[chubot]

As far as I understand, one important difference is that JavaScript doesn't have __getattr__ or __setattr__ (or at least earlier versions didn't).

You might not use those hooks a lot in your application code, but it seems that web frameworks and the like do a lot of reflection, which makes the code difficult/impossible to optimize (even at runtime with a JIT).

Python also has __add__ (operator overloading) and JavaScript doesn't. This is a good talk about how subtle or crufty the semantics of something like "a+b" is in Python:

https://www.youtube.com/watch?v=IeSu_odkI5I

The PyPy developers had to copy a lot of the implementation details of CPython, which doesn't always result in the fastest code.

[mschaef]

You should take a look at the Chambers and Ungar papers on the implementation of Self. In Self, _everything_ is theoretically done through message send/dispatch. This includes field access, numerical operations, flow control... the works. (In a language with prototypical inheritance too.)

What's interesting is that by the time they're done with their optimizations, they manage to get relatively close (x2, iirc) to native speed by essentially inlining everything they can and keeping enough metadata around that they can retain the dynamic properties of the languauge. Impressive stuff. (Which is probably why Sun hired them early in the Java/JIT days.)