It is. Description says: Assume the second parameter will be a string holding one of two values: 'value' and 'index'.

So we don't have to consider other cases. That behaviour is undefined, we can return whatever.

This solution is too slow anyway (if we are talking about performance) to bother about such minor optimization at all.

not really, i have almost similar benchmarks

Use of char constants instead of String constants will make this solution faster.

I consider solving this Kata with the use of Math.log() and Math.ceil() as a good practice and fastest code.

Solution treats any value of toReturn different of "index" as "value" which in my view is not a valid solution.