The specifications do say that JavaScript only has floating point numbers (and no integers) and that Python has infinite precision integers. But in implementation, for whole numbers small enough to fit into a regular 32-bit integer both Python and Node.js use regular integers as an optimization. fib(35) comfortably fits that.

I guess node.js might have the edge here with the necessary overflow checks in the addition. Both languages have to do them to fall back to either doubles or bigints respectively, but Node.js's JIT can probably do them faster.