Hello, world!

  • 5 minutes

The most common way to start any programming course is to display the text "Hello, world!" Continuing with this tradition, we'll use React to display the famous text.

We'll create two items for the base of our project:

  • The index.html file hosts our React application.
  • The index.jsx file mounts our application.

Create the application host

  1. In Visual Studio Code, create a new file in the public folder. Name it index.html.

  2. Add the following HTML:

    <!DOCTYPE html>
<html lang="en">
<head>
    <title>Recipes</title>
</head>
<body>
    <div id="app"></div>
    <script type="module" src="/dist/index.js"></script>
</body>
</html>

Explore the code

Notice two important lines in our HTML:

  • <div id="app"></div>
    • This line creates the HTML element that hosts our React application.
    • We call this element by its ID to render our application.
  • <script type="module" src="/dist/index.js"></script>
    • This line loads the JavaScript.

Note

The name of the file we're importing is index.js. We don't use index.jsx, because browsers can't render JSX files. We always need a bundler (such as Snowpack) to generate JavaScript. We reference the JavaScript rather than the JSX.

The type="module" attribute allows us to use import statements in our JavaScript (or JSX) files. This functionality is relatively new to browsers. It helps us import the necessary packages and components.

Create the entry point for the React application

We need code to render our React application inside the HTML. Traditionally, the index.jsx file is used to render the application.

  1. Create a new file inside the src folder. Name it index.jsx.

  2. Add the following code:

    import React from 'react';
import ReactDOM from 'react-dom';

ReactDOM.render(
    <h1>Hello, world!</h1>,
    document.getElementById('app')
);

Explore the code

Our index.jsx file starts by importing two important libraries. The first library, React, allows us to use JSX. It will be imported in every component or JSX file we create. The second library, ReactDOM, renders our application inside the HTML.

render takes two parameters:

  • The HTML we want to display. In this case, the HTML is an h1 heading.
  • The HTML element we want to use to display our HTML. We'll use the element that has an ID of app. We created this element earlier.

The ability to use HTML inside our JavaScript coding is part of the power of JSX.

Check the page

Now that we've created our code, let's see our site in action!

  1. Open the integrated terminal in Visual Studio Code by selecting View > Terminal or by selecting Ctrl+`. On a Mac, select Cmd+` instead.

  2. Use the following command to start Snowpack's development server:

    npm start

Your default browser should automatically open and display your page. If the page doesn't appear automatically, open your browser and go to http://localhost:8080. You should now see your page!

Screenshot showing a "Hello, world!" page.

Explore the generated code

Our JSX code is converted into the appropriate HTML and JavaScript for display in the browser. Open the JavaScript file generated by Snowpack: http://localhost:8080/dist/index.js. You'll see the following code:

import React from "../web_modules/react.js";
import ReactDOM from "../web_modules/react-dom.js";
ReactDOM.render(/* @__PURE__ */ React.createElement("h1", null, "Hello, world!"), document.getElementById("app"));

Focus on the line of code that generates the h1 element and places the text inside it:

React.createElement("h1", null, "Hello, world!")

Using this code is similar to using document.createElement with vanilla JavaScript. The tooling provided by Snowpack and other bundlers allows us to use JSX to automatically generate the appropriate browser-friendly code.