there's not a single variable name. So to read it, you have to figure out yourself what's going on. That's not readable code imo

how is this unreadable? It makes it so much clearer than, e.g., the kata description.

read about prototype.

by prototype you can add a property or a function to an object classes.

string.toUpperCase() is a prototpe function on string class

spoiler alert!

how does this work I don't even get it

Not best practice: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#never_use_eval!
But clever

Can you explain this? I can't read it

In closures, is the outer function only called once while the inner function is called however many times?

Why is this highly rated? Isn't readability more important than anything. This is unreadable to me

Initial solution code gives a hint how to do this. Did you see how solution setup looks and do you know what it means?