OBJECT ORIENTED JAVASCRIPTGetting Started

OBJECT ORIENTED JAVASCRIPT

The purpose of this presentation is to elaborate how to use object oriented approach when codding in JavaScriptCreated By :

Usman MehmoodSenior software EngineerSystems Limited97-Aziz Avenue Canal Bank, Gulberg V, Lahore, PakistanOffice: +92-42- 3577 5582 - 85 EXT: 245VOIP: + 1 732 994 6492 - 94 EXT: 245 Fax: +92 423 587 7238Email: usman.tufail@systemsltd.comWebsite: http://www.systemsltd.com

OBJECT-ORIENTED PROGRAMMING

Object-oriented programming is a programming paradigm that uses abstraction to create models based on the real world. It uses several techniques from previously established paradigms, including modularity, polymorphism, and encapsulation. Today, many popular programming languages (such as Java, JavaScript, C#, C++, Python, PHP, Ruby and Objective-C) support object-oriented programming (OOP).

Object-oriented programming may be seen as the design of software using a collection of cooperating objects, as opposed to a traditional view in which a program may be seen as a collection of functions, or simply as a list of instructions to the computer. In OOP, each object is capable of receiving messages, processing data, and sending messages to other objects. Each object can be viewed as an independent little machine with a distinct role or responsibility.

Object-oriented programming is intended to promote greater flexibility and maintainability in programming, and is widely popular in large-scale software engineering. By virtue of its strong emphasis on modularity, object oriented code is intended to be simpler to develop and easier to understand later on, lending itself to more direct analysis, coding, and understanding of complex situations and procedures than less modular programming methods.

TERMINOLOGY

Class: Defines the characteristics of the Object. An Instance of a Class. Property An Object characteristic, such as color. Method: An Object capability, such as walk. Constructor: A method called at the moment of

instantiation. Inheritance: A Class can inherit characteristics from

another Class. Encapsulation: A Class defines only the characteristics of

the Object, a method defines only how the method executes.

Abstraction: The conjunction of complex inheritance, methods, properties of an Object must be able to simulate a reality model.

Polymorphism Different Classes might define the same method or property.

PROTOTYPE-BASED PROGRAMMING

Prototype-based programming is a style of object-oriented programming in which classes are not present, and behavior reuse (known as inheritance in class-based languages) is accomplished through a process of decorating existing objects which serve as prototypes. This model is also known as class-less, prototype-oriented, or instance-based programming.

The original (and most canonical) example of a prototype-based language is the programming language Self developed by David Ungar and Randall Smith. However, the class-less programming style has recently grown increasingly popular, and has been adopted for programming languages such as JavaScript, Cecil, NewtonScript, Io, MOO, REBOL, Kevo, Squeak (when using the Viewer framework to manipulate Morphic components), and several others

ENUMERATING ALL PROPERTIES

Enumerating all properties of an object Starting with ECMAScript 5, there are three native

ways to list/traverse object properties: for...in loops

This method traverses all enumerable properties of an object and its prototype chain

Object.keys(o)This method returns an array with all the own (not in the prototype chain) enumerable properties' names ("keys") of an object o.

Object.getOwnPropertyNames(o)This method returns an array containing all own properties' names (enumerable or not) of an object o.

In ECMAScript 5, there is no native way to list all properties of an object. However, this can be achieved with the following function:

CORE OBJECTS

JavaScript has several objects included in its core; for example, there are objects like Math, Object, Array, and String. The example below shows how to use the Math object to get a random number by using its random() method.

you can look here for all core objects

NOTE

Every object in JavaScript is an instance of the object Object and therefore inherits all its properties and methods.

CUSTOM OBJECTS

The Class:JavaScript is a prototype-based language which contains no class statement, such as is found in C++ or Java. This is sometimes confusing for programmers accustomed to languages with a class statement. Instead, JavaScript uses functions as classes. Defining a class is as easy as defining a function. In the example below we define a new class called Person.

THE OBJECT (CLASS INSTANCE)

To create a new instance of an object obj we use the statement new obj, assigning the result (which is of type obj) to a variable to access it later. In the example below we define a class named Person and we create two instances (person1 and person2).

Please also see Object.create for a new and alternative instantiation method.

THE CONSTRUCTOR

The constructor is called at the moment of instantiation (the moment when the object instance is created). The constructor is a method of the class. In JavaScript, the function serves as the constructor of the object; therefore, there is no need to explicitly define a constructor method. Every action declared in the class gets executed at the time of instantiation. The constructor is used to set the object's properties or to call methods to prepare the object for use. Adding class methods and their definitions occurs using a different syntax described later in this article.In the example below, the constructor of the class Person displays an alert when a Person is instantiated.

THE PROPERTY (OBJECT ATTRIBUTE)

Properties are variables contained in the class; every instance of the object has those properties. Properties should be set in the prototype property of the class (function) so that inheritance works correctly.

Working with properties from within the class is done by the keyword this, which refers to the current object. Accessing (reading or writing) a property outside of the class is done with the syntax: InstanceName.Property; this is the same syntax used by C++, Java, and a number of other languages. (Inside the class the syntax this.Property is used to get or set the property's value.)

IN THE EXAMPLE BELOW WE DEFINE THE GENDER PROPERTY FOR THE PERSON CLASS AND WE DEFINE IT AT INSTANTIATION

THE METHODS

Methods follow the same logic as properties; the difference is that they are functions and they are defined as functions. Calling a method is similar to accessing a property, but you add () at the end of the method name, possibly with arguments. To define a method, assign a function to a named property of the class's prototype property; the name that the function is assigned to is the name that the method is called by on the object.

IN THE EXAMPLE BELOW WE DEFINE AND USE THE METHOD SAYHELLO() FOR THE PERSON CLASS.

IN JAVASCRIPT METHODS ARE REGULAR FUNCTION OBJECTS THAT ARE BOUND TO A CLASS/OBJECT AS A PROPERTY WHICH MEANS THEY CAN BE INVOKED "OUT OF THE CONTEXT". CONSIDER THE FOLLOWING EXAMPLE CODE:

This example demonstrates many concepts at once. It shows that there are no "per-object methods" in JavaScript since all references to the method point to the exact same function, the one we have defined in the first place on the prototype. JavaScript "binds" the current "object context" to the special "this" variable when a function is invoked as a method(or property to be exact) of an object. This is equal to calling the function object's "call" method as follows:

INHERITANCE

Inheritance is a way to create a class as a specialized version of one or more classes (JavaScript only supports single class inheritance). The specialized class is commonly called the child, and the other class is commonly called the parent. In JavaScript you do this by assigning an instance of the parent class to the child class, and then specializing it. In modern browsers you can also use Object.create to implement inheritance.

IMPORTANT

JavaScript does not detect the child class prototype.constructor see Core JavaScript 1.5 Reference:Global Objects:Object:prototype property, so we must state that manually.

IN THE EXAMPLE BELOW, WE DEFINE THE CLASS STUDENT AS A CHILD CLASS OF PERSON. THEN WE REDEFINE THE SAYHELLO() METHOD AND ADD THESAYGOODBYE() METHOD.

Click here to open code

ENCAPSULATION

In the previous example, Student does not need to know how the Person class's walk() method is implemented, but still can use that method; theStudent class doesn't need to explicitly define that method unless we want to change it. This is called encapsulation, by which every class inherits the methods of its parent and only needs to define things it wishes to change.

Example

ABSTRACTION

Abstraction is a mechanism that permits modeling the current part of the working problem. This can be achieved by inheritance (specialization), or composition. JavaScript achieves specialization by inheritance, and composition by letting instances of classes be the values of attributes of other objects.

The JavaScript Function class inherits from the Object class (this demonstrates specialization of the model). and the Function. prototype property is an instance of Object (this demonstrates composition) Abstraction example

CREATING OBJECTS

http://jsfiddle.net/mehmoodusman786/JLQFY/

INNER FUNCTIONS

JavaScript function declarations are allowed inside other functions. We've seen this once before, with an earlier makePerson() function. An important detail of nested functions in JavaScript is that they can access variables in their parent function's scope:

http://jsfiddle.net/mehmoodusman786/GFQha/1/

CLOSURES

function makeAdder(a) { return function(b) { return a + b; } } x = makeAdder(5); y = makeAdder(20); x(6) ? y(7) ?

CLOSURES

What's happening here is pretty much the same as was happening with the inner functions earlier on: a function defined inside another function has access to the outer function's variables. The only difference here is that the outer function has returned, and hence common sense would seem to dictate that its local variables no longer exist. But they do still exist — otherwise the adder functions would be unable to work. What's more, there are two different "copies" ofmakeAdder's local variables — one in which a is 5 and one in which a is 20. So the result of those function calls is as follows:

CLOSURES

Whenever JavaScript executes a function, a 'scope' object is created to hold the local variables created within that function. It is initialised with any variables passed in as function parameters. This is similar to the global object that all global variables and functions live in, but with a couple of important differences: firstly, a brand new scope object is created every time a function starts executing, and secondly, unlike the global object (which in browsers is accessible as window) these scope objects cannot be directly accessed from your JavaScript code. There is no mechanism for iterating over the properties of the current scope object for example.

CLOSURES

x(6) // returns 11 y(7) // returns 27

CLOSURES So when makeAdder is called, a scope object is created

with one property: a, which is the argument passed to the makeAdder function. makeAdder then returns a newly created function. Normally JavaScript's garbage collector would clean up the scope object created for makeAdder at this point, but the returned function maintains a reference back to that scope object. As a result, the scope object will not be garbage collected until there are no more references to the function object that makeAdder returned.

Scope objects form a chain called the scope chain, similar to the prototype chain used by JavaScript's object system.

A closure is the combination of a function and the scope object in which it was created.

Closures let you save state — as such, they can often be used in place of objects.

MEMORY LEAKS

An unfortunate side effect of closures is that they make it trivially easy to leak memory in Internet Explorer. JavaScript is a garbage collected language — objects are allocated memory upon their creation and that memory is reclaimed by the browser when no references to an object remain. Objects provided by the host environment are handled by that environment.

Browser hosts need to manage a large number of objects representing the HTML page being presented — the objects of the DOM. It is up to the browser to manage the allocation and recovery of these.

Internet Explorer uses its own garbage collection scheme for this, separate from the mechanism used by JavaScript. It is the interaction between the two that can cause memory leaks.

A memory leak in IE occurs any time a circular reference is formed between a JavaScript object and a native object. Consider the following:

MEMORY LEAKS

function leakMemory() { var el = document.getElementById('el'); var o = { 'el': el }; el.o = o; }

The circular reference formed above creates a memory leak; IE will not free the memory used by el and o until the browser is completely restarted.

MEMORY LEAKS

Closures make it easy to create a memory leak without meaning to. Consider this:

function addHandler() { var el = document.getElementById('el'); el.onclick = function() { this.style.backgroundColor = 'red'; } }

The above code sets up the element to turn red when it is clicked. It also creates a memory leak. Why? Because the reference to el is inadvertently caught in the closure created for the anonymous inner function. This creates a circular reference between a JavaScript object (the function) and a native object (el).

There are a number of workarounds for this problem. The simplest is not to use the el variable:

MEMORY LEAKS

There are a number of workarounds for this problem. The simplest is not to use the el variable:

function addHandler(){ document.getElementById('el').onclick = function(){ this.style.backgroundColor = 'red'; } }

Surprisingly, one trick for breaking circular references introduced by a closure is to add another closure:

MEMORY LEAKS

function addHandler() { var clickHandler = function() { this.style.backgroundColor = 'red'; }; (function() { var el = document.getElementById('el'); el.onclick = clickHandler; })(); }

The inner function is executed straight away, and hides its contents from the closure created with clickHandler.

Another good trick for avoiding closures is breaking circular references during the window.onunload event. Many event libraries will do this for you. Note that doing so disables bfcache in Firefox 1.5, so you should not register an unload listener in Firefox, unless you have other reasons to do so.

A JAVASCRIPT EXAMPLE USING OBJECT.DEFINEPROPERTY

A JavaScript example using Object.defineProperty var Person = function() { };

Object.defineProperty(Person.prototype, "name", { get: function() { return this._name ? this._name : "John Doe"; } }); var someone = new Person(); console.log( someone.name ); Please note that in the example above, I add the name property on the prototype object and not directly on the Person object (to increase performance).

In my opinion this is really simple and very elegant. You decrease boilerplate code throughout your whole solution, but still achieve data data encapsulation - and that's very cool :-)