jsx-info https://www.npmjs.com/package/jsx-info is a really neat project that will report how much a property is used on JSX components.
It's a really nice generalisation of dead code analysis: which properties are used the most?
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What are the most popular languages that have used an AST walker for their implementation?
I know Ruby used to do this, but there must be others.
(I'm interested in the lowest PL speed that users will tolerate if you have awesome features.)
It's really satisfying to use a profiler for the first time on a project. I always find a big performance win with only a small code change.
It's never the code that I expected to be slow, however!
Chekhov's repro: If a line of code is included in a bug report, it should contribute to the debugging somewhere.