The latest articles on DEV Community by Jonathan Wallace (@jonathan_wallace_c6087363). https://dev.to/jonathan_wallace_c6087363 https://media2.dev.to/dynamic/image/width=90,height=90,fit=cover,gravity=auto,format=auto/https:%2F%2Fdev-to-uploads.s3.amazonaws.com%2Fuploads%2Fuser%2Fprofile_image%2F3810551%2F638e7b63-86b3-496a-bfa5-8254c46ec9ba.jpg https://dev.to/jonathan_wallace_c6087363 en Jonathan Wallace Fri, 06 Mar 2026 20:29:57 +0000 https://dev.to/jonathan_wallace_c6087363/building-a-privacy-first-video-converter-with-webassembly-4pi4 https://dev.to/jonathan_wallace_c6087363/building-a-privacy-first-video-converter-with-webassembly-4pi4 <h2> The Problem </h2> <p>Every time you use a typical online video converter, your file gets uploaded to a remote server. For most people that is mildly concerning. For anyone working in healthcare, law, or finance it can be a compliance violation. HIPAA, GDPR, and internal data policies all have something to say about sending sensitive media to a third party.</p> <p>I wanted a converter that was truly private — one where the file never leaves the user's machine.</p> <h2> The Solution: FFmpeg + WebAssembly </h2> <p>FFmpeg is the gold standard for media processing. Thanks to Emscripten, it can be compiled to WebAssembly and run inside a browser tab. That means: </p> <ul> <li> <strong>Zero server uploads.</strong> The file stays on your device from start to finish.</li> <li> <strong>Full FFmpeg power.</strong> 30+ video and audio formats, codec control, bitrate tuning.</li> <li> <strong>Works offline</strong> once the WASM binary is cached.</li> </ul> <h2> How It Works Under the Hood </h2> <ol> <li>The user drops a file onto the page. A <code>File</code> object is created — no network request.</li> <li>The file bytes are written into an in-memory virtual filesystem that FFmpeg WASM exposes.</li> <li>FFmpeg runs the conversion entirely in a Web Worker, keeping the UI responsive.</li> <li>The output file is read from the virtual FS and offered as a Blob download.</li> </ol> <p>At no point does any byte leave the browser. You can verify this yourself by opening DevTools → Network and watching the requests during a conversion — there are none.</p> <h2> Challenges </h2> <ul> <li> <strong>WASM binary size.</strong> The FFmpeg WASM core is ~30 MB. Lazy-loading and caching with a service worker keeps repeat visits fast.</li> <li> <strong>Memory limits.</strong> Browsers cap WASM memory. Very large files (multi-GB) can hit this ceiling, so I surface a clear error instead of crashing silently.</li> <li> <strong>SharedArrayBuffer.</strong> Some FFmpeg WASM builds require cross-origin isolation headers (<code>COOP</code>/<code>COEP</code>), which can conflict with third-party scripts. </li> </ul> <h2> Try It Out </h2> <p>The tool is live at <a href="https://getconvertfree.com" rel="noopener noreferrer">getconvertfree.com</a>. It is completely free — no account, no limits, no ads.</p> <p>If you handle sensitive media and need a conversion tool you can trust, give it a spin. Feedback and feature requests are welcome.</p> <p><em>Happy converting.</em></p> webassembly privacy javascript webdev