What is the correct way to concat (merge) two audio (or video) files?

Basically, I want to concat two audio files (lets say, file1.mp3 and file2.mp3) , what is the correct code to achieve that?

(as a bonus, will be nice a way to concat mp4 files too). [I've only seen [this thread](https://github.com/ffmpegwasm/ffmpeg.wasm/issues/177), and even closed without answer.]