Microsoft.NET

……………………………………………….Expertise in .NET Technologies

Overview of JavaScript

Posted by Ravi Varma Thumati on January 21, 2009

         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 will save 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

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

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 the body section will be executed WHILE the page loads.

JavaScripts in the head section will be executed when CALLED.

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>

 

In JavaScript we can create three kind of popup boxes: Alert box, Confirm box, and Prompt box.


Examples

Alert box

Alert box with line breaks

Confirm box

Prompt box


Alert Box

An alert box is often used if you want to make sure information comes through to the user.

When an alert box pops up, the user will have to click “OK” to proceed.

Syntax:

alert("sometext")

 


Confirm Box

A confirm box is often used if you want the user to verify or accept something.

When a confirm box pops up, the user will have to click either “OK” or “Cancel” to proceed.

If the user clicks “OK”, the box returns true. If the user clicks “Cancel”, the box returns false.

Syntax:

confirm("sometext")

 


Prompt Box

A prompt box is often used if you want the user to input a value before entering a page.

When a prompt box pops up, the user will have to click either “OK” or “Cancel” to proceed after entering an input value.

If the user clicks “OK” the box returns the input value. If the user clicks “Cancel” the box returns null.

Syntax:

prompt("sometext","defaultvalue")

 

A function is a reusable code-block that will be executed by an event, or when the function is called.


Examples

Function
How to call a function.

Function with arguments
How to pass a variable to a function, and use the variable in the function.

Function with arguments 2
How to pass variables to a function, and use these variables in the function.

Function that returns a value
How to let the function return a value.

 

How to let the function find the product of 2 arguments and return the result.


JavaScript Functions

To keep the browser from executing a script as soon as the page is loaded, you can write your script as a function.

A function contains some code that will be executed only by an event or by a call to that function.

You may call a function from anywhere within the page (or even from other pages if the function is embedded in an external .js file).

Functions are defined at the beginning of a page, in the <head> section.

Example

<html>
<head>
<script type="text/javascript">
function displaymessage()
{
alert("Hello World!")
}
</script>
</head>
<body>
<form>
<input type="button" value="Click me!"
onclick="displaymessage()" >
</form>
</body>
</html>


If the line: alert(“Hello world!!”), in the example above had not been written within a function, it would have been executed as soon as the line was loaded. Now, the script is not executed before the user hits the button. We have added an onClick event to the button that will execute the function displaymessage() when the button is clicked.

You will learn more about JavaScript events in the JS Events chapter.


How to Define a Function

The syntax for creating a function is:

function functionname(var1,var2,...,varX)
{
some code
}

var1, var2, etc are variables or values passed into the function. The { and the } defines the start and end of the function.

Note: A function with no parameters must include the parentheses () after the function name:

function functionname()
{
some code
}

Note: Do not forget about the importance of capitals in JavaScript! The word function must be written in lowercase letters, otherwise a JavaScript error occurs! Also note that you must call a function with the exact same capitals as in the function name.


The return Statement

The return statement is used to specify the value that is returned from the function.

So, functions that are going to return a value must use the return statement.

Example

The function below should return the product of two numbers (a and b):

function prod(a,b)
{
x=a*b
return x
}

When you call the function above, you must pass along two parameters:

product=prod(2,3)

The returned value from the prod() function is 6, and it will be stored in the variable called product.

 

Events are actions that can be detected by JavaScript.


Events

By using JavaScript, we have the ability to create dynamic web pages. Events are actions that can be detected by JavaScript.

Every element on a web page has certain events, which can trigger JavaScript functions. For example, we can use the onClick event of a button element to indicate that a function will run when a user clicks on the button. We define the events in the HTML tags.

Examples of events:

  • A mouse click
  • A web page or an image loading
  • Mousing over a hot spot on the web page
  • Selecting an input box in an HTML form
  • Submitting an HTML form
  • A keystroke

The following table lists the events recognized by JavaScript:

Note: Events are normally used in combination with functions, and the function will not be executed before the event occurs!

For a complete reference of the events recognized by JavaScript, go to our complete Event reference.


onload and onUnload

The onload and onUnload events are triggered when the user enters or leaves the page.

The onload event is often used to check the visitor’s browser type and browser version, and load the proper version of the web page based on the information.

Both the onload and onUnload events are also often used to deal with cookies that should be set when a user enters or leaves a page. For example, you could have a popup asking for the user’s name upon his first arrival to your page. The name is then stored in a cookie. Next time the visitor arrives at your page, you could have another popup saying something like: “Welcome John Doe!”.


onFocus, onBlur and onChange

The onFocus, onBlur and onChange events are often used in combination with validation of form fields.

Below is an example of how to use the onChange event. The checkEmail() function will be called whenever the user changes the content of the field:

<input type="text" size="30"
id="email" onchange="checkEmail()">;

 


onSubmit

The onSubmit event is used to validate ALL form fields before submitting it.

Below is an example of how to use the onSubmit event. The checkForm() function will be called when the user clicks the submit button in the form. If the field values are not accepted, the submit should be cancelled. The function checkForm() returns either true or false. If it returns true the form will be submitted, otherwise the submit will be cancelled:

<form method="post" action="xxx.htm"
onsubmit="return checkForm()">

 


onMouseOver and onMouseOut

onMouseOver and onMouseOut are often used to create “animated” buttons.

Below is an example of an onMouseOver event. An alert box appears when an onMouseOver event is detected:

<a href="http://www.w3schools.com"
onmouseover="alert('An onMouseOver event');return false">
<img src="w3schools.gif" width="100" height="30">
</a>

 

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One Response to “Overview of JavaScript”

  1. chandrika said

    Respected Sir,
    After completing my btech iam just attending the interviews especially looking for a .net job so please can u please provide technical questions on the c#.net vb.net vc#.net asp.net and sql server2005
    awaiting for reply

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