Variables are a fundamental concept in programming because it allows us to store and modify data depending on where or how our program is used. In a React app, variables are made using something called the useState hook. React hooks are functions that can be called from within our functional components like App to use various core React features.
The tutorial starts getting a bit tricky here. If you get lost at any point, take a look at the sample implementation at github.com/dqna64/reactjs-project-tutorial/tree/full-walkthrough.
To use the useState hook, first import it into our App.jsx file. Then inside the App functional component and above the return statement, write the following:
import './styles/App.css'
import './styles/Card.css'
import logo from './assets/logo512.png'
import { ReactComponent as Star } from './assets/star.svg'
import { useState } from 'react'
const App = () => {
const [name, setName] = useState("Machiavelli Ristorante Italiano")
return (
<>
<header>
<nav>
Food
<img src={logo} alt="logo" />
Places
</nav>
</header>
...
Calling the useState function returns two things: a state value and a function that can be used to change this state value while our app is running. We use the const [name, setname] JavaScript "destructuring" syntax to assign the variable name to the first return value and the variable setName to the second return value.
useState takes in an argument -- the initial value of the state. We give it the string "Machiavelli Ristorante Italiano" because we want the state name to store the name of our restaurant.
We can now use this name state in our app. Replace our hard-coded restaurant name with the name state:
const App = () => {
const [name, setName] = useState("Machiavelli Ristorante Italiano")
return (
<>
<header>
<nav>
Food
<img src={logo} alt="logo" />
Places
</nav>
</header>
<main>
<h1>My List</h1>
<p>
Here is a list of all the restaurants, cafes, dessert bars and other eateries that I want
to keep a record of either because I like them, dislike them, or would like to find out.
</p>
<div className="card">
<h3>{name}</h3>
<h4>Pizza · Pasta</h4>
<h4>123 Clarence St, Sydney NSW 2000</h4>
<h4>
<a href="https://www.machiavelli.com.au" target="blank">
www.machiavelli.com.au
</a>
</h4>
Our app should still be working as it was before. Let's turn all the other restaurant information into state.
const App = () => {
const [name, setName] = useState("Machiavelli Ristorante Italiano")
const [tags, setTags] = useState(["Pizza", "Pasta"])
const [address, setAddress] = useState("123 Clarence St, Sydney NSW 2000")
const [website, setWebsite] = useState("https://www.machiavelli.com.au")
const [notes, setNotes] = useState("Authentic Italian restaurant serving a traditional Neapolitan cuisine. The Linguine Gamberi was good value but a little bland in flavour.")
const [rating, setRating] = useState(4)
const [visits, setVisits] = useState(3)
return (
<>
<header>
<nav>
Food
<img src={logo} alt="logo" />
Places
</nav>
</header>
<main>
<h1>My List</h1>
<p>
Here is a list of all the restaurants, cafes, dessert bars and other eateries that I want
to keep a record of either because I like them, dislike them, or would like to find out.
</p>
<div className="card">
<h3>{name}</h3>
<h4>{tags}</h4>
<h4>{address}</h4>
<h4>
<a href={website} target="blank">
{website}
</a>
</h4>
<p>{notes}</p>
<div className="metricsContainer">
<div className="ratingContainer">
Rating {rating}/5
<Star className='starIcon' />
<Star className='starIcon' />
<Star className='starIcon' />
<Star className='starIcon' />
</div>
<div className="visitsContainer">Visited {visits} times</div>
</div>
</div>
</main>
</>
)
}
We are using a few different data types to store different types of state. Our restaurant's name, address, website and notes are just JavaScript Strings. tags is a JS Array, which is an ordered collection of items, in this case Strings. ratingand visits are Numbers.
Let's account for these different data types in our rendering.
Firstly the tags array can be printed as a string of tags separated by bullets (•) with {tags.join(" • ")}.
<div className="card">
<h3>{name}</h3>
<h4>{tags.join(" • ")}</h4>
<h4>{address}</h4>
<h4>
<a href={website} target="blank">
{website}
</a>
</h4>
Our rating is currently just being rendered as a number but the four stars are still hard-coded. To render a number of stars based on our rating we'll make a function that returns a list of <Star /> components:
const App = () => {
const [name, setName] = useState("Machiavelli Ristorante Italiano")
const [tags, setTags] = useState(["Pizza", "Pasta"])
const [address, setAddress] = useState("123 Clarence St, Sydney NSW 2000")
const [website, setWebsite] = useState("https://www.machiavelli.com.au")
const [notes, setNotes] = useState("Authentic Italian restaurant serving a traditional Neapolitan cuisine. The Linguine Gamberi was good value but a little bland in flavour.")
const [rating, setRating] = useState(4)
const [visits, setVisits] = useState(3)
const renderStars = (numStars) => {
let stars = [] // Declare an empty array
for (let i = 0; i < numStars; i++) {
// Add a new Star component to the stars array.
stars.push(
<Star className='starIcon' />
)
}
return stars
}
return (
<>
<header>
<nav>
Food
<img src={logo} alt="logo" />
Places
</nav>
</header>
This is an arrow function that takes in an integer numStars, then loops for numStars times, each time adding a new <Star /> component to the stars array. Finally it returns the stars array.
Call the renderStars function in place of where we hard-coded our stars, passing in the rating state as an argument:
<div className="metricsContainer">
<div className="ratingContainer">
Rating {rating}/5
{renderStars(rating)}
</div>
<div className="visitsContainer">Visited {visits} times</div>
</div>
We should now see four stars rendered as before, but if we open the browser console (Ctrl/Cmd + Shift + i) we'll see a warning "Each child in a list should have a unique "key" prop.".
In React, all components have a special key attribute, and whenever we render a list of components, they each need a unique value for the special key property. To fix this issue, add a key={i} to our Star /> components in the for loop:
const renderStars = (numStars) => {
let stars = [] // Declare an empty array
for (let i = 0; i < numStars; i++) {
// Add a new Star component to the stars array.
stars.push(
<Star key={i} className='starIcon' />
)
}
return stars
}
In this tutorial we learned how to store state and render it. In the children we'll also learn how to modify state based on user input.