Why is it bad to add methods to a constructor?

Question

JavaScript

I have a constructor for some object. Is it necessary to throw a method into its prototype? Can I just put it in the constructor as usual? Or is it bad manners? Or what problems can I have? Why do they say that it is better to create methods outside the constructor, that is, in prototypes?

Answer 1

Because the constructor will duplicate the same methods each time it is instantiated. And so - each instance inherits from the prototype. With a large number of instances, there will be a significant difference in resources.

Answer 2

Why do you need to rewrite the same method n times, if you can assign it to the prototype once? This is the essence of OOP. One of.

Answer 3

Here is a good example https://jsfiddle.net/4r9k4vfq/9/
In this case, each instance creates its own object in memory with the methodOne method, but all instances use a common methodTwo object.
The first case is used only when you need to use the variables created in the scope constructor (if you try to refer to _somePrivateVar in the second method, you will get an error) but you should not abuse this, since the second method gives an advantage in performance and memory consumption.