This was written in 2014, when Crankshaft was still the JIT in V8. Turbofan was shipped in late 2016 and made primary in 2017. It's a much better JIT - it is tolerant to many more code patterns, including ES6 features - but IRHydra2 no longer works with Turbofan, so this type of introspection is more difficult to do.

A few years ago I hand-tuned some kdb+/q interop code on Crankshaft using IRHydra2 and d8. The results were impressive: about 10x by just shuffling code around to be friendlier to the JIT. It was fun, but more than a little mysterious.

Tuning on Crankshaft was all about avoiding bailout (https://github.com/vhf/v8-bailout-reasons/blob/master/README...). Tuning for Turbofan is less about "gotchas" and more about making sure your functions don't go megamorphic. Using TypeScript or Flow helps you do this just to get your code to typecheck anyway so it's a good habit.

As mentioned in this comment (https://news.ycombinator.com/item?id=15067462) there is still some minor analysis of the IR possible in IRHydra2 on TurboFan.