClick={onPlayMovie}>
Play Movie
</Button>
<Button onClick={onUploadImage}>
Upload Image
</Button>
</div>
);
}
function Button({ onClick, children }) {
return (
<button onClick={onClick}>
{children}
</button>
);
}
button { margin-right: 10px; }
Read Responding to Events to learn how to add event handlers.
State: a component's memory
Components often need to change what's on the screen as a result of an interaction. Typing into the form should update the input field, clicking "next" on an image carousel should change which image is displayed, clicking "buy" puts a product in the shopping cart. Components need to "remember" things: the current input value, the current image, the shopping cart. In React, this kind of component-specific memory is called state.
You can add state to a component with a useState Hook. Hooks are special functions that let your components use React features (state is one of those features). The useState Hook lets you declare a state variable. It takes the initial state and returns a pair of values: the current state, and a state setter function that lets you update it.
const [index, setIndex] = useState(0);
const [showMore, setShowMore] = useState(false);
Here is how an image gallery uses and updates state on click:
import { useState } from 'react';
import { sculptureList } from './data.js';
export default function Gallery() {
const [index, setIndex] = useState(0);
const [showMore, setShowMore] = useState(false);
const hasNext = index < sculptureList.length - 1;
function handleNextClick() {
if (hasNext) {
setIndex(index + 1);
} else {
setIndex(0);
}
}
function handleMoreClick() {
setShowMore(!showMore);
}
let sculpture = sculptureList[index];
return (
<>
<button onClick={handleNextClick}>
Next
</button>
<h2>
<i>{sculpture.name} </i>
by {sculpture.artist}
</h2>
<h3>
({index + 1} of {sculptureList.length})
</h3>
<button onClick={handleMoreClick}>
{showMore ? 'Hide' : 'Show'} details
</button>
{showMore && <p>{sculpture.description}</p>}
<img
src={sculpture.url}
alt={sculpture.alt}
/>
</>
);
}
export const sculptureList = [{
name: 'Homenaje a la Neurocirugía',
artist: 'Marta Colvin Andrade',
description: 'Although Colvin is predominantly known for abstract themes that allude to pre-Hispanic symbols, this gigantic sculpture, an homage to neurosurgery, is one of her most recognizable public art pieces.',
url: 'https://i.imgur.com/Mx7dA2Y.jpg',
alt: 'A bronze statue of two crossed hands delicately holding a human brain in their fingertips.'
}, {
name: 'Floralis Genérica',
artist: 'Eduardo Catalano',
description: 'This enormous (75 ft. or 23m) silver flower is located in Buenos Aires. It is designed to move, closing its petals in the evening or when strong winds blow and opening them in the morning.',
url: 'https://i.imgur.com/ZF6s192m.jpg',
alt: 'A gigantic metallic flower sculpture with reflective mirror-like petals and strong stamens.'
}, {
name: 'Eternal Presence',
artist: 'John Woodrow Wilson',
description: 'Wilson was known for his preoccupation with equality, social justice, as well as the essential and spiritual qualities of humankind. This massive (7ft. or 2,13m) bronze represents what he described as "a symbolic Black presence infused with a sense of universal humanity."',
url: 'https://i.imgur.com/aTtVpES.jpg',
alt: 'The sculpture depicting a human head seems ever-present and solemn. It radiates calm and serenity.'
}, {
name: 'Moai',
artist: 'Unknown Artist',
description: 'Located on the Easter Island, there are 1,000 moai, or extant monumental statues, created by the early Rapa Nui people, which some believe represented deified ancestors.',
url: 'https://i.imgur.com/RCwLEoQm.jpg',
alt: 'Three monumental stone busts with the heads that are disproportionately large with somber faces.'
}, {
name: 'Blue Nana',
artist: 'Niki de Saint Phalle',
description: 'The Nanas are triumphant creatures, symbols of femininity and maternity. Initially, Saint Phalle used fabric and found objects for the Nanas, and later on introduced polyester to achieve a more vibrant effect.',
url: 'https://i.imgur.com/Sd1AgUOm.jpg',
alt: 'A large mosaic sculpture of a whimsical dancing female figure in a colorful costume emanating joy.'
}, {
name: 'Ultimate Form',
artist: 'Barbara Hepworth',
description: 'This abstract bronze sculpture is a part of The Family of Man series located at Yorkshire Sculpture Park. Hepworth chose not to create literal representations of the world but developed abstract forms inspired by people and landscapes.',
url: 'https://i.imgur.com/2heNQDcm.jpg',
alt: 'A tall sculpture made of three elements stacked on each other reminding of a human figure.'
}, {
name: 'Cavaliere',
artist: 'Lamidi Olonade Fakeye',
description: "Descended from four generations of woodcarvers, Fakeye's work blended traditional and contemporary Yoruba themes.",
url: 'https://i.imgur.com/wIdGuZwm.png',
alt: 'An intricate wood sculpture of a warrior with a focused face on a horse adorned with patterns.'
}, {
name: 'Big Bellies',
artist: 'Alina Szapocznikow',
description: "Szapocznikow is known for her sculptures of the fragmented body as a metaphor for the fragility and impermanence of youth and beauty. This sculpture depicts two very realistic large bellies stacked on top of each other, each around five feet (1,5m) tall.",
url: 'https://i.imgur.com/AlHTAdDm.jpg',
alt: 'The sculpture reminds a cascade of folds, quite different from bellies in classical sculptures.'
}, {
name: 'Terracotta Army',
artist: 'Unknown Artist',
description: 'The Terracotta Army is a collection of terracotta sculptures depicting the armies of Qin Shi Huang, the first Emperor of China. The army consisted of more than 8,000 soldiers, 130 chariots with 520 horses, and 150 cavalry horses.',
url: 'https://i.imgur.com/HMFmH6m.jpg',
alt: '12 terracotta sculptures of solemn warriors, each with a unique facial expression and armor.'
}, {
name: 'Lunar Landscape',
artist: 'Louise Nevelson',
description: 'Nevelson was known for scavenging objects from New York City debris, which she would later assemble into monumental constructions. In this one, she used disparate parts like a bedpost, juggling pin, and seat fragment, nailing and gluing them into boxes that reflect the influence of Cubism’s geometric abstraction of space and form.',
url: 'https://i.imgur.com/rN7hY6om.jpg',
alt: 'A black matte sculpture where the individual elements are initially indistinguishable.'
}, {
name: 'Aureole',
artist: 'Ranjani Shettar',
description: 'Shettar merges the traditional and the modern, the natural and the industrial. Her art focuses on the relationship between man and nature. Her work was described as compelling both abstractly and figuratively, gravity defying, and a "fine synthesis of unlikely materials."',
url: 'https://i.imgur.com/okTpbHhm.jpg',
alt: 'A pale wire-like sculpture mounted on concrete wall and descending on the floor. It appears light.'
}, {
name: 'Hippos',
artist: 'Taipei Zoo',
description: 'The Taipei Zoo commissioned a Hippo Square featuring submerged hippos at play.',
url: 'https://i.imgur.com/6o5Vuyu.jpg',
alt: 'A group of bronze hippo sculptures emerging from the sett sidewalk as if they were swimming.'
}];
h2 { margin-top: 10px; margin-bottom: 0; }
h3 {
margin-top: 5px;
font-weight: normal;
font-size: 100%;
}
img { width: 120px; height: 120px; }
button {
display: block;
margin-top: 10px;
margin-bottom: 10px;
}
Read State: A Component's Memory to learn how to remember a value and update it on interaction.
Render and commit
Before your components are displayed on the screen, they must be rendered by React. Understanding the steps in this process will help you think about how your code executes and explain its behavior.
Imagine that your components are cooks in the kitchen, assembling tasty dishes from ingredients. In this scenario, React is the waiter who puts in requests from customers and brings them their orders. This process of requesting and serving UI has three steps:
- Triggering a render (delivering the diner's order to the kitchen)
- Rendering the component (preparing the order in the kitchen)
- Committing to the DOM (placing the order on the table)
Read Render and Commit to learn the lifecycle of a UI update.
State as a snapshot
Unlike regular JavaScript variables, React state behaves more like a snapshot. Setting it does not change the state variable you already have, but instead triggers a re-render. This can be surprising at first!
console.log(count); // 0
setCount(count + 1); // Request a re-render with 1
console.log(count); // Still 0!
This behavior helps you avoid subtle bugs. Here is a little chat app. Try to guess what happens if you press "Send" first and then change the recipient to Bob. Whose name will appear in the alert five seconds later?
import { useState } from 'react';
export default function Form() {
const [to, setTo] = useState('Alice');
const [message, setMessage] = useState('Hello');
function handleSubmit(e) {
e.preventDefault();
setTimeout(() => {
alert(`You said ${message} to ${to}`);
}, 5000);
}
return (
<form onSubmit={handleSubmit}>
<label>
To:{' '}
<select
value={to}
onChange={e => setTo(e.target.value)}>
<option value="Alice">Alice</option>
<option value="Bob">Bob</option>
</select>
</label>
<textarea
placeholder="Message"
value={message}
onChange={e => setMessage(e.target.value)}
/>
<button type="submit">Send</button>
</form>
);
}
label, textarea { margin-bottom: 10px; display: block; }
Read State as a Snapshot to learn why state appears "fixed" and unchanging inside the event handlers.
Queueing a series of state updates
This component is buggy: clicking "+3" increments the score only once.
import { useState } from 'react';
export default function Counter() {
const [score, setScore] = useState(0);
function increment() {
setScore(score + 1);
}
return (
<>
<button onClick={() => increment()}>+1</button>
<button onClick={() => {
increment();
increment();
increment();
}}>+3</button>
<h1>Score: {score}</h1>
</>
)
}
button { display: inline-block; margin: 10px; font-size: 20px; }
State as a Snapshot explains why this is happening. Setting state requests a new re-render, but does not change it in the already running code. So score continues to be 0 right after you call setScore(score + 1).
console.log(score); // 0
setScore(score + 1); // setScore(0 + 1);
console.log(score); // 0
setScore(score + 1); // setScore(0 + 1);
console.log(score); // 0
setScore(score + 1); // setScore(0 + 1);
console.log(score); // 0
You can fix this by passing an updater function when setting state. Notice how replacing setScore(score + 1) with setScore(s => s + 1) fixes the "+3" button. This lets you queue multiple state updates.
import { useState } from 'react';
export default function Counter() {
const [score, setScore] = useState(0);
function increment() {
setScore(s => s + 1);
}
return (
<>
<button onClick={() => increment()}>+1</button>
<button onClick={() => {
increment();
increment();
increment();
}}>+3</button>
<h1>Score: {score}</h1>
</>
)
}
button { display: inline-block; margin: 10px; font-size: 20px; }
Read Queueing a Series of State Updates to learn how to queue a sequence of state updates.
Updating objects in state
State can hold any kind of JavaScript value, including objects. But you shouldn't change objects and arrays that you hold in the React state directly. Instead, when you want to update an object and array, you need to create a new one (or make a copy of an existing one), and then update the state to use that copy.
Usually, you will use the ... spread syntax to copy objects and arrays that you want to change. For example, updating a nested object could look like this:
import { useState } from 'react';
export default function Form() {
const [person, setPerson] = useState({
name: 'Niki de Saint Phalle',
artwork: {
title: 'Blue Nana',
city: 'Hamburg',
image: 'https://i.imgur.com/Sd1AgUOm.jpg',
}
});
function handleNameChange(e) {
setPerson({
...person,
name: e.target.value
});
}
function handleTitleChange(e) {
setPerson({
...person,
artwork: {
...person.artwork,
title: e.target.value
}
});
}
function handleCityChange(e) {
setPerson({
...person,
artwork: {
...person.artwork,
city: e.target.value
}
});
}
function handleImageChange(e) {
setPerson({
...person,
artwork: {
...person.artwork,
image: e.target.value
}
});
}
return (
<>
<label>
Name:
<input
value={person.name}
onChange={handleNameChange}
/>
</label>
<label>
Title:
<input
value={person.artwork.title}
onChange={handleTitleChange}
/>
</label>
<label>
City:
<input
value={person.artwork.city}
onChange={handleCityChange}
/>
</label>
<label>
Image:
<input
value={person.artwork.image}
onChange={handleImageChange}
/>
</label>
<p>
<i>{person.artwork.title}</i>
{' by '}
{person.name}
<br />
(located in {person.artwork.city})
</p>
<img
src={person.artwork.image}
alt={person.artwork.title}
/>
</>
);
}
label { display: block; }
input { margin-left: 5px; margin-bottom: 5px; }
img { width: 200px; height: 200px; }
If copying objects in code gets tedious, you can use a library like Immer to reduce repetitive code:
import { useImmer } from 'use-immer';
export default function Form() {
const [person, updatePerson] = useImmer({
name: 'Niki de Saint Phalle',
artwork: {
title: 'Blue Nana',
city: 'Hamburg',
image: 'https://i.imgur.com/Sd1AgUOm.jpg',
}
});
function handleNameChange(e) {
updatePerson(draft => {
draft.name = e.target.value;
});
}
function handleTitleChange(e) {
updatePerson(draft => {
draft.artwork.title = e.target.value;
});
}
function handleCityChange(e) {
updatePerson(draft => {
draft.artwork.city = e.target.value;
});
}
function handleImageChange(e) {
updatePerson(draft => {
draft.artwork.image = e.target.value;
});
}
return (
<>
<label>
Name:
<input
value={person.name}
onChange={handleNameChange}
/>
</label>
<label>
Title:
<input
value={person.artwork.title}
onChange={handleTitleChange}
/>
</label>
<label>
City:
<input
value={person.artwork.city}
onChange={handleCityChange}
/>
</label>
<label>
Image:
<input
value={person.artwork.image}
onChange={handleImageChange}
/>
</label>
<p>
<i>{person.artwork.title}</i>
{' by '}
{person.name}
<br />
(located in {person.artwork.city})
</p>
<img
src={person.artwork.image}
alt={person.artwork.title}
/>
</>
);
}
{
"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"
}
}
label { display: block; }
input { margin-left: 5px; margin-bottom: 5px; }
img { width: 200px; height: 200px; }
Read Updating Objects in State to learn how to update objects correctly.
Updating arrays in state
Arrays are another type of mutable JavaScript objects you can store in state and should treat as read-only. Just like with objects, when you want to update an array stored in state, you need to create a new one (or make a copy of an existing one), and then set state to use the new array:
import { useState } from 'react';
const initialList = [
{ id: 0, title: 'Big Bellies', seen: false },
{ id: 1, title: 'Lunar Landscape', seen: false },
{ id: 2, title: 'Terracotta Army', seen: true },
];
export default function BucketList() {
const [list, setList] = useState(
initialList
);
function handleToggle(artworkId, nextSeen) {
setList(list.map(artwork => {
if (artwork.id === artworkId) {
return { ...artwork, seen: nextSeen };
} else {
return artwork;
}
}));
}
return (
<>
<h1>Art Bucket List</h1>
<h2>My list of art to see:</h2>
<ItemList
artworks={list}
onToggle={handleToggle} />
</>
);
}
function ItemList({ artworks, onToggle }) {
return (
<ul>
{artworks.map(artwork => (
<li key={artwork.id}>
<label>
<input
type="checkbox"
checked={artwork.seen}
onChange={e => {
onToggle(
artwork.id,
e.target.checked
);
}}
/>
{artwork.title}
</label>
</li>
))}
</ul>
);
}
If copying arrays in code gets tedious, you can use a library like Immer to reduce repetitive code:
import { useState } from 'react';
import { useImmer } from 'use-immer';
const initialList = [
{ id: 0, title: 'Big Bellies', seen: false },
{ id: 1, title: 'Lunar Landscape', seen: false },
{ id: 2, title: 'Terracotta Army', seen: true },
];
export default function BucketList() {
const [list, updateList] = useImmer(initialList);
function handleToggle(artworkId, nextSeen) {
updateList(draft => {
const artwork = draft.find(a =>
a.id === artworkId
);
artwork.seen = nextSeen;
});
}
return (
<>
<h1>Art Bucket List</h1>
<h2>My list of art to see:</h2>
<ItemList
artworks={list}
onToggle={handleToggle} />
</>
);
}
function ItemList({ artworks, onToggle }) {
return (
<ul>
{artworks.map(artwork => (
<li key={artwork.id}>
<label>
<input
type="checkbox"
checked={artwork.seen}
onChange={e => {
onToggle(
artwork.id,
e.target.checked
);
}}
/>
{artwork.title}
</label>
</li>
))}
</ul>
);
}
{
"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"
}
}
Read Updating Arrays in State to learn how to update arrays correctly.
What's next?
Head over to Responding to Events to start reading this chapter page by page!
Or, if you're already familiar with these topics, why not read about Managing State?