What is prototype in javascript? What does it do?

Answer 1

Prototype allow to declare method for objects that will be shared between each instance of the objects
Lets take this code

var myObj = function() {};

myObj.prototype.test = function() {
  alert('test');
};

You will call it with
var instance = new myObj();
instance.test();
So test() is now a method of myObj
But since you declared it with prototype, even if you create 100 myObj, the test function will simply be 1 time in memory.

So if we didn't use prototype like so

var myObj = function() {};

myObj.test = function() {
  alert('test');
};

And create 100 new myObj, in memory you will have 100 time the test function

So most of the time (99% of the time) you will want to declare your method as prototype