I had fun building this and experiencing the joyful side of JS (or at least the browser ecosystem). Boarding a plane I'd had the idea of finishing a simple tetris before arrival. The game was in a working state after ~2 hours, even though I had made a few bad decisions. (it was subsequently polished, investing another few hours).
It was made with a "get things done" mindset, and the experience from dabbling with purely functional programming in Haskell was very helpful (mostly the "pure", as in "non-destructive", part).
It's nice that in Javascript we have an environment where we can code a fun game in <400 lines, without any library dependencies (not even internet connection needed) or build systems. On the downside, if it had stronger dynamic types and errors (like Python) that would have saved maybe 25% of my debugging time, probably without coding overhead.