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Build your first WebContainer app

In this tutorial, you'll build a plain Vite-based JavaScript app featuring a barebones WebContainers app that loads the whole file system into the container and then runs the Express app. You don't need prior experience with Vite, Express, or WebContainers.

Your app will feature an editable textarea on the left and a Preview window on the right. This is where you will be able to see the output of the Express server.

An app with a textarea showing express js index.js code on left, and its output on right side using an iframe

Before you begin

You can follow along this tutorial using our Vite starter which you can later share with friends, colleagues, or on social media:

Open in StackBlitz

Alternatively, you can develop the app locally, which requires additional setup:

Setup instructions for local development


Before you proceed, make sure to have the following installed:

  • Node v14 or higher
  • npm (or pnpm or yarn)

Initialize a new Vite app with the vanilla JavaScript template:

If you're using the StackBlitz editor, you can skip this step.

In the terminal, run the following command to initialize a new Vite app:

npm create vite webcontainers-express-app --template vanilla

You'll see a new directory called webcontainers-express-app with a Vite app. Enter that directory, install the dependencies, and run the dev server:

cd webcontainers-express-app
npm install
npm run dev

When the dev server starts, you will see a localhost URL in the terminal - click on the link to see the preview page. You should see the following screen:

A Vite boilerplate welcome page featuring the logos of Vite and JS, a sign "Hello Vite!" and a counter

If you're already curious about the outcome of the tutorial, check out this demo 👀

1. Prepare the files in the Vite app

In your Vite app, there are only two files that you will need: index.html and main.js. Replace the contents of main.js with the following code:

import './style.css';

document.querySelector('#app').innerHTML = ``;

You can now remove counter.js as you won't need it anymore. Now your app looks very minimalistic:

A browser window with empty page and a tab featuring the title "My first WebContainers app" and StackBlitz logo

You might have noticed that we have also changed the name and the logo on the browser tab. You can do so in index.html.

2. Create and style a textarea and an iframe

Now let's set up HTML and styles for your app. You'll need to build two areas: a textarea in which the code will be featured, and an iframe to show the rendered output.

First, let's create a file called loading.html the entire contents of which are as follows:

Installing dependencies...

We will add this file to an iframe that will be visible in the preview only when the WebContainer is getting ready. After that, it will be replaced with the WebContainer output.

Now, let's build a scaffolding by adding HTML to the #app element in main.js:

document.querySelector('#app').innerHTML = `
  <div class="container">
    <div class="editor">
      <textarea>I am a textarea</textarea>
    <div class="preview">
      <iframe src="loading.html"></iframe>

Our page now features two boxes:

A page with two rectanglular boxes: a small one on top of a bigger one. The top one reads "I'm a textarea". The bottom one reads "Installing dependencies..."

Let's make it look nicer now. Replace the contents of style.css with the following code:

* {
  box-sizing: border-box;

body {
  margin: 0;
  height: 100vh;

.container {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 1rem;
  height: 100%;
  width: 100%;

textarea {
  width: 100%;
  height: 100%;
  resize: none;
  border-radius: 0.5rem;
  background: black;
  color: white;
  padding: 0.5rem 1rem;

iframe {
  height: 100%;
  width: 100%;
  border-radius: 0.5rem;

You should see something like this:

A page with two rectangles next to each other. The left one reads "I'm a textarea"

3. Setting references for elements

Before you add WebContainers, you need to enable access to the textarea and iframe elements. At the bottom of the main.js file, add this code that will locate the two elements:

/** @type {HTMLIFrameElement | null} */
const iframeEl = document.querySelector('iframe');

/** @type {HTMLTextAreaElement | null} */
const textareaEl = document.querySelector('textarea');

Next step

The scaffolding for the app is ready 👏 In the next step, you'll set up your WebContainer app.

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