Running your app

So far your project is a set of files sitting in a folder. To actually use it, you need to run it — turn those files into a working thing you can click around in your browser. This module is about doing that on your own computer, where it’s private and instant.

”On my computer” vs “on the internet”

Two phrases that’ll come up a lot:

• A local app runs on your computer. Only you can see it. It’s like cooking in your own kitchen — fast, private, nobody else invited.
• A web app runs on the internet, at an address anyone can visit. That’s Module 7. We don’t go there until your thing works locally first.

Always build and test locally first. It’s faster, it’s free, and mistakes are invisible to the world.

The dev server: a fresh copy on demand

To see a web project locally, you start a dev server A little program that runs your project on your computer and serves it to your browser while you build. Stop it anytime. (“dev” is short for “development”). Picture a chef who plates a fresh copy of your page every time you ask: the dev server keeps your project running and hands it to your browser, updating instantly as you and your assistant make changes.

It serves your project to a special private address called localhost Your computer's private address for things running on it. Looks like localhost:4321. Only you can reach it. . You’ll see something like localhost:4321. That’s not on the internet — it’s a door that only opens from your own machine. The number on the end is just which door.

Start it (just ask)

You don’t have to remember how to start the server for each kind of project — ask your assistant. With Claude Code running in your project folder, say:

Start the dev server so I can see my project in the browser,
and tell me the exact address to open.

It will start the server and show you a line with an address. Many projects use a command like the one below under the hood — you’ll get to recognize it:

PS> npm run dev

Copy the address it gives you (for example http://localhost:4321), paste it into your browser’s address bar, and press Enter. There’s your project, running.

Testing = clicking around

“Testing” sounds formal. For you, right now, it just means: use the thing and see if it does what you wanted. Click the buttons. Read the text. Try it on your phone-sized window by making the browser narrow. When something’s off, you’ve found your next instruction for the assistant: “the button doesn’t do anything when I click it — please fix that.”

Stopping the server

When you’re done for now, you stop the dev server in the PowerShell window where it’s running by holding Ctrl and pressing C. (Remember that PowerShell quirk from Module 2: here Ctrl + C means “stop,” which is exactly what we want.)

Ctrl + C

The server stops and localhost goes quiet. Nothing is lost — your files are all still there. Next time you want to see it, start the server again. Stopping and starting the dev server is completely routine; you’ll do it dozens of times.

Where you are now

You can build a thing and run it to see it work. That’s a complete loop on your own machine. The only thing left is to share it with the world — which is the next module, and the big payoff.