Sam Thorogood on Dev.to writes, Why did I write this post? Emscripten is a wonderful tool, but it has a long history (for asm.js), and isn’t perfect. I think it errs too much on the side of “magic”, and many posts rave about how it’s so easy to EMASM or use binding-fu, but this all comes at a cost, and can introduce huge amounts of inadvertent overhead—think copying huge memory buffers around because we’re trying to make them immutable or easily exposed.
FFMPEG.js is an amazing project and it helped me building one of my latest projects: Device Frame. It basically builds ffmpeg (with a good set of defaults to keep the size small — as small as it can be). If the default build doesn’t support the filters and encoders you need, then you will need to build it yourself. This is more of a note for me in the future, but this is what I did to get it working.