Saturday, April 23, 2011

Javascript Tutorials ( Class 4 of Javascript )

JavaScript Operators

= is used to assign values.

+ is used to add values.

The assignment operator = is used to assign values to JavaScript variables.
The arithmetic operator + is used to add values together.

y=5;
z=2;
x=y+z;

The value of x, after the execution of the statements above is 7.

JavaScript Arithmetic Operators

Arithmetic operators are used to perform arithmetic between variables and/or values.
Given that y=5, the table below explains the arithmetic operators:

Operator    |    Description            |     Example       |       Result   |

+                    Addition                          x=y+2                     x=7            y=5
-                    Substraction                     x=y-2                      x=3            y=5
*                    Multiplication                   x=y*2                      x=10          y=5
/                     Division                           x=y/2                       x=2.5         y=5
%                   Moduls                           x=y%2                     x=1            y=5
++                  Increment                       x=++y                      x=6            y=6
--                   Decrement                      x=y--                        x=5            y=4

JavaScript Assignment Operators

Assignment operators are used to assign values to JavaScript variables.
Given that x=10 and y=5, the table below explains the assignment operators:

Operator |    Example   |    Same As   |   Result
=                   x=y                                       x=5
+=                  x+=y             x=x+y             x=15
-=                   x-=y              x-y                  x=5
*=                  x*=y             x=x*y               x=50
/=                   x/=y              x=x/y              x=2
%=                x%=y           x=x%y             x=0

The + Operator Used on Strings

The + operator can also be used to add string variables or text values together.
To add two or more string variables together, use the + operator.

txt1="What a very";
txt2="nice day";
txt3=txt1+txt2;

After the execution of the statements above, the variable txt3 contains "What a verynice day".
To add a space between the two strings, insert a space into one of the strings:

txt1="What a very ";
txt2="nice day";
txt3=txt1+txt2;

or insert a space into the expression:

After the execution of the statements above, the variable txt3 contains:
"What a very nice day"

Adding Strings and Numbers

The rule is: If you add a number and a string, the result will be a string!

x=5+5;
document.write(x);

x="5"+"5";
document.write(x);

x=5+"5";
document.write(x);

x="5"+5;
document.write(x);

JavaScript Comparison and Logical Operators

Comparison and Logical operators are used to test for true or false.

Comparison Operators

Comparison operators are used in logical statements to determine equality or difference between variables or values.
Given that x=5, the table below explains the comparison operators:

Operator | Descriptioin                                          |   Example
==              is equal to                                                  x==8 is false
                                                                                       x==5 is true
===             is exactly equal to (value and type           x===5 is true
                                                                                       x==="5" is false
!=                 is not equal                                                x!= 8 is true
>                  is greater than                                           x>8 is false
<                  is less than                                                 x<8 is true
>=                is greater than or equal to                        x>=8 is false
<=                is less than or equal to                              x<=8 is true

How Can it be Used

Comparison operators can be used in conditional statements to compare values and take action depending on the result:

if (age<18) document.write("Too young");

You will learn more about the use of conditional statements in the next chapter of this tutorial.

Logical Operators

Logical operators are used to determine the logic between variables or values.
Given that x=6 and y=3, the table below explains the logical operators:

Operator    |     Description        |   Example
&&                   and                        (x<10 && y>1) is true
||                        or                           (x==5 || y ==5 is false
!                        not                         !(x==y) is true

Conditional Operator

JavaScript also contains a conditional operator that assigns a value to a variable based on some condition.

Syntax

variablename=(condition)?value1:value2

Example

greeting=(visitor=="PRES")?"Dear President ":"Dear ";

If the variable visitor has the value of "PRES", then the variable greeting will be assigned the value "Dear President " else it will be assigned "Dear".

Next Class -->>

Javascript Tutorials ( Class 3 of Javascript )

JavaScript Comments

JavaScript comments can be used to make the code more readable.

JavaScript Comments
Comments can be added to explain the JavaScript, or to make the code more readable.
Single line comments start with //.
The following example uses single line comments to explain the code:

<script type="text/javascript">
// Write a heading
document.write("<h1>This is a heading</h1>");
// Write two paragraphs:
document.write("<p>This is a paragraph.</p>");
document.write("<p>This is another paragraph.</p>");
</script>

JavaScript Multi-Line Comments
Multi line comments start with /* and end with /.
The following example uses a multi line comment to explain the code:

<script type="text/javascript">
/
The code below will write
one heading and two paragraphs
/
document.write("<h1>This is a heading</h1>");
document.write("<p>This is a paragraph.</p>");
document.write("<p>This is another paragraph.</p>");
</script>

Using Comments to Prevent Execution
In the following example the comment is used to prevent the execution of a single code line (can be suitable for debugging):

<script type="text/javascript">
//document.write("<h1>This is a heading</h1>");
document.write("<p>This is a paragraph.</p>");
document.write("<p>This is another paragraph.</p>");
</script>

In the following example the comment is used to prevent the execution of a code block (can be suitable for debugging):

<script type="text/javascript">
/
document.write("<h1>This is a heading</h1>");
document.write("<p>This is a paragraph.</p>");
document.write("<p>This is another paragraph.</p>");
*/
</script>

Using Comments at the End of a Line
In the following example the comment is placed at the end of a code line:

<script type="text/javascript">
document.write("Hello"); // Write "Hello"
document.write(" Dolly!"); // Write " Dolly!"
</script>

JavaScript Variables

Variables are "containers" for storing information.

Do You Remember Algebra From School?
Do you remember algebra from school? x=5, y=6, z=x+y
Do you remember that a letter (like x) could be used to hold a value (like 5), and that you could use the information above to calculate the value of z to be 11?
These letters are called variables, and variables can be used to hold values (x=5) or expressions (z=x+y).

Javascript Variables
As with algebra, JavaScript variables are used to hold values or expressions.
A variable can have a short name, like x, or a more descriptive name, like carname.
Rules for JavaScript variable names:

Variable names are case sensitive (y and Y are two different variables)

Variable names must begin with a letter or the underscore character

Note: Because JavaScript is case-sensitive, variable names are case-sensitive.

Example
A variable's value can change during the execution of a script. You can refer to a variable by its name to display or change its value.

Declaring (Creating) JavaScript Variables
Creating variables in JavaScript is most often referred to as "declaring" variables.
You declare JavaScript variables with the var keyword:

var x;
var carname;

After the execution of the statements above, the variable x will hold the value 5, and carname will hold the value Volvo.
Note: When you assign a text value to a variable, use quotes around the value.
Note: If you redeclare a JavaScript variable, it will not lose its value.

Local JavaScript Variables
A variable declared within a JavaScript function becomes LOCAL and can only be accessed within that function. (the variable has local scope).
You can have local variables with the same name in different functions, because local variables are only recognized by the function in which they are declared.
Local variables are destroyed when you exit the function.
You will learn more about functions in a later chapter of this tutorial.

Global JavaScript Variables
Variables declared outside a function becomes GLOBAL, and all scripts and functions on the web page can access it.
Global variables are destroyed when you close the page.
If you declare a variable, without using "var", the variable always becomes GLOBAL.

Assigning Values to Undeclared JavaScript Variables
If you assign values to variables that have not yet been declared, the variables will automatically be declared as global variables.
These statements:

x=5;
carname="Volvo";

will declare the variables x and carname as global variables (if they don't already exist).

JavaScript Arithmetic
As with algebra, you can do arithmetic operations with JavaScript variables:

y=x-5;
z=y+5;

You will learn more about the operators that can be used in the next chapter of this tutorial.

Next Class -->>

Javascript Tutorials ( Class 2 of Javascript )

JavaScripts can be put in the <body> and in the <head> sections of an HTML page.

JavaScript in <body>

The example below writes the current date into an existing <p> element when the page loads:

<html>
<body>
<h1>My First Web Page</h1>
<p id="demo"></p>
<script type="text/javascript">
document.getElementById("demo").innerHTML=Date();
</script>
</body>
</html>

Note that the JavaScript is placed at the bottom of the page to make sure it is not executed before the <p> element is created.

JavaScript Functions and Events

JavaScripts in an HTML page will be executed when the page loads. This is not always what we want.
Sometimes we want to execute a JavaScript when an event occurs, such as when a user clicks a button. When this is the case we can put the script inside a function.
Events are normally used in combination with functions (like calling a function when an event occurs).
You will learn more about JavaScript functions and events in later chapters.

JavaScript in <head>

The example below calls a function when a button is clicked:

<html>
<head>
<script type="text/javascript">
function displayDate()
{
document.getElementById("demo").innerHTML=Date();
}
</script>
</head>
<body>
<h1>My First Web Page</h1>
<p id="demo"></p>
<button type="button" onclick="displayDate()">Display Date</button>
</body>
</html>

Scripts in <head> and <body>

You can place an unlimited number of scripts in your document, and you can have scripts in both the body and the head section at the same time.
It is a common practice to put all functions in the head section, or at the bottom of the page. This way they are all in one place and do not interfere with page content.

Using an External JavaScript

JavaScript can also be placed in external files.
External JavaScript files often contains code to be used on several different web pages.
External JavaScript files have the file extension .js.
Note: External script cannot contain the <script></script> tags!
To use an external script, point to the .js file in the "src" attribute of the <script> tag:

<html>
<head>
<script type="text/javascript" src="xxx.js"></script>
</head>
<body>
</body>
</html>

Note: Remember to place the script exactly where you normally would write the script!

JavaScript Statements

A JavaScript statement is a command to a browser. The purpose of the command is to tell the browser what to do.
This JavaScript statement tells the browser to write "Hello Dolly" to the web page:

document.write("Hello Dolly");

It is normal to add a semicolon at the end of each executable statement. Most people think this is a good programming practice, and most often you will see this in JavaScript examples on the web.
The semicolon is optional (according to the JavaScript standard), and the browser is supposed to interpret the end of the line as the end of the statement. Because of this you will often see examples without the semicolon at the end.
Note: Using semicolons makes it possible to write multiple statements on one line.

JavaScript Code

JavaScript code (or just JavaScript) is a sequence of JavaScript statements.
Each statement is executed by the browser in the sequence they are written.
This example will write a heading and two paragraphs to a web page:

<script type="text/javascript">
document.write("<h1>This is a heading</h1>");
document.write("<p>This is a paragraph.</p>");
document.write("<p>This is another paragraph.</p>");
</script>

JavaScript Blocks

JavaScript statements can be grouped together in blocks.
Blocks start with a left curly bracket {, and ends with a right curly bracket }.
The purpose of a block is to make the sequence of statements execute together.
This example will write a heading and two paragraphs to a web page:

<script type="text/javascript">
{
document.write("<h1>This is a heading</h1>");
document.write("<p>This is a paragraph.</p>");
document.write("<p>This is another paragraph.</p>");
}
</script>

The example above is not very useful. It just demonstrates the use of a block. Normally a block is used to group statements together in a function or in a condition (where a group of statements should be executed if a condition is met).
You will learn more about functions and conditions in later chapters.

Next Class -->>

Javascript Tutorials ( Class 1 of Javascript )

Javascript Introduction

JavaScript is the most popular scripting language on the internet, and works in all major browsers, such as Internet Explorer, Firefox, Chrome, Opera, and Safari.

What is JavaScript?

JavaScript was designed to add interactivity to HTML pages
JavaScript is a scripting language
A scripting language is a lightweight programming language
JavaScript is usually embedded directly into HTML pages
JavaScript is an interpreted language (means that scripts execute without preliminary compilation)
Everyone can use JavaScript without purchasing a license

Are Java and JavaScript the same?

NO!
Java and JavaScript are two completely different languages in both concept and design!
Java (developed by Sun Microsystems) is a powerful and much more complex programming language - in the same category as C and C++.

What can a JavaScript do?

JavaScript gives HTML designers a programming tool - HTML authors are normally not programmers, but JavaScript is a scripting language with a very simple syntax! Almost anyone can put small "snippets" of code into their HTML pages
JavaScript can put dynamic text into an HTML page - A JavaScript statement like this: document.write("<h1>" + name + "</h1>") can write a variable text into an HTML page
JavaScript can react to events - A JavaScript can be set to execute when something happens, like when a page has finished loading or when a user clicks on an HTML element
JavaScript can read and write HTML elements - A JavaScript can read and change the content of an HTML element
JavaScript can be used to validate data - A JavaScript can be used to validate form data before it is submitted to a server. This saves the server from extra processing
JavaScript can be used to detect the visitor's browser - A JavaScript can be used to detect the visitor's browser, and - depending on the browser - load another page specifically designed for that browser
JavaScript can be used to create cookies - A JavaScript can be used to store and retrieve information on the visitor's computer

JavaScript = ECMAScript

JavaScript is an implementation of the ECMAScript language standard. ECMA-262 is the official JavaScript standard.
JavaScript was invented by Brendan Eich at Netscape (with Navigator 2.0), and has appeared in all browsers since 1996.
The official standardization was adopted by the ECMA organization (an industry standardization association) in 1997.
The ECMA standard (called ECMAScript-262) was approved as an international ISO (ISO/IEC 16262) standard in 1998.
The development is still in progress.

Javascript How to

The HTML <script> tag is used to insert a JavaScript into an HTML page.

Writing to The HTML Document

The example below writes a <p> element with current date information to the HTML document:

<html>
<body>

<h1>My First Web Page</h1>

<script type="text/javascript">
document.write("<p>" + Date() + "</p>");
</script>

</body>
</html>

Note: To manipulate HTML elements JavaScript uses the DOM method getElementById(). This method access the element with the specified id.

Examples Explained

To insert a JavaScript into an HTML page, use the <script> tag.
Inside the <script> tag use the type attribute to define the scripting language.
The <script> and </script> tells where the JavaScript starts and ends:

<html>
<body>
<h1>My First Web Page</h1>

<p id="demo">This is a paragraph.</p>

<script type="text/javascript">
... some JavaScript code ...
</script></body>
</html>

The lines between the <script> and </script> contains the JavaScript and are executed by the browser.
In this case the browser will replace the content of the HTML element with id="demo", with the current date:

<html>
<body>
<h1>My First Web Page</h1>

<p id="demo">This is a paragraph.</p>

<script type="text/javascript">
document.getElementById("demo").innerHTML=Date();
</script></body>
</html>

Without the <script> tag(s), the browser will treat "document.getElementById("demo").innerHTML=Date();" as pure text and just write it to the page:

Some Browsers do Not Support JavaScript

Browsers that do not support JavaScript, will display JavaScript as page content.
To prevent them from doing this, and as a part of the JavaScript standard, the HTML comment tag should be used to "hide" the JavaScript.
Just add an HTML comment tag  (end of comment) after the last JavaScript statement, like this:

<html>
<body>
<script type="text/javascript">

</script></body>
</html>

The two forward slashes at the end of comment line (//) is the JavaScript comment symbol. This prevents JavaScript from executing the --> tag.

Next Class -->>