I know there are JavaScript ports of FFmpeg and I would love to use them. But so far, I never got it working. I tried it with AI and this prompt: Make a simple example of speeding up an mp4 video in the browser using a version of ffmpeg that runs in the browser. Don't use any server side tech like node. Make it a single html file. But so far every LLM I tried failed to come up with a working solution.
Maybe you need to actually learn how it works instead of deferring to LLMs that have no understanding of what you are specifically requesting.
Just read the fine documentation.