Which is ultimately what separates a good language from a bad language. A good language should have ONE best way to do things. With the prototypical inheritance, there are impossible numbers of way to declare/define and inherit a class in JavaScript. That is just a trap for bugs and defects. How could you effectively understand what the code does when you can't read it in one glance? Especially code that is not yours.
ES6's class system is infinitely better.