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2.6’s JIT is entirely at the method level currently, and unfortunately for certain types of workloads (e.g. Rails) this means the performance benefit is largely outweighed by the call counters and deoptimization checks on every method invocation.

There’s definitely room for improvement, and the inclusion of JIT infrastructure is awesome, it’s just not making much of a performance impact. Yet.

V8 is also a method-based JIT as far as I know.

The overheads from the call counters and deoptimization checks are tiny. The real problem is that right now MJIT doesn't allow much optimization beyond generating native code equivalent to simply executing the instructions. It's ridiculously simple compared to V8.

Out of curiosity, what are the major advantages Ruby has to offer over JavaScript?

Isn't there a Ruby->JS transpiler?

Opal: https://github.com/opal/opal