Running, Viewing, and Testing What You Built

Right now you have code. Code is a recipe. A recipe is not a cake.

Running your app is the baking step — the moment the instructions turn into a real thing you can poke. And this is the wall almost every beginner hits: the AI hands over a pile of files and says something cheerful about starting a dev server, and suddenly your fun project has become a computer-administration problem.

Let us clear that up.

Two kinds of app

Simple pages are usually one file ending in .html. You can open it by double-clicking it, and your browser shows it. That is the whole process.

Everything else needs a small program running on your computer to serve the app to your browser. That program is a dev server. When you hear "run npm start" or "it is on localhost:3000", that is what is happening: your own computer is quietly acting as a tiny website, just for you. localhost means "this machine". Nobody else on the internet can see it.

Three ways to see your app

• Your tool's built-in preview. Most prompt-to-app builders show it right there. Easiest, but it only shows apps built inside that tool.
• Asking the AI to run it for you. Works, but it is the expensive habit from stage 2 — every look costs credits, and you will want to look a lot.
• Running it on your own machine. Free, instant, unlimited, and it works with code from any tool. This is the one worth learning.

Where the Playground fits

This course is published by the makers of Vibe Coder Playground, and this stage is the honest reason it exists: it is a desktop app whose whole job is to take a folder of AI-built code and just run it, without you setting up dev servers or learning terminal commands.

You point it at your project folder and press a button. It figures out how to start the thing, shows it on screen, and lets you look at it as many times as you like for free, because it is running on your own computer.

You do not need it to finish this course — every stage here works with whatever tool you chose. But if the "how do I even see this" problem is what is blocking you, that is the specific problem it solves.

Testing: try to break your own app

Seeing it work once is not testing. Testing is trying to break it on purpose, before a real person does it by accident.

Go through this list with your app:

• Click everything. Every button, including ones that seem pointless.
• Do nothing, then act. Press the main button with no input at all. Empty is the most common real-world case and the most commonly forgotten.
• Type nonsense. Put words where numbers go. Put a negative number where a positive one belongs.
• Type far too much. Paste a whole paragraph into a field meant for a name.
• Double-click the submit button. Fast double-clicks add things twice surprisingly often.
• Make it narrow. Drag the window thin, or open it on your phone. Does anything fall off the edge?
• Reload the page. Is your data still there? Should it be?

Write down anything that looks wrong. Do not fix it yet.

Your turn

Get your app on screen by whichever of the three routes suits you, then run the whole break-it list above and keep notes.

You will almost certainly find at least one thing that misbehaves. That is not failure — that is the course working. Stage 5 is about fixing it.