Judging by the comments here I guess I'm the only one that feels extremely awkward about this being written in Javascript. I don't mind wrappers but is this essentially just an ffmpeg wrapper in JS?
If you don't like that it's a wrapper, what is wrong with adding a wrapper that adds new functionality?
If you don't like that it's Javascript, how would another language somehow make this better? Especially with the sheer number of people that are already comfortable using Javascript.
Left-pad, is-promise.