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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.