This is the github repo: https://github.com/Microsoft/pxt/tree/master/pxtcompiler

PXT's underlying programming language is a subset of TypeScript (leaving out JavaScript dynamic features).

The main features of PXT are: [...] an ARM Thumb machine code emitter

In fact there is more dynamic features in now, including accessing arbitrary fields through 'any' type and typeof expression. What's missing is mostly eval and prototype inheritance (instead we use classes). There's also a precise GC now.

See also the open access paper:

http://delivery.acm.org/10.1145/3220000/3211335/lctes18main-...

paper name is MakeCode and CODAL: Intuitive and Efficient Embedded Systems Programming for Education

https://www.microsoft.com/en-us/research/uploads/prod/2018/0...

https://doi.org/10.1145/3211332.3211335

I wonder if this could be retargeted to retro CPUs like the 6502 and Z80? Seems the assumption of 32-bit integers might make things less efficient though. (EDIT: never mind, it does support smaller int types!)