This answer is what the kata asked for, not the top voted solution. I think this should be top voted.

Oh! Now I see it! The starting code was with a constructor function so I assumed this problem was also with one. My bad, next time I should double check ;)

This code does create a method on the prototype. It does not create a separate function for every Ship instance. Test it for yourself if you don't believe me :)

It's better to attach the function on the prototype : Ship.prototype.isWorthIt = function(){ ... } because you create the function once and every instance of the Ship constructor can use it while in your code you create a separate function for every instance :)