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Running dev server in WebContainers

Now that your app is able to install dependencies, you can run the dev server. Let's add some code that will:

  1. run npm run dev,
  2. listen to server-ready event,
  3. and then assign URL to the iframe.

WebContainers expose the server-ready event, which is emitted when the server is ready to accept requests. You can listen to this event using webcontainerInstance.on.

Let's add this code to the main.js file

async function startDevServer() {
  // Run `npm run start` to start the Express app
  await webcontainerInstance.spawn('npm', ['run', 'start']);

  // Wait for `server-ready` event
  webcontainerInstance.on('server-ready', (port, url) => {
    iframeEl.src = url;

In your browser dev tools, after you run startDevServer() in the next step, you will be able to see the dev server running:

Browser console shows dependency installation output and reads: "App is live at http://localhost:3111"

Remember the iframeEl you defined at the beginning of this tutorial? Now is the time for it to shine! You are setting the iframeEl's src to the URL of the Express app, which will print its output in the iframe.

Let's call the function in the event listener:

window.addEventListener('load', async () => {
  textareaEl.value = files['index.js'].file.contents;
  // Call only once
  webcontainerInstance = await WebContainer.boot();
  await webcontainerInstance.mount(files);

  const exitCode = await installDependencies();
  if (exitCode !== 0) {
    throw new Error('Installation failed');


Your Express app is running inside WebContainers!

Now the right box, so far empty, features the output of the Express server: "Welcome to a WebContainers app! 🥳"

Next step

Congratulations! You have an Express app running in your browser with WebContainers 🥳 In the next step, you'll edit the files.

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