he/him 
I want to learn more about ASTs, codemods and compilers.
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Do you think writing codemods using an existing compiler or writing a compiler would be a better place to start?
Use an existing compiler, then you can slowly rewrite the pieces. It's exactly what I did with @babeljs. I use escodegen, estraverse, esprima and other libraries, then slowly began rewriting those libraries to add features I wanted and better ergonomics.
