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Know about JavaScript by Parimal Sudas, Assistant Professor, Dronacharya College of Engineering, Gurgaon

Know about JavaScript

By : Parimal Sudas, Assistant Professor, Dronacharya College of Engineering, Gurgaon
Industry : Teaching/Education Functional Area : Open Source
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Know

JAVASCRIPT

Activity:  6 comments  368 views  last activity : 07 06 2010 20:18:04 +0000
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JavaScript JavaScript is used in millions of Web pages to improve the design, validate forms, detect browsers, create cookies, and much more.JavaScript is the most popular scripting language on the internet, and works in all major browsers, such as Internet Explorer, Mozilla, Firefox, Netscape, and Opera.

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)
  • A JavaScript consists of lines of executable computer code
  • A 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

How to Put a JavaScript Into an HTML Page

<html>
<body>
<script type="text/javascript">
document.write("Hello World!")
</script>
</body>
</html>
The code above will produce this output on an HTML page:
Hello World!

Example Explained

To insert a JavaScript into an HTML page, we use the <script> tag (also use the type attribute to define the scripting language).So, the <script type="text/javascript"> and </script> tells where the JavaScript starts and ends: 
<html>
<body>
<script type="text/javascript">
...
</script>
</body>
</html>
The word document.write is a standard JavaScript command for writing output to a page.By entering the document.write command between the <script type="text/javascript"> and </script> tags, the browser will recognize it as a JavaScript command and execute the code line. In this case the browser will write Hello World! to the page: 
<html>
<body>
<script type="text/javascript">
document.write("Hello World!")
</script>
</body>
</html>
Note: If we had not entered the <script> tag, the browser would have treated the document.write("Hello World!") command as pure text, and just write the entire line on the page.

Ending Statements With a Semicolon?

With traditional programming languages, like C++ and Java, each code statement has to end with a semicolon.Many programmers continue this habit when writing JavaScript, but in general, semicolons are optional! However, semicolons are required if you want to put more than one statement on a single line.

How to Handle Older Browsers

Browsers that do not support JavaScript will display the script as page content. To prevent them from doing this, we may use the HTML comment tag: 
<script type="text/javascript">
<!--
document.write("Hello World!")
//-->
</script>
The two forward slashes at the end of comment line (//) are a JavaScript comment symbol. This prevents the JavaScript compiler from compiling the line.

Where to Put the JavaScript

JavaScripts in a page will be executed immediately while the page loads into the browser. This is not always what we want. Sometimes we want to execute a script when a page loads, other times when a user triggers an event.Scripts in the head section: Scripts to be executed when they are called, or when an event is triggered, go in the head section. When you place a script in the head section, you will ensure that the script is loaded before anyone uses it.  
<html>
<head>
<script type="text/javascript">
....
</script>
</head>
Scripts in the body section: Scripts to be executed when the page loads go in the body section. When you place a script in the body section it generates the content of the page. 
<html>
<head>
</head>
<body>
<script type="text/javascript">
....
</script>
</body>
Scripts in both the body and the head section: You can place an unlimited number of scripts in your document, so you can have scripts in both the body and the head section. 
<html>
<head>
<script type="text/javascript">
....
</script>
</head>
<body>
<script type="text/javascript">
....
</script>
</body>
 

Using an External JavaScript

Sometimes you might want to run the same JavaScript on several pages, without having to write the same script on every page.To simplify this, you can write a JavaScript in an external file. Save the external JavaScript file with a .js file extension.Note: The external script cannot contain the <script> tag!To use the external script, point to the .js file in the "src" attribute of the <script> tag: 
<html>
<head>
<script src="xxx.js"></script>
</head>
<body>
</body>
</html>
Note: Remember to place the script exactly where you normally would write the script!

Variables

A variable is a "container" for information you want to store. A variable's value can change during the script. You can refer to a variable by name to see its value or to change its value.Rules for variable names:
  • Variable names are case sensitive
  • They must begin with a letter or the underscore character
IMPORTANT! JavaScript is case-sensitive! A variable named strname is not the same as a variable named STRNAME!

Declare a Variable

You can create a variable with the var statement: 
var strname = some value
You can also create a variable without the var statement: 
strname = some value
 

Assign a Value to a Variable

You can assign a value to a variable like this: 
var strname = "Hege"
Or like this: 
strname = "Hege"
The variable name is on the left side of the expression and the value you want to assign to the variable is on the right. Now the variable "strname" has the value "Hege".

Lifetime of Variables

When you declare a variable within a function, the variable can only be accessed within that function. When you exit the function, the variable is destroyed. These variables are called local variables. You can have local variables with the same name in different functions, because each is recognized only by the function in which it is declared.If you declare a variable outside a function, all the functions on your page can access it. The lifetime of these variables starts when they are declared, and ends when the page is closed.

Conditional Statements

Very often when you write code, you want to perform different actions for different decisions. You can use conditional statements in your code to do this.In JavaScript we have the following conditional statements:
  • if statement - use this statement if you want to execute some code only if a specified condition is true
  • if...else statement - use this statement if you want to execute some code if the condition is true and another code if the condition is false
  • if...else if....else statement - use this statement if you want to select one of many blocks of code to be executed
  • switch statement - use this statement if you want to select one of many blocks of code to be executed

If Statement

You should use the if statement if you want to execute some code only if a specified condition is true.

Syntax

if (condition)
{
code to be executed if condition is true
}
Note that if is written in lowercase letters. Using uppercase letters (IF) will generate a JavaScript error!

Example 1

<script type="text/javascript">
//Write a "Good morning" greeting if
//the time is less than 10
var d=new Date()
var time=d.getHours()
 
if (time<10) 
{
document.write("<b>Good morning</b>")
}
</script>

Example 2

<script type="text/javascript">
//Write "Lunch-time!" if the time is 11
var d=new Date()
var time=d.getHours()
 
if (time==11) 
{
document.write("<b>Lunch-time!</b>")
}
</script>
Note: When comparing variables you must always use two equals signs next to each other (==)!Notice that there is no ..else.. in this syntax. You just tell the code to execute some code only if the specified condition is true.

If...else Statement

If you want to execute some code if a condition is true and another code if the condition is not true, use the if....else statement.

Syntax

if (condition)
{
code to be executed if condition is true
}
else
{
code to be executed if condition is not true
}

Example

<script type="text/javascript">
//If the time is less than 10,
//you will get a "Good morning" greeting.
//Otherwise you will get a "Good day" greeting.
var d = new Date()
var time = d.getHours()
 
if (time < 10) 
{
document.write("Good morning!")
}
else
{
document.write("Good day!")
}
</script>
 

If...else if...else Statement

You should use the if....else if...else statement if you want to select one of many sets of lines to execute.

Syntax

if (condition1)
{
code to be executed if condition1 is true
}
else if (condition2)
{
code to be executed if condition2 is true
}
else
{
code to be executed if condition1 and
condition2 are not true
}

Example

<script type="text/javascript">
var d = new Date()
var time = d.getHours()
if (time<10)
{
document.write("<b>Good morning</b>")
}
else if (time>10 && time<16)
{
document.write("<b>Good day</b>")
}
else
{
document.write("<b>Hello World!</b>")
}
</script>
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6 comments on "Know about JavaScript"
Sort by: Most Recent Top Rated
  Commented by  Viktor Stephen, COO, Business Mashup/Partner Get.Next.Job    | 04 02 2009 10:33:26 +0000 
Nice article. Would appreciate if you could write about using JavaScript to write widgets. Thanks.
  Commented by  Prashant Gajjar, QA Engineer, Cybage Software Pvt Ltd.    | 09 03 2008 02:27:18 +0000 
good information.
  Commented by  Saket Vishal, Software Developer, Xebia    | 09 03 2008 01:11:44 +0000 
If you have written this article, good work, else just post the link. I would really appreciate if you can write up something on Object Orienting JavaScript.
  Commented by  Darpan Sinha, Solution Architect, Fujitsu Consulting India Pvt Ltd    | 09 02 2008 00:24:23 +0000 
Good Basics of JavaScript for starters
  Commented by  Samir Nigam, Sr. Software Engineer , SRM TECHSOL Pvt. Ltd.    | 07 27 2008 22:41:15 +0000 
nice. but basic information.
  Commented by  Anurag Mitra, Product Marketing Manager (Asia Pacific)    | 05 24 2008 10:25:16 +0000 
good tutorial introduction
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