From JavaScript to React

Building UI with Components

User interfaces can be broken down into smaller building blocks called components.

Components allow you to build self-contained, reusable snippets of code. If you think of components as LEGO bricks, you can take these individual bricks and combine them together to form larger structures. If you need to update a piece of the UI, you can update the specific component or brick.

This modularity allows your code to be more maintainable as it grows because you can easily add, update, and delete components without touching the rest of our application.

The nice thing about React components is that they are just JavaScript. Let's see how you can write a React component, from a JavaScript perspective:

Creating components

In React, components are functions. Inside your script tag, write a function called header:

<script type="text/jsx">
  const app = document.getElementById("app")


  function header() {
  }

  ReactDOM.render(<h1>Develop. Preview. Ship. ๐Ÿš€</h1>, app)
</script>

A component is a function that returns UI elements. Inside the return statement of the function, you can write JSX:

<script type="text/jsx">
  const app = document.getElementById("app")

  function header() {
     return (<h1>Develop. Preview. Ship. ๐Ÿš€</h1>)
   }

  ReactDOM.render(, app)
</script>

To render this component to the DOM, you can pass it as the first argument in the ReactDOM.render() method:

<script type="text/jsx">

  const app = document.getElementById("app")

  function header() {
     return (<h1>Develop. Preview. Ship. ๐Ÿš€</h1>)
   }


   ReactDOM.render(header, app)
</script>

But, wait a second. If you try to run the code above in your browser, youโ€™ll get an error. To get this to work, there are two things you have to do:

First, React components should be capitalized to distinguish them from plain HTML and JavaScript.

function Header() {
  return <h1>Develop. Preview. Ship. ๐Ÿš€</h1>;
}

// Capitalize the React Component
ReactDOM.render(Header, app);

Second, you use React components the same way youโ€™d use regular HTML tags, with angle brackets <>.

function Header() {
  return <h1>Develop. Preview. Ship. ๐Ÿš€</h1>;
}

ReactDOM.render(<Header />, app);

Nesting Components

Applications usually include more content than a single component. You can nest React components inside each other like you would regular HTML elements.

In your example, create a new component called HomePage:

function Header() {
  return <h1>Develop. Preview. Ship. ๐Ÿš€</h1>;
}
function HomePage() {
  return <div></div>;
}

ReactDOM.render(<Header />, app);

Then nest the <Header> component inside the new <HomePage>component:

function Header() {
  return <h1>Develop. Preview. Ship. ๐Ÿš€</h1>;
}

function HomePage() {
  return (
    <div>
      {/* Nesting the Header component */}
      <Header />
    </div>
  );
}

ReactDOM.render(<Header />, app);

Component Trees

You can keep nesting React components this way to form component trees.

For example, your top-level HomePage component could hold a Header, an Article, and a Footer Component. And each of those components could in turn have their own child components and so on. For example, the Header component could contain a Logo, Title and Navigation component.

This modular format allows you to reuse components in different places inside your app.

In your project, since <HomePage> is now your top-level component, you can pass it to the ReactDOM.render() method:

function Header() {
  return <h1>Develop. Preview. Ship. ๐Ÿš€</h1>;
}

function HomePage() {
  return (
    <div>
      <Header />
    </div>
  );
}

ReactDOM.render(<HomePage />, app);

In the next section, we will discuss props and how you can use them to pass data between your components.

Quick Review: How would you nest a Header component inside a Layout component in React?

Additional Resources: