JSBOX
Challenge
I created a black box for you to guess the flag. It's using secure comparison, so don't bother with a side channel!
Approach
The obvious first move is to call .toString() on it and read the source directly.
That gets you a "nice try :)" instead of real code.
Overriding an instance's own toString only shadows what you get from calling fn.toString() directly. It doesn't touch the real, original Function.prototype.toString, which still knows how to serialize the actual function body. Calling that original method and pointing it at the function, instead of asking the function to stringify itself, walks straight past the fake override.
Solution
Function.prototype.toString.call(fn)
This calls the real, un-overridden toString implementation with fn as its target, which returns the function's genuine source instead of whatever the instance-level override was set to return.