The language is limited by IE dragging their feet on unbreaking their implementation or advancing standards, Webkit and V8 staying at a very conservative middle ground (Google pushing Dart and compilation as substitutes), and Mozilla being the only engine developer showing willingness to evolve the language, because they are the only ones using it to implement the rest of their browser. Node.js is held back by a language implementation they don't control and feels underpowered in the language department, indirectly being a victim of resistance to change in the browser landscape.
Python and other non-browser languages are far more powerful and expressive, and — since you refer to Python 3 — willing to further improve.
Python and other non-browser languages are far more powerful and expressive, and — since you refer to Python 3 — willing to further improve.