How JustTrimVideo Works

A technical explanation of client-side video trimming — and how to verify it yourself.

Browser-Based Processing

JustTrimVideo processes your video files entirely within your web browser using WebAssembly. Your files never leave your device — no upload, no server processing, no third-party access to your content.

Technical Implementation

JustTrimVideo uses the following technologies to trim video locally:

ffmpeg.wasm

A WebAssembly port of FFmpeg that runs natively in the browser. Handles all video reading, timestamp seeking, and stream-copy trimming operations entirely in your local memory.

File API

The browser's built-in File API reads your selected video directly into memory from your local filesystem. No network request is made to load the file.

Stream Copy Mode (-c copy)

FFmpeg's stream copy mode extracts the requested time range without re-encoding the video or audio streams. This preserves 100% of the original quality and completes in seconds regardless of file length.

What This Means for Privacy

  • ✓ Your video file is never uploaded to any server.
  • ✓ No third party has access to your video content at any point.
  • ✓ File names, video metadata, and content are not transmitted or logged.
  • ✓ Processing happens on your own device using your own CPU and RAM.
  • ✓ You can disconnect from the internet after loading the page — the trimmer continues to work.

How to Verify This Yourself

You don't have to take our word for it. Here's how to confirm no file data leaves your browser:

  1. Open your browser's developer tools (press F12, or right-click anywhere and choose Inspect).
  2. Go to the Network tab.
  3. Load a video file into JustTrimVideo and perform a trim.
  4. Watch the Network tab — you will see no requests containing your video data being sent anywhere.
Alternatively: Load the page fully, then disconnect from the internet. The trimmer will still work, because all processing runs locally.

Technical Trade-offs

Because processing runs locally on your device, very large files may require more memory than your browser can allocate. The browser tool handles files up to approximately 4 GB. For larger files, the free JustTrimVideo Desktop app uses native FFmpeg with no size limits — and no internet connection required.

Ready to trim your video?

Go to the Video Trimmer →