Why I Built an Image Converter That Never Touches a Server

The problem: every "free" image converter wants your files If you've ever needed to quickly convert a batch of photos to WebP or shrink a folder of PNGs before shipping them to production, you've probably run into the same annoyance I did: most " free online converters " require you to upload your files to a remote server first. That's fine for a random screenshot. It's not fine when the images are: Unreleased product shots under NDA Client assets you're not supposed to redistribute Personal photos you'd rather not hand to a third-party server you know nothing about So I started looking at what the browser can actually do on its own — and it turns out, more than most people assume. What the browser can already do Modern browsers ship with everything needed to decode, resize, re-encode, and compress images entirely client-side: + toBlob() / toDataURL() for re-encoding to JPG, PNG, or WebP The File API for drag-and-drop and batch uploads Web Workers to keep the UI thread responsive during batch conversion JSZip (or similar) to bundle multiple converted files into a single downloadable ZIP None of this requires a backend. No image ever has to leave the user's machine. Why this matters beyond privacy Besides the obvious privacy win, doing conversion in-browser has some nice side effects: No server costs that scale with usage. A traditional image-conversion API has to provision compute for every request. A client-side tool scales for free — the user's own CPU does the work. No upload/download round trip. For large batches, skipping the network entirely is often faster than uploading to a server and waiting for a processed file back. Works offline once loaded. A PWA-style client-side converter keeps working even with a flaky connection. The trade-offs It's not free lunch: Very large batches (hundreds of high-res images) can strain the main thread if you're not careful with Web Workers. WebP/AVIF encoder quality and speed vary by browser engine, so you can't guarantee byte