You can't change the rules interpreting your source code so that the parser will expand a small command into an entire program, or that it will read a more fitting DSL that does not resemble your original language.
You can not inspect an object and change its type or the set of available properties based on some calculation.
You can not run some code in a controlled environment separated from your main code.
Could you please give examples of expressiveness of Python or Ruby that Javascript lacks?