it removes the character that was there previously i think

Humm? Is not your own solution read through the string a lot mor of times? doing a cicle for you are reading the string mor than once for each character.

Why set such conditions in the first place?
Kata is only for experience, in reality you get slapped on hand for doing like this solution.
How I can blocked exersise from this 'coder'?

I'm new to coding, this is sick! THANKS!

The this key word refers to the object that changes what it refers to based on when it's invoked. In the example, it's referring to the string that .toJadenCase is being called upon.

It's a tricky concept starting out, MDN probably explains it much better than most of us could: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this

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This solution is at least 8 years old. Probably let or const weren't a thing when it was written.

I advise using const or let ao instead of var friend