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A prototype is a property present in every object that allows to add methods and properties.

In Javascript there is no real distinction between class and object.

Every object is declared as a function.

	var myObj = function() {}

Or

	function myFunc() {}

You can then add new methods using the prototype property:

	myObj.prototype.doSomething = function() {document.write("Writing something");}

You can also add properties and methods without prototype:

	myObj.doSomething() {document.write("Dummy method");}

But in this case the property or object exist only for the current instance and not the others:

	var myObj = function() {}
	myObj.doSomething() {document.write("Writing something");}

	var otherObj = new myObj();
	otherObj.doSomething(); // You get an error here!

By using prototype, the properties and the methods are automatically propagated to all the objects, as demonstrated in this code:

	var obj1 = function() {};

	obj1.prototype.saySomething = function() {
		document.write("Saying something");
	};

	var obj2 = function() {};

	obj2.prototype = new obj1();

	new obj2().saySomething();

This is an other example:

	var obj1 = function() {};
	var obj2 = new obj1();

	obj1.prototype.saySomething = function() {
		document.write("Saying something");
	};

	obj2.saySomething();

However a prototype is examined only when a new object is created, so this code generate error:

	var obj1 = function() {};

	obj1.prototype.saySomething = function() {
		document.write("Saying something");
	};

	new obj1.saySomething();  // This is fine
	obj1.saySomething(); //This generate error

A chain of prototypes is formed with each creation, where a prototype constitute a template only for the derivated objects and not the current one.