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<h1 id="6501">Introduction to Javascript</h1>

This guide is based on the <a href="http://www.sergiopereira.com/articles/advjs.html">

Quick guide to somewhat advanced JavaScript tour of some OO features</a> by Sergio Pereira.

<h2 id="6502">Hey, I didn't know you could do that</h2>

<p id="820693" class="block-content">

    If you are a web developer and come from the same place I do, you have probably

    used quite a bit of Javascript in your web pages, mostly as UI glue.

</p>

<p id="820694" class="block-content">

    Until recently, I knew that Javascript had more OO capabilities than I was employing,

    but I did not feel like I needed to use it. As the browsers started to support a more

    standardized featureset of Javascript and the DOM, it became viable to write more

    complex and functional code to run on the client. That helped giving birth to the

    AJAX phenomena.

</p>

<p id="820695" class="block-content">

    As we all start to learn what it takes to write our cool, AJAX applications, we begin

    to notice that the Javascript we used to know was really just the tip of the iceberg.

    We now see Javascript being used beyond simple UI chores like input validation and frivolous

    tasks. The client code now is far more advanced and layered, much like a real desktop

    application or a client-server thick client. We see class libraries, object models,

    hierarchies, patterns, and many other things we got used to seeing only in our server

    side code.

</p>

<p id="820696" class="block-content">

    In many ways we can say that suddenly the bar was put much higher than before. It takes

    a heck lot more proficiency to write applications for the new Web and we need to improve

    our Javascript skills to get there.

    If you try to use many of the existing javascript libraries out there, like

    <a href="http://prototype.conio.net/">Prototype.js</a>,

    <a href="http://script.aculo.us/">Scriptaculous</a>,

    <a href="http://moofx.mad4milk.net/">moo.fx</a>,

    <a href="http://bennolan.com/behaviour/">Behaviour</a>,

    <a href="http://developer.yahoo.net/yui/">YUI</a>,

    etc you'll eventually find yourself reading the JS code. Maybe because you want

    to learn how they do it, or because you're curious, or more often because that's the

    only way to figure out how to use it, since documentation does not seem to be highly

    regarded with most of these libraries. Whatever the case may be, you'll face some

    kung-fu techniques that will be foreign and scary if you haven't seen anything like

    that before.

</p>

<p id="820697" class="block-content">

    The purpose of this article is precisely explaining the types of constructs that

    many of us are not familiar with yet.

</p>

<h2 id="6503">JSON (JavaScript Object Notation)</h2>

<p id="820698" class="block-content">

    JavaScript Object Notation (<a href="http://www.json.org/">JSON</a>,) is one of the new

    buzzwords popping up around the AJAX theme. JSON, simply put, is a way of

    declaring an object in Javascript. Let's see an example right away and note

    how simple it is.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820233">

var myPet = { color: 'black', leg_count: 4, communicate: function(repeatCount){

for(i=0;i<repeatCount;i++) alert('Woof!');} };

</com:TTextHighlighter>

<p id="820699" class="block-content">

    Let's just add little bit of formatting so it looks more like how we usually find out there:

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820234">

var myPet =

{

    color: 'black',

    legCount: 4,

    communicate: function(repeatCount)

    {

        for(i=0;i<repeatCount;i++)

            alert('Woof!');

    }

};

</com:TTextHighlighter>

<p id="820700" class="block-content">

    Here we created a reference to an object with two properties (<tt>color</tt>

    and <tt>legCount</tt>) and a method (<tt>communicate</tt>.)

    It's not hard to figure out that the object's properties and methods

    are defined as a comma delimited list. Each of the members is introduced by name, followed

    by a colon and then the definition. In the case of the properties it is easy, just the value

    of the property. The methods are created by assigning an anonymous function, which we will

    explain better down the line.

    After the object is created and assigned to the variable <tt>myPet</tt>,

    we can use it like this:

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820235">

alert('my pet is ' + myPet.color);

alert('my pet has ' + myPet.legCount + ' legs');

//if you are a dog, bark three times:

myPet.communicate(3);

</com:TTextHighlighter>

<p id="820701" class="block-content">

    You'll see JSON used pretty much everywhere in JS these days, as arguments to functions,

    as return values, as server responses (in strings,) etc.

</p>

<h2 id="6504">What do you mean? A function is an object too?</h2>

<p id="820702" class="block-content">

    This might be unusual to developers that never thought about that, but in JS a function is

    also an object. You can pass a function around as an argument to another function just like

    you can pass a string, for example. This is extensively used and very handy.

</p>

<p id="820703" class="block-content">

    Take a look at this example. We will pass functions to another function that will use them.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820236">

var myDog =

{

    bark: function()

    {

        alert('Woof!');

    }

};

var myCat =

{

    meow: function()

    {

        alert('I am a lazy cat. I will not meow for you.');

    }

};

function annoyThePet(petFunction)

{

    //let's see what the pet can do

    petFunction();

}

//annoy the dog:

annoyThePet(myDog.bark);

//annoy the cat:

annoyThePet(myCat.meow);

</com:TTextHighlighter>

<p id="820704" class="block-content">

    Note that we pass myDog.bark and myCat.meow without appending parenthesis

    <tt>"()"</tt> to them. If we did that we would not be passing

    the function, rather we would be calling the method and passing the return value,

    <tt>undefined</tt> in both cases here.

</p>

<p id="820705" class="block-content">

    If you want to make my lazy cat start barking, you can easily do this:

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820237">

myCat.meow = myDog.bark;

myCat.meow(); //alerts 'Woof!'

</com:TTextHighlighter>

<h2 id="6505">Arrays, items, and object members</h2>

<p id="820706" class="block-content">

    The following two lines in JS do the same thing.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820238">

var a = new Array();

var b = [];

</com:TTextHighlighter>

<p id="820707" class="block-content">

    As I'm sure you already know, you can access individual items in an array

    by using the square brackets:

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820239">

var a = ['first', 'second', 'third'];

var v1 = a[0];

var v2 = a[1];

var v3 = a[2];

</com:TTextHighlighter>

<p id="820708" class="block-content">

    But you are not limited to numeric indices. You can access any member of a JS

    object by using its name, in a string. The following example creates an empty

    object, and adds some members by name.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820240">

var obj = {}; //new, empty object

obj['member_1'] = 'this is the member value';

obj['flag_2'] = false;

obj['some_function'] = function(){ /* do something */};

</com:TTextHighlighter>

<p id="820709" class="block-content">

    The above code has identical effect as the following:

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820241">

var obj =

{

    member_1:'this is the member value',

    flag_2: false,

    some_function: function(){ /* do something */}

};

</com:TTextHighlighter>

<p id="820710" class="block-content">

    In many ways, the idea of objects and associative arrays (hashes) in JS are not

    distiguishable. The following two lines do the same thing too.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820242">

obj.some_function();

obj['some_function']();

</com:TTextHighlighter>

<h2 id="6506">Enough about objects, may I have a class now?</h2>

<p id="820711" class="block-content">

    The great power of object oriented programming languages derive from the use

    of classes. I don't think I would have guessed how classes are defined in JS

    using only my previous experience with other languages. Judge for yourself.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820243">

//defining a new class called Pet

var Pet = function(petName, age)

{

    this.name = petName;

    this.age = age;

};

//let's create an object of the Pet class

var famousDog = new Pet('Santa\'s Little Helper', 15);

alert('This pet is called ' + famousDog.name);

</com:TTextHighlighter>

<p id="820712" class="block-content">

    Let's see how we add a method to our <tt>Pet</tt> class. We will be using the

    <tt>prototype</tt> property that all classes have. The <tt>prototype</tt>

    property is an object that contains all the members that any object of the class will have.

    Even the default JS classes, like <tt>String</tt>, <tt>Number</tt>,

    and <tt>Date</tt> have a <tt>prototype</tt> object that we

    can add methods and properties to and make any object of that class automatically gain this new member.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820244">

Pet.prototype.communicate = function()

{

    alert('I do not know what I should say, but my name is ' + this.name);

};

</com:TTextHighlighter>

<p id="820713" class="block-content">

    That's when a library like <a href="http://www.sergiopereira.com/articles/prototype.js.html">prototype.js</a> comes in

    handy. If we are using prototype.js, we can make our code look cleaner (at least in my opinion.)

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820245">

var Pet = Class.create();

Pet.prototype =

{

    //our 'constructor'

    initialize: function(petName, age)

    {

        this.name = petName;

        this.age = age;

    },

    communicate: function()

    {

        alert('I do not know what I should say, but my name is ' + this.name);

    }

};

</com:TTextHighlighter>

<h2 id="6507">Functions as arguments, an interesting pattern</h2>

<p id="820714" class="block-content">

    If you have never worked with languages that support closures

     you may find the following idiom too funky.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820246">

var myArray = ['first', 'second', 'third'];

myArray.each( function(item, index)

{

    alert('The item in the position #' + index + ' is:' + item);

});

</com:TTextHighlighter>

<p id="820715" class="block-content">

    Whoa! Let's explain what is going on here before you decide I've gone too

    far and navigate to a better article than this one.

</p>

<p id="820716" class="block-content">

    First of all, in the above example we are using the prototype.js library, which

    adds the each function to the Array class. The each function accepts one

    argument that is a function object. This function, in turn, will be called once

    for each item in the array, passing two arguments when called, the item and the index

    for the current item. Let's call this function our iterator function.

    We could have also written the code like this.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820247">

function myIterator(item, index)

{

    alert('The item in the position #' + index + ' is:' + item);

}

var myArray = ['first', 'second', 'third'];

myArray.each( myIterator );

</com:TTextHighlighter>

<p id="820717" class="block-content">

    But then we would not be doing like all the cool kids in school, right?

    More seriously, though, this last format is simpler to understand but causes

    us to jump around in the code looking for the myIterator function. It's nice

    to have the logic of the iterator function right there in the same place

    it's called. Also, in this case, we will not need the iterator function anywhere

    else in our code, so we can transform it into an anonymous function without penalty.

</p>

<h2 id="6508">This is <tt>this</tt> but sometimes <tt>this</tt> is also that</h2>

<p id="820718" class="block-content">

    One of the most common troubles we have with JS when we start writing our code

    it the use of the <tt>this</tt> keyword. It could be a real

    tripwire.

</p>

<p id="820719" class="block-content">

    As we mentioned before, a function is also an object in JS, and sometimes we

    do not notice that we are passing a function around.

</p>

<p id="820720" class="block-content">

    Take this code snippet as an example.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820248">

function buttonClicked()

{

    alert('button ' + this.id + ' was clicked');

}

var myButton = document.getElementById('someButtonID');

var myButton2 = document.getElementById('someOtherButtonID');

myButton.onclick = buttonClicked;

myButton2.onclick = buttonClicked;

</com:TTextHighlighter>

<p id="820721" class="block-content">

    Because the buttonClicked function is defined outside any object we may tend to

    think the <tt>this</tt> keyword will contain a reference to

    the <tt>window</tt> or <tt>document</tt>

    object (assuming this code is in the middle of an HTML page viewed in a browser.)

</p>

<p id="820722" class="block-content">

    But when we run this code we see that it works as intended and displays the <tt>id</tt> of

    the clicked button. What happened here is that we made the onclick method of each button contain the

    <tt>buttonClicked</tt> object reference, replacing whatever was there before. Now

    whenever the button is clicked, the browser will execute something similar to the following line.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820249">

myButton.onclick();

</com:TTextHighlighter>

<p id="820723" class="block-content">

    That isn't so confusing afterall, is it? But see what happens you start having other

    objects to deal with and you want to act on these object upon events like the button's click.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820250">

var myHelper =

{

    formFields: [ ],

    emptyAllFields: function()

    {

        for(i=0; i < this.formFields.length; i++)

        {

            var elementID = this.formFields[i];

            var field = document.getElementById(elementID);

            field.value = '';

        }

    }

};

//tell which form fields we want to work with

myHelper.formFields.push('txtName');

myHelper.formFields.push('txtEmail');

myHelper.formFields.push('txtAddress');

//clearing the text boxes:

myHelper.emptyAllFields();

var clearButton = document.getElementById('btnClear');

clearButton.onclick = myHelper.emptyAllFields;

</com:TTextHighlighter>

<p id="820724" class="block-content">

    So you think, nice, now I can click the Clear button on my page and those three text boxes

    will be emptied. Then you try clicking the button only to get a runtime error. The error

    will be related to (guess what?) the <tt>this</tt> keyword.

    The problem is that <tt>this.formFields</tt> is not defined if

    <tt>this</tt> contains a referece to the button, which is

    precisely what's happening. One quick solution would be to rewrite our last line of code.

</p>

<com:TTextHighlighter Language="javascript" CssClass="source block-content" id="code_820251">

clearButton.onclick = function()

{

    myHelper.emptyAllFields();

};

</com:TTextHighlighter>

<p id="820725" class="block-content">

    That way we create a brand new function that calls our helper method within the helper object's context.

</p>

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