State: A Component's Memory
Components often need to change whatâs on the screen as a result of an interaction. Typing into the form should update the input field, clicking ânextâ on an image carousel should change which image is displayed, clicking âbuyâ should put a product in the shopping cart. Components need to ârememberâ things: the current input value, the current image, the shopping cart. In React, this kind of component-specific memory is called state.
You will learn
- How to add a state variable with the
useState
Hook - What pair of values the
useState
Hook returns - How to add more than one state variable
- Why state is called local
When a regular variable isnât enough
Hereâs a component that renders a sculpture image. Clicking the âNextâ button should show the next sculpture by changing the index
to 1
, then 2
, and so on. However, this wonât work (you can try it!):
import { sculptureList } from './data.js'; export default function Gallery() { let index = 0; function handleClick() { index = index + 1; } let sculpture = sculptureList[index]; return ( <> <button onClick={handleClick}> Next </button> <h2> <i>{sculpture.name} </i> by {sculpture.artist} </h2> <h3> ({index + 1} of {sculptureList.length}) </h3> <img src={sculpture.url} alt={sculpture.alt} /> <p> {sculpture.description} </p> </> ); }
The handleClick
event handler is updating a local variable, index
. But two things prevent that change from being visible:
- Local variables donât persist between renders. When React renders this component a second time, it renders it from scratchâit doesnât consider any changes to the local variables.
- Changes to local variables wonât trigger renders. React doesnât realize it needs to render the component again with the new data.
To update a component with new data, two things need to happen:
- Retain the data between renders.
- Trigger React to render the component with new data (re-rendering).
The useState
Hook provides those two things:
- A state variable to retain the data between renders.
- A state setter function to update the variable and trigger React to render the component again.
Adding a state variable
To add a state variable, import useState
from React at the top of the file:
import { useState } from 'react';
Then, replace this line:
let index = 0;
with
const [index, setIndex] = useState(0);
index
is a state variable and setIndex
is the setter function.
The
[
and]
syntax here is called array destructuring and it lets you read values from an array. The array returned byuseState
always has exactly two items.
This is how they work together in handleClick
:
function handleClick() {
setIndex(index + 1);
}
Now clicking the âNextâ button switches the current sculpture:
import { useState } from 'react'; import { sculptureList } from './data.js'; export default function Gallery() { const [index, setIndex] = useState(0); function handleClick() { setIndex(index + 1); } let sculpture = sculptureList[index]; return ( <> <button onClick={handleClick}> Next </button> <h2> <i>{sculpture.name} </i> by {sculpture.artist} </h2> <h3> ({index + 1} of {sculptureList.length}) </h3> <img src={sculpture.url} alt={sculpture.alt} /> <p> {sculpture.description} </p> </> ); }
Meet your first Hook
In React, useState
, as well as any other function starting with âuse
â, is called a Hook.
Hooks are special functions that are only available while React is rendering (which weâll get into in more detail on the next page). They let you âhook intoâ different React features.
State is just one of those features, but you will meet the other Hooks later.
Anatomy of useState
When you call useState
, you are telling React that you want this component to remember something:
const [index, setIndex] = useState(0);
In this case, you want React to remember index
.
The convention is to name this pair like
const [something, setSomething]
. You could name it anything you like, but conventions make things easier to understand across projects.
The only argument to useState
is the initial value of your state variable. In this example, the index
âs initial value is set to 0
with useState(0)
.
Every time your component renders, useState
gives you an array containing two values:
- The state variable (
index
) with the value you stored. - The state setter function (
setIndex
) which can update the state variable and trigger React to render the component again.
Hereâs how that happens in action:
const [index, setIndex] = useState(0);
- Your component renders the first time. Because you passed
0
touseState
as the initial value forindex
, it will return[0, setIndex]
. React remembers0
is the latest state value. - You update the state. When a user clicks the button, it calls
setIndex(index + 1)
.index
is0
, so itâssetIndex(1)
. This tells React to rememberindex
is1
now and triggers another render. - Your componentâs second render. React still sees
useState(0)
, but because React remembers that you setindex
to1
, it returns[1, setIndex]
instead. - And so on!
Giving a component multiple state variables
You can have as many state variables of as many types as you like in one component. This component has two state variables, a number index
and a boolean showMore
thatâs toggled when you click âShow detailsâ:
import { useState } from 'react'; import { sculptureList } from './data.js'; export default function Gallery() { const [index, setIndex] = useState(0); const [showMore, setShowMore] = useState(false); function handleNextClick() { setIndex(index + 1); } 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} /> </> ); }
It is a good idea to have multiple state variables if their state is unrelated, like index
and showMore
in this example. But if you find that you often change two state variables together, it might be better to combine them into a single one. For example, if you have a form with many fields, itâs more convenient to have a single state variable that holds an object than state variable per field. Choosing the State Structure has more tips on this.
Deep Dive
How does React know which state to return?
How does React know which state to return?
You might have noticed that the useState
call does not receive any information about which state variable it refers to. There is no âidentifierâ that is passed to useState
, so how does it know which of the state variables to return? Does it rely on some magic like parsing your functions? The answer is no.
Instead, to enable their concise syntax, Hooks rely on a stable call order on every render of the same component. This works well in practice because if you follow the rule above (âonly call Hooks at the top levelâ), Hooks will always be called in the same order. Additionally, a linter plugin catches most mistakes.
Internally, React holds an array of state pairs for every component. It also maintains the current pair index, which is set to 0
before rendering. Each time you call useState
, React gives you the next state pair and increments the index. You can read more about this mechanism in React Hooks: Not Magic, Just Arrays.
This example doesnât use React but it gives you an idea of how useState
works internally:
let componentHooks = []; let currentHookIndex = 0; // How useState works inside React (simplified). function useState(initialState) { let pair = componentHooks[currentHookIndex]; if (pair) { // This is not the first render, // so the state pair already exists. // Return it and prepare for next Hook call. currentHookIndex++; return pair; } // This is the first time we're rendering, // so create a state pair and store it. pair = [initialState, setState]; function setState(nextState) { // When the user requests a state change, // put the new value into the pair. pair[0] = nextState; updateDOM(); } // Store the pair for future renders // and prepare for the next Hook call. componentHooks[currentHookIndex] = pair; currentHookIndex++; return pair; } function Gallery() { // Each useState() call will get the next pair. const [index, setIndex] = useState(0); const [showMore, setShowMore] = useState(false); function handleNextClick() { setIndex(index + 1); } function handleMoreClick() { setShowMore(!showMore); } let sculpture = sculptureList[index]; // This example doesn't use React, so // return an output object instead of JSX. return { onNextClick: handleNextClick, onMoreClick: handleMoreClick, header: `${sculpture.name} by ${sculpture.artist}`, counter: `${index + 1} of ${sculptureList.length}`, more: `${showMore ? 'Hide' : 'Show'} details`, description: showMore ? sculpture.description : null, imageSrc: sculpture.url, imageAlt: sculpture.alt }; } function updateDOM() { // Reset the current Hook index // before rendering the component. currentHookIndex = 0; let output = Gallery(); // Update the DOM to match the output. // This is the part React does for you. nextButton.onclick = output.onNextClick; header.textContent = output.header; moreButton.onclick = output.onMoreClick; moreButton.textContent = output.more; image.src = output.imageSrc; image.alt = output.imageAlt; if (output.description !== null) { description.textContent = output.description; description.style.display = ''; } else { description.style.display = 'none'; } } let nextButton = document.getElementById('nextButton'); let header = document.getElementById('header'); let moreButton = document.getElementById('moreButton'); let description = document.getElementById('description'); let image = document.getElementById('image'); let sculptureList = [{ name: 'Homenaje a la NeurocirugĂa', 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 GenĂ©rica', 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 Cubismâs 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.' }]; // Make UI match the initial state. updateDOM();
You donât have to understand it to use React, but you might find this a helpful mental model.
State is isolated and private
State is local to a component instance on the screen. In other words, if you render the same component twice, each copy will have completely isolated state! Changing one of them will not affect the other.
In this example, the Gallery
component from earlier is rendered twice with no changes to its logic. Try clicking the buttons inside each of the galleries. Notice that their state is independent:
import Gallery from './Gallery.js'; export default function Page() { return ( <div className="Page"> <Gallery /> <Gallery /> </div> ); }
This is what makes state different from regular variables that you might declare at the top of your module. State is not tied to a particular function call or a place in the code, but itâs âlocalâ to the specific place on the screen. You rendered two <Gallery />
components, so their state is stored separately.
Also notice how the Page
component doesnât âknowâ anything about the Gallery
state or even whether it has any. Unlike props, state is fully private to the component declaring it. The parent component canât change it. This lets you add state to any component or remove it without impacting the rest of the components.
What if you wanted both galleries to keep their states in sync? The right way to do it in React is to remove state from child components and add it to their closest shared parent. The next few pages will focus on organizing state of a single component, but we will return to this topic in Sharing State Between Components.
Recap
- Use a state variable when a component needs to ârememberâ some information between renders.
- State variables are declared by calling the
useState
Hook. - Hooks are special functions that start with
use
. They let you âhook intoâ React features like state. - Hooks might remind you of imports: they need to be called unconditionally. Calling Hooks, including
useState
, is only valid at the top level of a component or another Hook. - The
useState
Hook returns a pair of values: the current state and the function to update it. - You can have more than one state variable. Internally, React matches them up by their order.
- State is private to the component. If you render it in two places, each copy gets its own state.
Challenge 1 of 4: Complete the gallery
When you press âNextâ on the last sculpture, the code crashes. Fix the logic to prevent the crash. You may do this by adding extra logic to event handler or by disabling the button when the action is not possible.
After fixing the crash, add a âPreviousâ button that shows the previous sculpture. It shouldnât crash on the first sculpture.
import { useState } from 'react'; import { sculptureList } from './data.js'; export default function Gallery() { const [index, setIndex] = useState(0); const [showMore, setShowMore] = useState(false); function handleNextClick() { setIndex(index + 1); } 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} /> </> ); }