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Your First Component

Components are one of the core concepts of React. They are the foundation upon which you build user interfaces (UI), which makes them the perfect place to start your React journey!

  • What a component is
  • What role components play in a React application
  • How to write your first React component

Components: UI building blocks ​

On the Web, HTML lets us create rich structured documents with its built-in set of tags like <h1> and <li>:

<article>
  <h1>My First Component</h1>
  <ol>
    <li>Components: UI Building Blocks</li>
    <li>Defining a Component</li>
    <li>Using a Component</li>
  </ol>
</article>

This markup represents this article <article>, its heading <h1>, and an (abbreviated) table of contents as an ordered list <ol>. Markup like this, combined with CSS for style, and JavaScript for interactivity, lies behind every sidebar, avatar, modal, dropdown—every piece of UI you see on the Web.

React lets you combine your markup, CSS, and JavaScript into custom "components", reusable UI elements for your app. The table of contents code you saw above could be turned into a <TableOfContents /> component you could render on every page. Under the hood, it still uses the same HTML tags like <article>, <h1>, etc.

Just like with HTML tags, you can compose, order and nest components to design whole pages. For example, the documentation page you're reading is made out of React components:

<PageLayout>
  <NavigationHeader>
    <SearchBar />
    <Link to="/docs">Docs</Link>
  </NavigationHeader>
  <Sidebar />
  <PageContent>
    <TableOfContents />
    <DocumentationText />
  </PageContent>
</PageLayout>

As your project grows, you will notice that many of your designs can be composed by reusing components you already wrote, speeding up your development. Our table of contents above could be added to any screen with <TableOfContents />! You can even jumpstart your project with the thousands of components shared by the React open source community like Chakra UI and Material UI.

Defining a component ​

Traditionally when creating web pages, web developers marked up their content and then added interaction by sprinkling on some JavaScript. This worked great when interaction was a nice-to-have on the web. Now it is expected for many sites and all apps. React puts interactivity first while still using the same technology: a React component is a JavaScript function that you can sprinkle with markup. Here's what that looks like (you can edit the example below):

export default function Profile() {
  return (
    <img
      src="https://i.imgur.com/MK3eW3Am.jpg"
      alt="Katherine Johnson"
    />
  )
}
img { height: 200px; }

And here's how to build a component:

Step 1: Export the component ​

The export default prefix is a standard JavaScript syntax (not specific to React). It lets you mark the main function in a file so that you can later import it from other files. (More on importing in Importing and Exporting Components!)

Step 2: Define the function ​

With function Profile() { } you define a JavaScript function with the name Profile.

React components are regular JavaScript functions, but their names must start with a capital letter or they won't work!

Step 3: Add markup ​

The component returns an <img /> tag with src and alt attributes. <img /> is written like HTML, but it is actually JavaScript under the hood! This syntax is called JSX, and it lets you embed markup inside JavaScript.

Return statements can be written all on one line, as in this component:

return <img src="https://i.imgur.com/MK3eW3As.jpg" alt="Katherine Johnson" />;

But if your markup isn't all on the same line as the return keyword, you must wrap it in a pair of parentheses:

return (
  <div>
    <img src="https://i.imgur.com/MK3eW3As.jpg" alt="Katherine Johnson" />
  </div>
);

Without parentheses, any code on the lines after return will be ignored!

Using a component ​

Now that you've defined your Profile component, you can nest it inside other components. For example, you can export a Gallery component that uses multiple Profile components:

function Profile() {
  return (
    <img
      src="https://i.imgur.com/MK3eW3As.jpg"
      alt="Katherine Johnson"
    />
  );
}

export default function Gallery() {
  return (
    <section>
      <h1>Amazing scientists</h1>
      <Profile />
      <Profile />
      <Profile />
    </section>
  );
}
img { margin: 0 10px 10px 0; height: 90px; }

What the browser sees ​

Notice the difference in casing:

  • <section> is lowercase, so React knows we refer to an HTML tag.
  • <Profile /> starts with a capital P, so React knows that we want to use our component called Profile.

And Profile contains even more HTML: <img />. In the end, this is what the browser sees:

<section>
  <h1>Amazing scientists</h1>
  <img src="https://i.imgur.com/MK3eW3As.jpg" alt="Katherine Johnson" />
  <img src="https://i.imgur.com/MK3eW3As.jpg" alt="Katherine Johnson" />
  <img src="https://i.imgur.com/MK3eW3As.jpg" alt="Katherine Johnson" />
</section>

Nesting and organizing components ​

Components are regular JavaScript functions, so you can keep multiple components in the same file. This is convenient when components are relatively small or tightly related to each other. If this file gets crowded, you can always move Profile to a separate file. You will learn how to do this shortly on the page about imports.

Because the Profile components are rendered inside Gallery—even several times!—we can say that Gallery is a parent component, rendering each Profile as a "child". This is part of the magic of React: you can define a component once, and then use it in as many places and as many times as you like.

Components can render other components, but you must never nest their definitions:

export default function Gallery() {
  // 🔴 Never define a component inside another component!
  function Profile() {
    // ...
  }
  // ...
}

The snippet above is very slow and causes bugs. Instead, define every component at the top level:

export default function Gallery() {
  // ...
}

// ✅ Declare components at the top level
function Profile() {
  // ...
}

When a child component needs some data from a parent, pass it by props instead of nesting definitions.

Components all the way down ​

Your React application begins at a "root" component. Usually, it is created automatically when you start a new project. For example, if you use CodeSandbox or if you use the framework Next.js, the root component is defined in pages/index.js. In these examples, you've been exporting root components.

Most React apps use components all the way down. This means that you won't only use components for reusable pieces like buttons, but also for larger pieces like sidebars, lists, and ultimately, complete pages! Components are a handy way to organize UI code and markup, even if some of them are only used once.

React-based frameworks take this a step further. Instead of using an empty HTML file and letting React "take over" managing the page with JavaScript, they also generate the HTML automatically from your React components. This allows your app to show some content before the JavaScript code loads.

Still, many websites only use React to add interactivity to existing HTML pages. They have many root components instead of a single one for the entire page. You can use as much—or as little—React as you need.

You've just gotten your first taste of React! Let's recap some key points.

  • React lets you create components, reusable UI elements for your app.

  • In a React app, every piece of UI is a component.

  • React components are regular JavaScript functions except:

    1. Their names always begin with a capital letter.
    2. They return JSX markup.

Export the component ​

This sandbox doesn't work because the root component is not exported:

function Profile() {
  return (
    <img
      src="https://i.imgur.com/lICfvbD.jpg"
      alt="Aklilu Lemma"
    />
  );
}
img { height: 181px; }

Try to fix it yourself before looking at the solution!

Add export default before the function definition like so:

export default function Profile() {
  return (
    <img
      src="https://i.imgur.com/lICfvbD.jpg"
      alt="Aklilu Lemma"
    />
  );
}
img { height: 181px; }

You might be wondering why writing export alone is not enough to fix this example. You can learn the difference between export and export default in Importing and Exporting Components.

Fix the return statement ​

Something isn't right about this return statement. Can you fix it?

You may get an "Unexpected token" error while trying to fix this. In that case, check that the semicolon appears after the closing parenthesis. Leaving a semicolon inside return ( ) will cause an error.

export default function Profile() {
  return
    <img src="https://i.imgur.com/jA8hHMpm.jpg" alt="Katsuko Saruhashi" />;
}
img { height: 180px; }

You can fix this component by moving the return statement to one line like so:

export default function Profile() {
  return <img src="https://i.imgur.com/jA8hHMpm.jpg" alt="Katsuko Saruhashi" />;
}
img { height: 180px; }

Or by wrapping the returned JSX markup in parentheses that open right after return:

export default function Profile() {
  return (
    <img 
      src="https://i.imgur.com/jA8hHMpm.jpg" 
      alt="Katsuko Saruhashi" 
    />
  );
}
img { height: 180px; }

Spot the mistake ​

Something's wrong with how the Profile component is declared and used. Can you spot the mistake? (Try to remember how React distinguishes components from the regular HTML tags!)

function profile() {
  return (
    <img
      src="https://i.imgur.com/QIrZWGIs.jpg"
      alt="Alan L. Hart"
    />
  );
}

export default function Gallery() {
  return (
    <section>
      <h1>Amazing scientists</h1>
      <profile />
      <profile />
      <profile />
    </section>
  );
}
img { margin: 0 10px 10px 0; height: 90px; }

React component names must start with a capital letter.

Change function profile() to function Profile(), and then change every <profile /> to <Profile />:

function Profile() {
  return (
    <img
      src="https://i.imgur.com/QIrZWGIs.jpg"
      alt="Alan L. Hart"
    />
  );
}

export default function Gallery() {
  return (
    <section>
      <h1>Amazing scientists</h1>
      <Profile />
      <Profile />
      <Profile />
    </section>
  );
}
img { margin: 0 10px 10px 0; }

Your own component ​

Write a component from scratch. You can give it any valid name and return any markup. If you're out of ideas, you can write a Congratulations component that shows <h1>Good job!</h1>. Don't forget to export it!

// Write your component below!

export default function Congratulations() {
  return (
    <h1>Good job!</h1>
  );
}