Rendering Lists
You will often want to display multiple similar components from a collection of data. You can use the JavaScript array methods to manipulate an array of data. On this page, you’ll use filter()
and map()
with React to filter and transform your array of data into an array of components.
You will learn
- How to render components from an array using JavaScript’s
map()
- How to render only specific components using JavaScript’s
filter()
- When and why to use React keys
Rendering data from arrays
Say that you have a list of content.
<ul>
<li>Creola Katherine Johnson: mathematician</li>
<li>Mario José Molina-Pasquel Henríquez: chemist</li>
<li>Mohammad Abdus Salam: physicist</li>
<li>Percy Lavon Julian: chemist</li>
<li>Subrahmanyan Chandrasekhar: astrophysicist</li>
</ul>
The only difference among those list items is their contents, their data. You will often need to show several instances of the same component using different data when building interfaces: from lists of comments to galleries of profile images. In these situations, you can store that data in JavaScript objects and arrays and use methods like map()
and filter()
to render lists of components from them.
Here’s a short example of how to generate a list of items from an array:
- Move the data into an array:
const people = [
'Creola Katherine Johnson: mathematician',
'Mario José Molina-Pasquel Henríquez: chemist',
'Mohammad Abdus Salam: physicist',
'Percy Lavon Julian: chemist',
'Subrahmanyan Chandrasekhar: astrophysicist'
];
- Map the
people
members into a new array of JSX nodes,listItems
:
const listItems = people.map(person => <li>{person}</li>);
- Return
listItems
from your component wrapped in a<ul>
:
return <ul>{listItems}</ul>;
Here is the result:
const people = [ 'Creola Katherine Johnson: mathematician', 'Mario José Molina-Pasquel Henríquez: chemist', 'Mohammad Abdus Salam: physicist', 'Percy Lavon Julian: chemist', 'Subrahmanyan Chandrasekhar: astrophysicist' ]; export default function List() { const listItems = people.map(person => <li>{person}</li> ); return <ul>{listItems}</ul>; }
Error
Extra 185 of 186 byte(s) found at buffer[1]
Notice the sandbox above displays a console error:
You’ll learn how to fix this error later on this page. Before we get to that, let’s add some structure to your data.
Filtering arrays of items
This data can be structured even more.
const people = [{
id: 0,
name: 'Creola Katherine Johnson',
profession: 'mathematician',
}, {
id: 1,
name: 'Mario José Molina-Pasquel Henríquez',
profession: 'chemist',
}, {
id: 2,
name: 'Mohammad Abdus Salam',
profession: 'physicist',
}, {
id: 3,
name: 'Percy Lavon Julian',
profession: 'chemist',
}, {
id: 4,
name: 'Subrahmanyan Chandrasekhar',
profession: 'astrophysicist',
}];
Let’s say you want a way to only show people whose profession is 'chemist'
. You can use JavaScript’s filter()
method to return just those people. This method takes an array of items, passes them through a “test” (a function that returns true
or false
), and returns a new array of only those items that passed the test (returned true
).
You only want the items where profession
is 'chemist'
. The “test” function for this looks like (person) => person.profession === 'chemist'
. Here’s how to put it together:
- Create a new array of just “chemist” people,
chemists
, by callingfilter()
on thepeople
filtering byperson.profession === 'chemist'
:
const chemists = people.filter(person =>
person.profession === 'chemist'
);
- Now map over
chemists
:
const listItems = chemists.map(person =>
<li>
<img
src={getImageUrl(person)}
alt={person.name}
/>
<p>
<b>{person.name}:</b>
{' ' + person.profession + ' '}
known for {person.accomplishment}
</p>
</li>
);
- Lastly, return the
listItems
from your component:
return <ul>{listItems}</ul>;
import { people } from './data.js'; import { getImageUrl } from './utils.js'; export default function List() { const chemists = people.filter(person => person.profession === 'chemist' ); const listItems = chemists.map(person => <li> <img src={getImageUrl(person)} alt={person.name} /> <p> <b>{person.name}:</b> {' ' + person.profession + ' '} known for {person.accomplishment} </p> </li> ); return <ul>{listItems}</ul>; }
Keeping list items in order with key
Notice that all the sandboxes above show an error in the console:
You need to give each array item a key
— a string or a number that uniquely identifies it among other items in that array:
<li key={person.id}>...</li>
Keys tell React which array item each component corresponds to, so that it can match them up later. This becomes important if your array items can move (e.g. due to sorting), get inserted, or get deleted. A well-chosen key
helps React infer what exactly has happened, and make the correct updates to the DOM tree.
Rather than generating keys on the fly, you should include them in your data:
export const people = [{ id: 0, // Used in JSX as a key name: 'Creola Katherine Johnson', profession: 'mathematician', accomplishment: 'spaceflight calculations', imageId: 'MK3eW3A' }, { id: 1, // Used in JSX as a key name: 'Mario José Molina-Pasquel Henríquez', profession: 'chemist', accomplishment: 'discovery of Arctic ozone hole', imageId: 'mynHUSa' }, { id: 2, // Used in JSX as a key name: 'Mohammad Abdus Salam', profession: 'physicist', accomplishment: 'electromagnetism theory', imageId: 'bE7W1ji' }, { id: 3, // Used in JSX as a key name: 'Percy Lavon Julian', profession: 'chemist', accomplishment: 'pioneering cortisone drugs, steroids and birth control pills', imageId: 'IOjWm71' }, { id: 4, // Used in JSX as a key name: 'Subrahmanyan Chandrasekhar', profession: 'astrophysicist', accomplishment: 'white dwarf star mass calculations', imageId: 'lrWQx8l' }];
Deep Dive
What do you do when each item needs to render not one, but several DOM nodes?
The short <>...</>
Fragment syntax won’t let you pass a key, so you need to either group them into a single <div>
, or use the slightly longer and more explicit <Fragment>
syntax:
import { Fragment } from 'react';
// ...
const listItems = people.map(person =>
<Fragment key={person.id}>
<h1>{person.name}</h1>
<p>{person.bio}</p>
</Fragment>
);
Fragments disappear from the DOM, so this will produce a flat list of <h1>
, <p>
, <h1>
, <p>
, and so on.
Where to get your key
Different sources of data provide different sources of keys:
- Data from a database: If your data is coming from a database, you can use the database keys/IDs, which are unique by nature.
- Locally generated data: If your data is generated and persisted locally (e.g. notes in a note-taking app), use an incrementing counter,
crypto.randomUUID()
or a package likeuuid
when creating items.
Rules of keys
- Keys must be unique among siblings. However, it’s okay to use the same keys for JSX nodes in different arrays.
- Keys must not change or that defeats their purpose! Don’t generate them while rendering.
Why does React need keys?
Imagine that files on your desktop didn’t have names. Instead, you’d refer to them by their order — the first file, the second file, and so on. You could get used to it, but once you delete a file, it would get confusing. The second file would become the first file, the third file would be the second file, and so on.
File names in a folder and JSX keys in an array serve a similar purpose. They let us uniquely identify an item between its siblings. A well-chosen key provides more information than the position within the array. Even if the position changes due to reordering, the key
lets React identify the item throughout its lifetime.
Recap
On this page you learned:
- How to move data out of components and into data structures like arrays and objects.
- How to generate sets of similar components with JavaScript’s
map()
. - How to create arrays of filtered items with JavaScript’s
filter()
. - Why and how to set
key
on each component in a collection so React can keep track of each of them even if their position or data changes.
Challenge 1 of 4: Splitting a list in two
This example shows a list of all people.
Change it to show two separate lists one after another: Chemists and Everyone Else. Like previously, you can determine whether a person is a chemist by checking if person.profession === 'chemist'
.
import { people } from './data.js'; import { getImageUrl } from './utils.js'; export default function List() { const listItems = people.map(person => <li key={person.id}> <img src={getImageUrl(person)} alt={person.name} /> <p> <b>{person.name}:</b> {' ' + person.profession + ' '} known for {person.accomplishment} </p> </li> ); return ( <article> <h1>Scientists</h1> <ul>{listItems}</ul> </article> ); }