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Javascript nested prototypes

The other day I was fiddling with Javascript, and I noticed that I can't write a prototype to an object within a prototype function. Like this:
var obj = function() { }
obj.prototype.First = function() {
this.prototype.Second = function() {
alert("Second Prototype");
}
}
obj.First();
obj.Second();
For some reason, the second prototype won't work and the code doesn't run. Is my syntax wrong or are there some prototype limitations that I don't know about? Thanks!
Edit:
I'm not trying to add a prototype to a prototype... that wouldn't make much sense. This is what I'm trying to do: Add two separate prototypes to obj. Prototype 2 is defined when prototype 1 is called. I thought that this would contain a reference to object, so this.prototype would be the same as obj.prototype, but it doesn't work as expected.

This is an old question, but I thought I'd add my two cents.
This code is trying to add functions on 'prototype'. However, this can only be done on the class name. What you have is a variable pointing to an anonymous class. To access the variable of an anonymous variable use 'proto'. The below is the same as your example, except, using proto it is 'successful'. Although, I don't see the benefit of using prototypes like this as the prototype added methods only apply to the anonymous instance 'obj'. http://jsbin.com/zapocamipi/edit?js,console
var obj = function() { }
obj.__proto__.First = function() {
console.log("First Prototype");
this.__proto__.Second = function() {
console.log(this);
}
}
obj.First();
obj.Second();

Maybe this could help you out understanding the role of a constructor function and the prototype.
Depending on what you're trying to do (obj, First and Second doesn't really show your intention) you could do:
A Person has Eyes. This can be done through composition.
An Employer is a Person but a Person is not necessarily an Employer (can be Client or Consultant too). This could be done through inheritance.
A Cat can move. In a Class based language Cat has to implement Movable but in JavaScript you can use mix ins and leave the implementation to the default implementation that Movable provides or override it. JavaScript does not compile time check if you do implement certain things.
If you would like to change the type of the object instance after calling a certain function then it's dangerous to meddle with the prototype because that will affect all instances of that type.
Maybe you should return an instance of another type.
var Customer = function(name) {
this.name=name || 'unknown';
};
Customer.createVipCustomer = function() {
return new VipCustomer(this);
}
var VipCustomer=function(customer){
//re use Customer constructor
Customer.call(this,customer.name);
this.isVip=true;
}
//inherit the protype defined members
VipCustomer.prototype=Object.create(Customer.prototype);
VipCustomer.prototype.constructor=VipCustomer;
VipCustomer.prototype.second=function(){
console.log('this is second');
}
var aCustomer = new Customer('Ben');
//update to VipCustomer
aCustomer = Customer.createVipCustomer(aCustomer);
aCustomer.second();

this.prototype doesn't exist.
If you want to add a property to the instance, use this.
If you want to add a property to the prototype, use Constructor.prototype.
Also, obj is a function (class), not an instance,
You want to create an instance using the new keyword, and you should name the constructor function as UpperCamelCase.

Is there a difference between constructor and constructor function in javascript?

I come from a Java background, completely understand what constructor does in java. I am new to javascript and trying to understand why a constructor is called constructor function. Do constructors and constructor functions mean different things in javascript ?

A constructor in JavaScript is a function that is used to construct an object. For example:
function Obj() {
// The constructed object is bound to the 'this' variable
this.property = "someValue";
}
var obj = new Obj();
console.log(obj.property); // Will print "someValue"
console.log(obj.constructor); // Will print Obj's code
Obj is now the constructor of the instance obj. Obj is just a normal function, but since it's invoked via new it acts as a constructor.
Note that constructor functions are by convention named like Java classes, i.e. Upper CamelCase. The language doesn't enforce it in any way, but it makes it clear that a function is intended as a constructor.
You can also achieve inheritance in JavaScript, even though the language lacks classes per se, by defining constructors' "prototypes":
// Base constructor, like a baseclass in Java
function Base() {
this.baseProperty = "baseValue";
}
// Base implementation of a method
Base.prototype.printSelf = function () {
console.log(this.baseProperty);
};
// Constructor, corresponding to a subclass in Java
function Obj() {
// Call "baseclass" constructor
Base.call(this);
this.property = "someValue";
}
// Inherit prototype from Base
Obj.prototype = Object.create(Base.prototype);
// Overload printSelf
Obj.prototype.printSelf = function () {
// Call base implementation
Base.prototype.printSelf.call(this);
console.log(this.property);
};
var obj = new Obj();
obj.printSelf();
See my fiddle for a live demonstration if you like.

Java constructors are special functions that can only be called at object creation; JavaScript "constructors" are just standard functions.

JS has no special handling or type called function constructor. It's a term used to imply that this function is used with the new keyword to create an object. Any function can be a "function constructor" by prefixing it with the new keyword.
Of course that is usually not the intention of the author. So the convention is to uppercase the first letter of the function name to denote that is a function constructor and should therefore be used in conjuction w/ new.
The term constructor is usually meant to mean function constructor. Most in the community use the term full term since JS has no classes, and function constructor is more clear.

Object vs Class vs Function

I was wondering - what's the difference between JavaScript objects, classes and functions?
Am I right in thinking that classes and functions are types of objects?
And what distinguishes a class from a function? Or are they really the same thing, just the term for them changes according to how they are used?
function func() { alert('foo'); } // a function
func(); // call the function - alerts 'foo'
var func2 = function () { alert('hello'); } // acts the same way as 'func' surely?
func2(); // alerts 'hello'
var Class = function() { alert('bar'); }; // a class
var c = new Class(); // an istance of a class - alerts 'bar'
Sure, classes have methods and properties and can be instantiated - but then, I could do the same with any old function - or not?

As you must already be aware by now there are no classes in JavaScript. Instead functions in JavaScript may be made to behave like constructors by preceding a function call with the new keyword. This is known as the constructor pattern.
In JavaScript, everything is an object except for the primitive data types (boolean, number, and string), and undefined. On the other hand null is actually an object reference even though you may at first believe otherwise. This is the reason typeof null returns "object".
Functions in JavaScript are similar to functables in Lua (i.e. they are callable objects). Hence a function can be used in place of an object. Similarly, arrays are also objects in JavaScript. On the other hand, objects can be thought of as associative arrays.
The most important point however is that there are no classes in JavaScript because JavaScript is a prototypal object-oriented language. This means that objects in JavaScript directly inherit from other objects. Hence we don't need classes. All we need is a way to create and extend objects.
Read the following thread to learn more about prototypal inheritance in JavaScript: Benefits of prototypal inheritance over classical?

Update 2015
There are classes in JavaScript they just aren't used on older browsers:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes
It has constructors, extensions, and the like.
class Cat {
constructor(name) {
this.name = name;
}
speak() {
console.log(this.name + ' makes a noise.');
}
}
class Lion extends Cat {
speak() {
super.speak();
console.log(this.name + ' roars.');
}
}

A Class in JS:
function Animal(){
// Private property
var alive=true;
// Private method
function fight(){ //... }
// Public method which can access private variables
this.isAlive = function() { return alive; }
// Public property
this.name = "Joe";
}
// Public method
Animal.prototype.play = function() { alert("Bow wow!"); }
// .. and so on
Now when you create it's object
var obj = new Animal();
You can expect anything of this object as you would from objects in other language. Just the efforts to achieve it, was a bit different. You should also be looking at inheritance in JS.
Getting back too your question, I'll reword it as:
//Class : A representation of a set with common properties.
//object : One from the set with the same properties.
var Class = function() {alert('bar');}; // A set of function which alert 'bar'
var object = new Class(); // One of the functions who alert's 'bar'.

JavaScript does not have classes, and functions are actually objects in JavaScript (first-class citizens).
The only difference that function objects have is that they are callable.
function func() { alert('foo'); } // a function - Correct
func(); // call the function - alerts 'foo' - Correct
var func2 = function () { alert('foo'); } // same as 'func' surely? - Nope, func2 is a different object, that apparently does the same thing when called.
var Class = function() { alert('bar'); }; - It's a function with no name stored in variable Class.
var c = new Class(); - Calls function stored in Class supplying new empty object as this and returning that object. Functions called as new functionA() are expected to work as constructors and prepare a newly created object (this). In your case - constructor does nothing with the object and just alerts bar.

You also get classes in ES6 that look like this:
//class
class Cat {
//constructor
constructor() {
this.name = 'Snowball';
}
//method
meow() {
console.log('Hello, nyah! My name is ' + this.name + ' nyah!~');
}
};

There are no classes in javascript. But, there are ways to make a function to behave like a class in other languages.
A very good explanation is given here 3 way to define a class in js
Also, found a very good reference for OOP in Javascript

Object is the base type in JavaScript i.e. all the user defined data types inherits from Object in one way or another. So if you define a function or a class [note as of now JS doesn't support class construct, but its proposed in ECMAScript version 6], it will implicitly inherit from Object type.
Classes are really used for encapsulating logical functions and properties into one type / entity and you can 'new' it up using constructor syntax. So if you define a 'Customer' class, you can instantiate it multiple times and each instance / object can have different values. They can even share the values if you define class level value using prototype.
Since JS doesn't support class construct at the moment, functions can really act as individual method as well as container for other functions or types.
I hope with ECMAScript 6 we will have clear separation between these constructs similar to what we have in other languages like C#, Java etc.

JavaScript inheritance: Object.create vs new

In JavaScript what is the difference between these two examples:
Prerequisite:
function SomeBaseClass(){
}
SomeBaseClass.prototype = {
doThis : function(){
},
doThat : function(){
}
}
Inheritance example A using Object.create:
function MyClass(){
}
MyClass.prototype = Object.create(SomeBaseClass.prototype);
Inheritance example B using the new keyword
function MyClass(){
}
MyClass.prototype = new SomeBaseClass();
Both examples seem to do the same thing. When would you chose one over the other?
An additional question:
Consider code in below link (line 15), where a reference to the the function's own constructor is stored in the prototype. Why is this useful?
https://github.com/mrdoob/three.js/blob/master/src/loaders/ImageLoader.js
Excerpt (if you don't want to open the link):
THREE.ImageLoader.prototype = {
constructor: THREE.ImageLoader
}

In your question you have mentioned that Both examples seem to do the same thing, It's not true at all, because
Your first example
function SomeBaseClass(){...}
SomeBaseClass.prototype = {
doThis : function(){...},
doThat : function(){...}
}
function MyClass(){...}
MyClass.prototype = Object.create(SomeBaseClass.prototype);
In this example, you are just inheriting SomeBaseClass' prototype but what if you have a property in your SomeBaseClass like
function SomeBaseClass(){
this.publicProperty='SomeValue';
}
and if you use it like
var obj=new MyClass();
console.log(obj.publicProperty); // undefined
​console.log(obj);​
The obj object won't have publicProperty property like in this example.
Your second example
MyClass.prototype = new SomeBaseClass();
It's executing the constructor function, making an instance of SomeBaseClass and inheriting the whole SomeBaseClass object. So, if you use
var obj=new MyClass();
console.log(obj.publicProperty); // SomeValue
console.log(obj);​
In this case its publicProperty property is also available to the obj object like in this example.
Since the Object.create is not available in some old browsers, in that case you can use
if(!Object.create)
{
Object.create=function(o){
function F(){}
F.prototype=o;
return new F();
}
}
Above code just adds Object.create function if it's not available so you can use Object.create function and I think the code above describes what Object.create actually does. Hope it'll help in some way.

Both examples seem to do the same thing.
That's true in your case.
When would you chose one over the other?
When SomeBaseClass has a function body, this would get executed with the new keyword. This usually is not intended - you only want to set up the prototype chain. In some cases it even could cause serious issues because you actually instantiate an object, whose private-scoped variables are shared by all MyClass instances as they inherit the same privileged methods. Other side effects are imaginable.
So, you should generally prefer Object.create. Yet, it is not supported in some legacy browsers; which is the reason you see the new-approach much too frequent as it often does no (obvious) harm. Also have a look at this answer.

The difference becomes obvious if you use Object.create() as it is intended. Actually, it does entirely hideout the prototype word from your code, it'll do the job under the hood. Using Object.create(), we can go like
var base = {
doThis : function(){
},
doThat : function(){
}
};
And then we can extend/inherit other objects from this
var myObject = Object.create( base );
// myObject will now link to "base" via the prototype chain internally
So this is another concept, a more "object oriented" way of inherting. There is no "constructor function" out of the box using Object.create() for instance. But of course you could just create and call a self defined constructor function within those objects.
One argument for using Object.create() is that it might look more natural to mix/*inherit* from other objects, than using Javascripts default way.

I am not an expert in java script but here is a simple example to understand difference between "Object.create" and "new" ..
step 1 : create the parent function with some properties and actions..
function Person() {
this.name = 'venkat';
this.address = 'dallas';
this.mobile='xxxxxxxxxx'
}
Person.prototype.func1 = function () {
return this.name + this.address;
}
step 2 : create a child function (PersonSalary) which extends above Person function using New keyword..
function PersonSalary() {
Person.call(this);
}
PersonSalary.prototype = new Person();
PersonSalary();
step 3 : create second child function (PersonLeaves) which extends above Person function using Object.create keyword..
function PersonLeaves() {
Person.call(this);
}
PersonLeaves.prototype = Object.create(Person.prototype);
PersonLeaves();
// Now check both child functions prototypes.
PersonSalary.prototype
PersonLeaves.prototype
both of these child functions will link to Person(parent function) prototype and can access it's methods but if you create child function using new it will return a brand new object with all parent properties which we don't need and also when you create any object or function using "New" that parent function is executed which we don't want to be.
Here are the takeaways
if you just want to delegate to some methods in parent function and don't want a new object to be created , using Object.create is best way.

A couple of additions to this answer set, mindful that JS obviously now has native classes:
In both Example A and Example B the static inheritance chain is not configured.
In Example B the superclass constructor is run at the "wrong time". It is run before the call to instantiate an instance of the subclass, before any arguments are known and perhaps before you have decided to instantiate an instance of the subclass. Note that constructors can contain any logic they like, including side-effectful logic, so this can be impactful.
Post-ES6 the inheritance chain can be configured in a standardised way using the class and extends keywords (which solve both of these issues).
See also.

Javascript Inheritance and Arrays

I am trying to define a javascript class with an array property, and its subclass. The problem is that all instances of the subclass somehow "share" the array property:
// class Test
function Test() {
this.array = [];
this.number = 0;
}
Test.prototype.push = function() {
this.array.push('hello');
this.number = 100;
}
// class Test2 : Test
function Test2() {
}
Test2.prototype = new Test();
var a = new Test2();
a.push(); // push 'hello' into a.array
var b = new Test2();
alert(b.number); // b.number is 0 - that's OK
alert(b.array); // but b.array is containing 'hello' instead of being empty. why?
As you can see I don't have this problem with primitive data types... Any suggestions?

Wen you write Test2.prototype = new Test(), you create a single Test instance, with a single array instance, which is shared by every Test2 instance.
Therefore, all of the Test2 instances are sharing the same array.
You can solve this problem by calling the base Test constructor from the Test2 constructor, which will create a new array instance for every Test2 instance.
For example:
function Test2() {
Test.call(this);
}

Another, rather inelegant, alternative is to move initialization code from the constructor to a method and call it from both constructors:
// class Test
function Test() {
this.init();
}
Test.prototype.init = function() {
this.array = [];
this.number = 0;
};
Test.prototype.push = function() {
this.array.push('hello');
this.number = 100;
};
// class Test2 : Test
function Test2() {
this.init();
}
Test2.prototype = new Test();

Only thing I can think of is that arrays are shared references. There should be an obvious solution since this kind of classic OOP code is implemented all the time in Javascript.

JavaScript does not have a classical inheritance system, it has a prototypical inheritance system. So in JavaScript technically there is no concept of "Class".
Objects inherit from other objects (the so called prototype object), not abstract classes. One big consequence of this is that if several objects share the same prototype and one of these objects change an attribute defined in the prototype, that change is effective immediately to all other objects.
Trying to use JavaScript inheritance as if it was based on classes usually causes big headaches, because you find this type of problems all the time.
I know this does not really answer your particular questions, but at a high level I recommend you embrace the prototypical inheritance instead of trying to use "pseudo-classes". Even if it looks weird at first, your code will be much more reliable and you will not lose time trying to understand strange bugs like this, cause by a strange chain of prototypes.
Watch this video where Douglas Crockford explains inheritance, it's available online in Yahoo UI Theater website. It changed the way I programmed in JavaScript :)
http://video.yahoo.com/watch/111585/1027823 (link to the first part)
http://developer.yahoo.com/yui/theater/ (for all the videos)