The Problem

Every freelancer needs invoices. Most "free" tools aren't really free — they limit you to 5 invoices/month, require an account, or add watermarks. I wanted to build the simplest possible invoice tool with zero ongoing costs.

The Architecture

The entire application runs client-side in the browser:

No backend — all logic runs in JavaScript

No database — form state lives in the DOM

No auth — no accounts to manage

No server costs — Firebase Hosting free tier handles everything

Key Technical Decisions

PDF Generation: I used jsPDF to generate invoices client-side. The PDF is created entirely in the browser and downloaded directly — no server round-trip needed.

Offline Support: The app is a Progressive Web App (PWA) with a service worker. Once loaded, it works without an internet connection.

Privacy: Since there's no backend, user data never leaves the browser.…