Precomputations enabled by a small input domain are not cheats, they're optimisations.

Newer JS versions have BigInt, which would allow for a larger input domain. That might actually solve this "problem". Scanning source code for patterns is a losing proposition.

Damn, I just wanted to post this after I completed the kata :D

verbatim what i wrote too! Party!

Just a small detail: \w refers to [a-zA-Z0-9_] (so it also contains the underscore and digits).

What? It's the most time efficient solution

The arity of the function does need to be taken into account. Since only functions which have already been declared may be called, the intepreter has all of the information it needs to make that call.

As a more general example, take the following expression in Polish (prefix) notation:

* + 1 2 2

A prefix interpreter knows that + is a binary operator, so it takes the 1 2 as the arguments to +, then * takes the resulting 3 2, resulting in 6.

Likewise, in your example the interpreter knows that echo is a unary function, so it takes only the first argument in the stack.

Hope that helps. If there was any particular part of the description that led you to believe that arity wouldn't need to be taken into account, please let me know and I will try to clarify that section.

Thank you very much. You have helped me alot. Not only you have helped me to improve the kata description but I have also gotten a English class for free. Thanks again!

Yes you're right: English is not my native tongue. I would be very grateful for your help. Thanks!

no real reason. probably still mentally stuck in if/else syntax from other languages. if I were to write the solution again, I would likely use if