How JustCompressVideo Works
Your video is processed entirely in your browser. Here is exactly what happens under the hood.
Browser-Side Processing
JustCompressVideo runs entirely in your web browser. When you compress a video, every step happens on your device — no file is ever sent to any server, and no external service processes your data.
Technical Implementation
The tool is built on these technologies and open-source libraries:
ffmpeg.wasm
A WebAssembly port of the FFmpeg library. Runs the full FFmpeg toolchain natively in your browser, including H.264 video encoding via libx264. The WASM binary is loaded once from the jsDelivr CDN and then runs entirely locally.
WebAssembly (WASM)
A binary instruction format that lets native C/C++ code run at near-native speed in the browser. ffmpeg.wasm compiles FFmpeg's entire C codebase to WASM so it executes client-side with no server involvement.
File API
A browser built-in API for reading files from your device directly into browser memory. Your video is read into memory locally — no network request containing your file data is made at any point.
H.264 / libx264
The video codec used for re-encoding. Compression quality is controlled via the CRF (Constant Rate Factor) parameter, which maps directly to the quality slider. Lower CRF = higher quality and larger file; higher CRF = smaller file with more compression.
What This Means for Your Privacy
- Your video is never uploaded to any server
- No third party receives or processes your video
- We have no access to the content of your files
- No video data, filenames, or metadata are logged or stored
- Your file exists only in your browser's memory during processing
How to Verify
You do not have to take our word for it. Here is how to confirm:
- 1 Open your browser's developer tools (press F12, or right-click and choose Inspect)
- 2 Go to the Network tab
- 3 Use the tool to compress a video
- 4 Observe: no outbound requests contain your video data
Alternatively, disconnect from the internet after the page loads. The tool continues to work — because it requires no server.
Trade-offs and Limitations
Because processing happens entirely in your browser using single-threaded WebAssembly, compression is slower than server-based tools or the native desktop app. A 100MB video may take 2–5 minutes on a typical desktop. Larger files or lower-powered devices will take longer. For files over 4GB or if speed is critical, the free desktop app uses native FFmpeg and is significantly faster.