A side project I've been working on for the last ten days is a new Z80 assembler, which I've written in TypeScript and PEG.js. I can't totally remember why I started it, other than a few bugs in ASM80, which I was using, but it's going quite well. It's at a state where it can compile the Z80 monitor for my computer, but there are loads of things I need to add. I also need to properly define how expressions are handled — at the moment it's fairly haphazard, especially when casting types.

It's currently available on npm, but I wouldn't recommend using it!