This is a compilation of the libraries associated with handling audio and video in ffmpeg—libavformat, libavcodec, libavfilter, libavutil, libswresample, and libswscale—for emscripten, and thus the web.
length is never (or at least should never be) undefined. It's just the exact length that the program requested. You can ignore it, but it'll always be defined.
length is never (or at least should never be) undefined. It's just the exact length that the program requested. You can ignore it, but it'll always be defined.