[eva1984]

Well, just want to say for one example here, why Python is hard to speedup than JS.

In Python, essentially every object is a dictionary. And behavior like, "x.y = 'z'" is legal, meaning pretty much everything could be put into object x without declaration ahead of time, at all. While, not very sure, JS doesn't allow such behavior, you can still do assignment, but assessing will yield undefined.

The above behavior come as default(you can use __slot__ for sure, but compiler can't assume that) is pretty problematic for optimization, compiler simply won't know where to locate a specific variable ahead-of-time, so it cannot substitute the variable access using a pointer offset, it will have to go through a hash lookup, which comparing to the offset alternative, is magnitude slower.

[trishume]

Except that LuaJIT does something close to just a pointer offset. The trick it uses is to use a hash function that is known by the compiler so when you have a constant key ("y" in this case) the hash table set is compiled down to 3 instructions (imagine hash("y") = 0xa7):

1. Load whatever necessary to check if x[0xa7] is occupied 2. Branch to slow path if occupied 3. Store x[0xa7] = 'z'

And a modern super-scalar CPU will even do some of this in parallel.

[pcwalton]

You can generally do that in JS as well. (Added properties are sometimes called "expandos".)

[eva1984]

True. Just find out. But Chrome's console yield undefined to me...Weird.

Edit: find the reason. It is because I am trying to assign a property to int. Seems JS won't allow this to primitive type?

Edit: Python won't allow assignment to primitive type bject either. The handling is the same.