Trying to understand the difference between prototype and constructor in JavaScript

Its a pretty hard thing to wrap your mind around if you are used to the ease of extending objects in other OOP languages, but I'll do my best to explain the uses of those and what is what. I am going to assume you are familiar with other OOP languages. Correct me if I'm wrong.

All functions have the prototype Function(). They are inheriting all base functionality from Function like toString() and valueOf().

Then there is a constructor. That is what you use to initialize an object with.

p = new Foo();

So in this case we have two things.

• A function Foo with Function as prototype(Foo)
• A Function object with Foo() as constructor(p)

(following me yet?)

The Foo() constructor can override some base functionality of the Function constructor, but also leave it as it is and make good use of it.

If you are familiar with OOP principles, The prototype is the base class, the constructor your current class. in OOP the above would be class Foo extends Function

You can also start inheritance with this entire setup of prototype and constructor making more complex objects as you go whilst sharing functionality.

For example this:

// make a object initialiser extending Function. in oop `class Foo extends Function`

function Foo(bar) {
    this.baz = bar;
}
Foo.prototype.append = function(what) {
    this.baz += " " + what;
};
Foo.prototype.get() {
    return this.baz
}

Now lets say we want different ways to get baz out of there. one for console logging and one for putting it on the title bar. We could make a big thing about our class Foo, but we dont do that, because we need to do wholly different things with the new classes but are made for different implementations. The only thing they need to share are the baz item and the setters and getters.

So we need to extend it to use an OOP term. in OOp this would be the desired end result class Title extends Foo(){}. So lets take a look how to get there.

function Title(what) {
    this.message = what;
}

At this point the Title function looks like this:

• prototype Function
• constructor Title

So, to make it extends Foo we need to change the prototype.

Title.prototype = new Foo();

• prototype Foo
• constructor Foo

This is done by initializing a new Foo() object against the prototype. Now its basically a Foo object called Title. That is not what we want because now we cant access the message part in Title. We can make it properly extend Foo() by resetting the constructor to Title

Title.prototype.constructor = Title;

• prototype Foo
• Constructor Title

Now we are faced with one more problem. The constructor of Foo doesn't get initialized so we end up with an undefined this.baz

To resolve that we need to call the parent. In java you would do that with super(vars), in php $parent->__construct($vars).

In javascript we have to modify the Title class constructor to call the constructor of the parent object.

So the Title class constructor would become

function Title(what) {
    Foo.call(this,what);
    this.message = what;
}

By using the Function object property Foo inherited we can initialism the Foo object in the Title object.

And now you have a properly inherited object.

So instead of using a keyword like extend like other OOP languages it uses prototype and constructor.

If you want to create a javascript object you can simply declare a new object and give it properties (I've chosen to objectify myself):

var myself= {
    name:"Niddro",
    age:32
};

This method allows you to make one object. If what you want to have is a prototype describing a person in general, where you can declare several people with the same setup. To create a prototype, you can use a constructor, as seen below:

//Constructor
function generalNameForObject(param1, param2,...) {
    //Give the object some properties...
}

I have a prototype (a recipe) in mind that I want to call person and it should contain the properties name and age and I'll use a constructor to make it:

function person(name,age) {
    this.name=name;
    this.age=age;
}

The above construct function describes the prototype for my person objects.

Create a new person by calling the construct function:

var myself = new person("Niddro",31);
var OP = new person("rajashekar thirumala",23);

Some time passes and I realise that I've had a birthday so I need to change the property of the prototype:

myself.age=32;

If you want to add properties to the construct, you need to manually add it into the construct function:

function person(name,age,rep) {
    this.name=name;
    this.age=age;
    this.reputation=rep;
}

Instead, you can add properties to the prototype by doing the following (here "prototype" is an actual command and not just a name):

function person(name,age,rep) {
    this.name=name;
    this.age=age;
}
person.prototype.reputation=105;

note that this will add a reputation of 105 for all objects created.

I hope this has given you some more insight on the relationship between the constructor and prototype.

employee.constructor //gives Function()

In JavaScript functions are also objects, which can be constructed using its own constructor which is Function . So you can write following code to get a instance of Function.

var employee2 = new Function('a', 'b', 'return a+b');

Same happens when you create function using function literal like in your case. And the constructor property of this object also refers to the same native Function object/class.

employee.prototype // gives Emp {}

Each object in JavaScript has a prototype associated with it. Though only function objects prototype is directly accessible with the .prototype. This same prototype is copied on its objects prototype when you create objects with new keyword. Primarily this copying is responsible for the inheritance/extension. Although the prototype is copied, it is not directly asseccible like in case of Function objects. It's available in non standard way with .__proto__ . Following code will return true.

jack.__proto__==employee.prototype

employee.prototype.constructor //gives Emp(name)

As said in the documentation of Object.prototype.constructor . This returns a reference to the Object function that created the instance's prototype. Here the object being refered is employee.prototype and not employee. This is bit complex but prototype of object employee.prototype was created by the function Emp(name)

jack.constructor //gives Emp(name)

As said in the previous point, this objects prototype was created by function Emp(name) when you created the object using new Emp(),

jack.prototype //gives undefined

jack is not a function object, so you cant access its prototype like that. You can access(not a standard way) the prototype of jack like following.

jack.__proto__