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-<com:TContent ID="body" >
-<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&lt;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&lt;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>
-</com:TContent>