Flash cards
Review the key moves
What is the main idea behind React Lists?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
___ MyCars() {Put the learning moves in the order that makes the concept easiest to apply.
In React, you will render lists with some type of loop.
The JavaScript map() array method is generally the preferred method.
If you need a refresher on the map() method, check out the ES6 Array map() section .
Example
Let's create a simple list using the map() method:
function MyCars() {
const cars = ['Ford', 'BMW', 'Audi'];
return ( <> <h1>My Cars:</h1> <ul> {cars.map((car) => <li>I am a { car }</li>)}
</ul> </> );
}
createRoot(document.getElementById('root')).render( <MyCars /> );When you run this code in your React environment, it will work but you will receive a warning that there is no "key" provided for the list items.
Keys in React Lists
Keys allow React to keep track of elements. This way, if an item is updated or removed, only that item will be re-rendered instead of the entire list.
Keys must be unique among siblings, but they don't have to be unique across the entire application.
Generally, the key should be a unique ID assigned to each item. As a last resort, you can use the array index as a key.
Example
Here the example from above, with keys:
function MyCars() {
const cars = [ {id: 1001, brand: 'Ford'}, {id: 1002, brand: 'BMW'}, {id: 1003, brand: 'Audi'}
];
return ( <> <h1>My Cars:</h1> <ul> {cars.map((car) => <li key={car.id}>I am a { car.brand }</li>)}
</ul> </> );
}
createRoot(document.getElementById('root')).render( <MyCars /> );Using Array Index as Keys
While it's possible to use the array index as a key, it's not recommended unless:
- The list is static (won't change)
- The list will never be reordered or filtered
- The items in the list have no IDs
Example
Using array indexes as keys (not recommended for dynamic lists):
function MyCars() {
const cars = ['Ford', 'BMW', 'Audi'];
return ( <> <h1>My Cars:</h1> <ul> {cars.map((car, index) => <li key={index}>I am a { car }</li>)}
</ul> </> );
}
createRoot(document.getElementById('root')).render( <MyCars /> );