JavaScript

What is JavaScript ?
JavaScript is a scripting language produced by Netscape for use within HTML Web pages.
JavaScript started life as LiveScript, but Netscape changed the name, possibly because of the excitement being generated by Java.to JavaScript. JavaScript made its first appearance in Netscape 2.0 in 1995 with a name LiveScript.

JavaScript is a lightweight, interpreted programming language with object-oriented capabilities that allows you to build interactivity into otherwise static HTML pages.

The general-purpose core of the language has been embedded in Netscape, Internet Explorer, and other web browsers

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)

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

Client-side JavaScript:

Client-side JavaScript is the most common form of the language. The script should be included in or referenced by an HTML document for the code to be interpreted by the browser.

It means that a web page need no longer be static HTML, but can include programs that interact with the user, control the browser, and dynamically create HTML content.

The JavaScript client-side mechanism features many advantages over traditional CGI server-side scripts. For example, you might use JavaScript to check if the user has entered a valid e-mail address in a form field.

The JavaScript code is executed when the user submits the form, and only if all the entries are valid they would be submitted to the Web Server.

JavaScript can be used to trap user-initiated events such as button clicks, link navigation, and other actions that the user explicitly or implicitly initiates.

Advantages of JavaScript:

The merits of using JavaScript are:

· Less server interaction: You can validate user input before sending the page off to the server. This saves server traffic, which means less load on your server.

· Immediate feedback to the visitors: They don't have to wait for a page reload to see if they have forgotten to enter something.

· Increased interactivity: You can create interfaces that react when the user hovers over them with a mouse or activates them via the keyboard.

· Richer interfaces: You can use JavaScript to include such items as drag-and-drop components and sliders to give a Rich Interface to your site visitors.

Limitations with JavaScript:

We can not treat JavaScript as a full fledged programming language. It lacks the following important features:

· Client-side JavaScript does not allow the reading or writing of files. This has been kept for security reason.

· JavaScript can not be used for Networking applications because there is no such support available.

· JavaScript doesn't have any multithreading or multiprocess capabilities.

Once again, JavaScript is a lightweight, interpreted programming language that allows you to build interactivity into otherwise static HTML pages.

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

<html>

<body>

<script type="text/javascript">

document.write("Hello World!");

</script>

</body>

</html>

To insert a JavaScript into an HTML page, we use the <script> tag.

Inside the <script> tag we use the type attribute to define the scripting language.

So, the <script type="text/javascript"> and </script> tells where the JavaScript starts and ends:

The word document.write is a standard JavaScript command for writing output to a page.

By entering the document.write command between the <script> 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:

JavaScripts in the body section will be executed WHILE the page loads.

JavaScripts in the head section will be executed when CALLED.

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 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

JavaScript is a sequence of statements to be executed by the browser.

JavaScript is Case Sensitive

Unlike HTML, JavaScript is case sensitive - therefore watch your capitalization closely when you write JavaScript statements, create or call variables, objects and functions.

JavaScript Statements

A JavaScript statement is a command to the 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 header and two paragraphs to a web page:

<script type="text/javascript">

document.write("<h1>This is a header</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 header and two paragraphs to a web page:

<script type="text/javascript">

document.write("<h1>This is a header</h1>");

document.write("<p>This is a paragraph</p>");

document.write("<p>This is another paragraph</p>");

</script>

JavaScript Comments

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

Comments can be added to explain the JavaScript, or to make it more readable.

Single line comments start with //.

<script type="text/javascript">

// This will write a header:

document.write("<h1>This is a header</h1>");

// This will 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 */.

This example uses a multi line comment to explain the code:

<script type="text/javascript">

/*

The code below will write

one header and two paragraphs

*/

document.write("<h1>This is a header</h1>");

document.write("<p>This is a paragraph</p>");

document.write("<p>This is another paragraph</p>");

</script>

Using Comments to Prevent Execution

Using Comments at the End of a Line

In this example the comment is placed at the end of a line:

<script type="text/javascript">

document.write("Hello"); // This will write "Hello"

document.write("Dolly"); // This will write "Dolly"

</script>

JavaScript Variables

· Variables are "containers" for storing information.

· Do You Remember Algebra From School?

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 can declare JavaScript variables with the var statement:

var x;

var carname;

After the declaration shown above, the variables are empty (they have no values yet).

However, you can also assign values to the variables when you declare them:

var x=5;

var carname="Volvo";

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.

Assigning Values to Undeclared JavaScript Variables

If you assign values to variables that have not yet been declared, the variables will automatically be declared.

x=5;

carname="Volvo";

have the same effect as:

var x=5;

var carname="Volvo";

Redeclaring JavaScript Variables

If you redeclare a JavaScript variable, it will not lose its original value.

var x=5;

var x;

After the execution of the statements above, the variable x will still have the value of 5. The value of x is not reset (or cleared) when you redeclare it.

JavaScript Operators

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
-	Subtraction	x=y-2	x=3
*	Multiplication	x=y*2	x=10
/	Division	x=y/2	x=2.5
%	Modulus (division remainder)	x=y%2	x=1
++	Increment	x=++y	x=6
--	Decrement	x=--y	x=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=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:

txt1="What a very";

txt2="nice day";

txt3=txt1+" "+txt2;

After the execution of the statements above, the variable txt3 contains:

"What a very nice day"

Adding Strings and Numbers

x=5+5;

document.write(x);

x="5"+"5";

document.write(x);

x=5+"5";

document.write(x);

x="5"+5;

document.write(x);

If you add a number and a string, the result will be a string.

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	Description	Example
==	is equal to	x==8 is false
===	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".

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>

JavaScript Switch Statement

You should use the switch statement if you want to select one of many blocks of code to be executed.

Syntax

switch(n)

case 1:

  execute code block 1

  break;

case 2:

  execute code block 2

  break;

default:

  code to be executed if n is

  different from case 1 and 2

This is how it works: First we have a single expression n (most often a variable), that is evaluated once. The value of the expression is then compared with the values for each case in the structure. If there is a match, the block of code associated with that case is executed. Use break to prevent the code from running into the next case automatically.

Example

<script type="text/javascript">

//You will receive a different greeting based

//on what day it is. Note that Sunday=0,

//Monday=1, Tuesday=2, etc.

var d=new Date();

theDay=d.getDay();

switch (theDay)

case 5:

  document.write("Finally Friday");

  break;

case 6:

  document.write("Super Saturday");

  break;

case 0:

  document.write("Sleepy Sunday");

  break;

default:

  document.write("I'm looking forward to this weekend!");

</script>

JavaScript Loops

Very often when you write code, you want the same block of code to run over and over again in a row. Instead of adding several almost equal lines in a script we can use loops to perform a task like this.

In JavaScript there are two different kind of loops:

for - loops through a block of code a specified number of times
while - loops through a block of code while a specified condition is true

The for Loop

The for loop is used when you know in advance how many times the script should run.

Syntax

for (var=startvalue;var<=endvalue;var=var+increment)

    code to be executed

Example

Explanation: The example below defines a loop that starts with i=0. The loop will continue to run as long as i is less than, or equal to 10. i will increase by 1 each time the loop runs.

Note: The increment parameter could also be negative, and the <= could be any comparing statement.

<html>

<body>

<script type="text/javascript">

var i=0;

for (i=0;i<=10;i++)

document.write("The number is " + i);

document.write("<br />");

</script>

</body>

</html>

The while loop

The while loop is used when you want the loop to execute and continue executing while the specified condition is true.

while (var<=endvalue)

    code to be executed

Note: The <= could be any comparing statement.

Example

Explanation: The example below defines a loop that starts with i=0. The loop will continue to run as long as i is less than, or equal to 10. i will increase by 1 each time the loop runs.

<html>

<body>

<script type="text/javascript">

var i=0;

while (i<=10)

document.write("The number is " + i);

document.write("<br />");

i=i+1;

</script>

</body>

</html>

The do...while Loop

The do...while loop is a variant of the while loop. This loop will always execute a block of code ONCE, and then it will repeat the loop as long as the specified condition is true. This loop will always be executed at least once, even if the condition is false, because the code is executed before the condition is tested.

do

    code to be executed

while (var<=endvalue);

Example

<html>

<body>

<script type="text/javascript">

var i=0;

do

document.write("The number is " + i);

document.write("<br />");

i=i+1;

while (i<0);

</script>

</body>

</html>

JavaScript Break and Continue

JavaScript break and continue Statements

There are two special statements that can be used inside loops: break and continue.

Break

The break command will break the loop and continue executing the code that follows after the loop (if any).

Example

<html>

<body>

<script type="text/javascript">

var i=0;

for (i=0;i<=10;i++)

if (i==3)

break;

document.write("The number is " + i);

document.write("<br />");

</script>

</body>

</html>

Continue

The continue command will break the current loop and continue with the next value.

Example

<html>

<body>

<script type="text/javascript">

var i=0

for (i=0;i<=10;i++)

if (i==3)

continue;

document.write("The number is " + i);

document.write("<br />");

</script>

</body>

</html>

JavaScript For...In Statement

The for...in statement is used to loop (iterate) through the elements of an array or through the properties of an object.

The code in the body of the for ... in loop is executed once for each element/property.

Syntax

for (variable in object)

    code to be executed

The variable argument can be a named variable, an array element, or a property of an object.

Example

Using for...in to loop through an array:

<html>

<body>

<script type="text/javascript">

var x;

var mycars = new Array();

mycars[0] = "Saab";

mycars[1] = "Volvo";

mycars[2] = "BMW";

for (x in mycars)

document.write(mycars[x] + "<br />");

</script>

</body>

</html>

JavaScript Popup Boxes

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");

JavaScript Functions

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

JavaScript Functions

To keep the browser from executing a script when the page loads, you can put your script into a function.

A function contains code that will be executed 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 can be defined both in the <head> and in the <body> section of a document. However, to assure that the function is read/loaded by the browser before it is called, it could be wise to put it 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 put 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.

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.

JavaScript Events

Events are actions that can be detected by JavaScript.

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.

JavaScript's interaction with HTML is handled through events that occur when the user or browser manipulates a page.

When the page loads, that is an event. When the user clicks a button, that click, too, is an event. Another example of events are like pressing any key, closing window, resizing window etc.

Developers can use these events to execute JavaScript coded responses, which cause buttons to close windows, messages to be displayed to users, data to be validated, and virtually any other type of response imaginable to occur.

Events are a part of the Document Object Model (DOM) Level 3 and every HTML element have a certain set of events which can trigger JavaScript Code.

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

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

onclick Event Type:

This is the most frequently used event type which occurs when a user clicks mouse eft button. You can put your validation, warning etc against this event type.

Example:

<html>

<head>

<script type="text/javascript">

<!--

function sayHello() {

   alert("Hello World")

//-->

</script>

</head>

<body>

<input type="button" onclick="sayHello()" value="Say Hello" />

</body>

</html>

onsubmit event type:

Another most important event type is onsubmit. This event occurs when you try to submit a form. So you can put your form validation against this event type.

Here is simple example showing its usage. Here we are calling a validate() function before submitting a form data to the webserver. If validate() function returns true the form will be submitted otherwise it will not submit the data.

Example:

<html>

<head>

<script type="text/javascript">

<!--

function validation() {

   all validation goes here

   .........

   return either true or false

//-->

</script>

</head>

<body>

<form method="POST" action="t.cgi" onsubmit="return validate()">

.......

<input type="submit" value="Submit" />

</form>

</body>

</html>

onmouseover and onmouseout:

These two event types will help you to create nice effects with images or even with text as well. The onmouseover event occurs when you bring your mouse over any element and the onmouseout occurs when you take your mouse out from that element.

Example:

Following example shows how a division reacts when we bring our mouse in that division:

<html>

<head>

<script type="text/javascript">

<!--

function over() {

   alert("Mouse Over");

function out() {

   alert("Mouse Out");

//-->

</script>

</head>

<body>

<div onmouseover="over()" onmouseout="out()">

<h2> This is inside the division </h2>

</div>

</body>

</html>

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()">

javaScript Try...Catch Statement

JavaScript - Catching Errors

When browsing Web pages on the internet, we all have seen a JavaScript alert box telling us there is a runtime error and asking "Do you wish to debug?". Error message like this may be useful for developers but not for users. When users see errors, they often leave the Web page.

This chapter will teach you how to trap and handle JavaScript error messages, so you don't lose your audience.

There are two ways of catching errors in a Web page:

By using the try...catch statement (available in IE5+, Mozilla 1.0, and Netscape 6)
By using the onerror event. This is the old standard solution to catch errors (available since Netscape 3)

Try...Catch Statement

The try...catch statement allows you to test a block of code for errors. The try block contains the code to be run, and the catch block contains the code to be executed if an error occurs.

Syntax

try


  
          


       Note that try...catch is written in lowercase letters. Using uppercase letters will generate a JavaScript error!       
    

  //Run some code here

catch(err)

//Handle errors here

Example 1

The example below contains a script that is supposed to display the message "Welcome guest!" when you click on a button. However, there's a typo in the message() function. alert() is misspelled as adddlert(). A JavaScript error occurs:

<html>

<head>

<script type="text/javascript">

function message()

alert("Welcome guest!");

</script>

</head>

<body>

<input type="button" value="View message" onclick="message()" />

</body>

</html>

To take more appropriate action when an error occurs, you can add a try...catch statement.

The example below contains the "Welcome guest!" example rewritten to use the try...catch statement. Since alert() is misspelled, a JavaScript error occurs. However, this time, the catch block catches the error and executes a custom code to handle it. The code displays a custom error message informing the user what happened:

<html>

<head>

<script type="text/javascript">

var txt=""

function message()

try

  adddlert("Welcome guest!");

catch(err)

  txt="There was an error on this page.\n\n";

  txt+="Error description: " + err.description + "\n\n";

  txt+="Click OK to continue.\n\n";

  alert(txt);

</script>

</head>

<body>

<input type="button" value="View message" onclick="message()" />

</body>

</html>

Example 2

The next example uses a confirm box to display a custom message telling users they can click OK to continue viewing the page or click Cancel to go to the homepage. If the confirm method returns false, the user clicked Cancel, and the code redirects the user. If the confirm method returns true, the code does nothing:

<html>

<head>

<script type="text/javascript">

var txt=""

function message()

try

  adddlert("Welcome guest!");

catch(err)

  txt="There was an error on this page.\n\n";

  txt+="Click OK to continue viewing this page,\n";

  txt+="or Cancel to return to the home page.\n\n";

  if(!confirm(txt))

    document.location.href="http://www.w3schools.com/";

    }   }

</script>

</head>

<body>

<input type="button" value="View message" onclick="message()" />

</body>

</html>

The Throw Statement

The throw statement allows you to create an exception. If you use this statement together with the try...catch statement, you can control program flow and generate accurate error messages.

Syntax

throw(exception)

The exception can be a string, integer, Boolean or an object.

Note that throw is written in lowercase letters. Using uppercase letters will generate a JavaScript error!

Example 1

The example below determines the value of a variable called x. If the value of x is higher than 10 or lower than 0 we are going to throw an error. The error is then caught by the catch argument and the proper error message is displayed:

<html>

<body>

<script type="text/javascript">

var x=prompt("Enter a number between 0 and 10:","");

try

if(x>10)

throw "Err1";

else if(x<0)

throw "Err2";

catch(er)

if(er=="Err1")

alert("Error! The value is too high");

if(er == "Err2")

alert("Error! The value is too low");

</script>

</body>

</html>

Insert Special Characters

The backslash (\) is used to insert apostrophes, new lines, quotes, and other special characters into a text string.

Look at the following JavaScript code:

var txt="We are the so-called "Vikings" from the north.";

document.write(txt);

In JavaScript, a string is started and stopped with either single or double quotes. This means that the string above will be chopped to: We are the so-called

To solve this problem, you must place a backslash (\) before each double quote in "Viking". This turns each double quote into a string literal:

var txt="We are the so-called \"Vikings\" from the north.";

document.write(txt);

JavaScript will now output the proper text string: We are the so-called "Vikings" from the north.

Here is another example:

document.write ("You \& I are singing!");

The example above will produce the following output:

You & I are singing!

The table below lists other special characters that can be added to a text string with the backslash sign:

Code	Outputs
\'	single quote
\"	double quote
\&	ampersand
\\	backslash
\n	new line
\r	carriage return
\t	tab
\b	backspace
\f	form feed

JavaScript Objects Introduction

· JavaScript is an Object Oriented Programming (OOP) language.

· An OOP language allows you to define your own objects and make your own variable types.

Object Oriented Programming

JavaScript is an Object Oriented Programming (OOP) language. An OOP language allows you to define your own objects and make your own variable types.

Note that an object is just a special kind of data. An object has properties and methods.

Properties

Properties are the values associated with an object.

Methods

Methods are the actions that can be performed on objects.

In the following example we are using the toUpperCase() method of the String object to display a text in uppercase letters:

<script type="text/javascript">

var str="Hello world!";

document.write(str.toUpperCase());

</script>

The output of the code above will be:

HELLO WORLD!

String object

The String object is used to manipulate a stored piece of text.

EXAMPLE 1:

<html>

<body>

var txt="Hello World!";

document.write(txt.length);

</script>

</body>

</html>

JavaScript Date Object

The Date object is used to work with dates and times.

Set Dates

We can easily manipulate the date by using the methods available for the Date object.

In the example below we set a Date object to a specific date (14th January 2010):

var myDate=new Date();

myDate.setFullYear(2010,0,14);

And in the following example we set a Date object to be 5 days into the future:

var myDate=new Date();

myDate.setDate(myDate.getDate()+5);

Note: If adding five days to a date shifts the month or year, the changes are handled automatically by the Date object itself!

Compare Two Dates

The Date object is also used to compare two dates.

The following example compares today's date with the 14th January 2010:

var myDate=new Date();

myDate.setFullYear(2010,0,14);

var today = new Date();

if (myDate>today)

alert("Today is before 14th January 2010");

else

alert("Today is after 14th January 2010");

JavaScript Array Object

The Array object is used to store multiple values in a single variable.

Create an Array

var   myCars=new Array();

There are two ways of adding values to an array (you can add as many values as you need to define as many variables you require).

var myCars=new Array();

myCars[0]="Saab";

myCars[1]="Volvo";

myCars[2]="BMW";

You could also pass an integer argument to control the array's size:

var myCars=new Array(3);

myCars[0]="Saab";

myCars[1]="Volvo";

myCars[2]="BMW";

var myCars=new Array("Saab","Volvo","BMW");

Note: If you specify numbers or true/false values inside the array then the type of variables will be numeric or Boolean instead of string.

Access an Array

You can refer to a particular element in an array by referring to the name of the array and the index number. The index number starts at 0.

The following code line:

document.write(myCars[0]);

will result in the following output:

Saab

EXAMPLE

<html>

<body>

var x;

var mycars = new Array();

mycars[0] = "Saab";

mycars[1] = "Volvo";

mycars[2] = "BMW";

for (x in mycars)

{

document.write(mycars[x] + "<br />");

}

</script>

</body>

</html>

JavaScript Boolean Object

The Boolean object is used to convert a non-Boolean value to a Boolean value (true or false).

Create a Boolean Object

The Boolean object represents two values: "true" or "false".

The following code creates a Boolean object called myBoolean:

var myBoolean=new Boolean();

Note: If the Boolean object has no initial value or if it is 0, -0, null, "", false, undefined, or NaN, the object is set to false. Otherwise it is true (even with the string "false")!

All the following lines of code create Boolean objects with an initial value of false:

var myBoolean=new Boolean();

var myBoolean=new Boolean(0);

var myBoolean=new Boolean(null);

var myBoolean=new Boolean("");

var myBoolean=new Boolean(false);

var myBoolean=new Boolean(NaN);

And all the following lines of code create Boolean objects with an initial value of true:

var myBoolean=new Boolean(true);

var myBoolean=new Boolean("true");

var myBoolean=new Boolean("false");

var myBoolean=new Boolean("Richard");

JavaScript Math Object

Math Object

The Math object allows you to perform mathematical tasks.

The Math object includes several mathematical constants and methods.

Syntax for using properties/methods of Math:

var pi_value=Math.PI;

var sqrt_value=Math.sqrt(16);

Note: Math is not a constructor. All properties and methods of Math can be called by using Math as an object without creating it.

Mathematical Constants

JavaScript provides eight mathematical constants that can be accessed from the Math object. These are: E, PI, square root of 2, square root of 1/2, natural log of 2, natural log of 10, base-2 log of E, and base-10 log of E.

You may reference these constants from your JavaScript like this:

Math.E

Math.PI

Math.SQRT2

Math.SQRT1_2

Math.LN2

Math.LN10

Math.LOG2E

Math.LOG10E

Mathematical Methods

In addition to the mathematical constants that can be accessed from the Math object there are also several methods available.

The following example uses the round() method of the Math object to round a number to the nearest integer:

document.write(Math.round(4.7));

output: 5

The following example uses the random() method of the Math object to return a random number between 0 and 1:

document.write(Math.random());

The code above can result in the following output:

0.571332362441485

The following example uses the floor() and random() methods of the Math object to return a random number between 0 and 10:

document.write(Math.floor(Math.random()*11));    output is 0

JavaScript Form Validation

JavaScript can be used to validate input data in HTML forms before sending off the content to a server.

JavaScript can be used to validate input data in HTML forms before sending off the content to a server.Form data that typically are checked by a JavaScript could be:

has the user left required fields empty?
has the user entered a valid e-mail address?
has the user entered a valid date?
has the user entered text in a numeric field?

Required Fields

The function below checks if a required field has been left empty. If the required field is blank, an alert box alerts a message and the function returns false. If a value is entered, the function returns true (means that data is OK):

function validate_required(field,alerttxt)

with (field)

  if (value==null||value=="")

  alert(alerttxt);return false;

  else

  return true;

  }  }   }

The entire script, with the HTML form could look something like this:

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

function validate_required(field,alerttxt)

with (field)

if (value==null||value=="")

  {alert(alerttxt);return false;}

else {return true}

function validate_form(thisform)

with (thisform)

if (validate_required(email,"Email must be filled out!")==false)

  {email.focus();return false;}

</script>

</head>

<body>

<form action="submitpage.htm"

onsubmit="return validate_form(this)"

method="post">

Email: <input type="text" name="email" size="30">

<input type="submit" value="Submit">

</form>

</body>

</html>

E-mail Validation

The function below checks if the content has the general syntax of an email.This means that the input data must contain at least an @ sign and a dot (.). Also, the @ must not be the first character of the email address, and the last dot must at least be one character after the @ sign:

EXAMPLE

<html>

<head>

<script type="text/javascript">

function validate_email(field,alerttxt)

with (field)

apos=value.indexOf("@");

dotpos=value.lastIndexOf(".");

if (apos<1||dotpos-apos<2)

  {alert(alerttxt);return false;}

else {return true;}

function validate_form(thisform)

with (thisform)

if (validate_email(email,"Not a valid e-mail address!")==false)

  {email.focus();return false;}

</script>

</head>

<body>

<form action="submitpage.htm"

onsubmit="return validate_form(this);"

method="post">

Email: <input type="text" name="email" size="30">

<input type="submit" value="Submit">

</form>

</body>

</html>

What are Cookies ?

Web Browser and Server use HTTP protocol to communicate and HTTP is a stateless protocol. But for a commercial website it is required to maintain session information among different pages. For example one user registration ends after completing many pages. But how to maintain user's session information across all the web pages.

In many situations, using cookies is the most efficient method of remembering and tracking preferences, purchases, commissions, and other information required for better visitor experience or site statistics.

How It Works ?

Your server sends some data to the visitor's browser in the form of a cookie. The browser may accept the cookie. If it does, it is stored as a plain text record on the visitor's hard drive. Now, when the visitor arrives at another page on your site, the browser sends the same cookie to the server for retrieval. Once retrieved, your server knows/remembers what was stored earlier.

Cookies are a plain text data record of 5 variable-length fields:

· Expires : The date the cookie will expire. If this is blank, the cookie will expire when the visitor quits the browser.

· Domain : The domain name of your site.

· Path : The path to the directory or web page that set the cookie. This may be blank if you want to retrieve the cookie from any directory or page.

· Secure : If this field contains the word "secure" then the cookie may only be retrieved with a secure server. If this field is blank, no such restriction exists.

· Name=Value : Cookies are set and retrieved in the form of key and value pairs.

Cookies were originally designed for CGI programming and cookies' data is automatically transmitted between the web browser and web server, so CGI scripts on the server can read and write cookie values that are stored on the client.

JavaScript can also manipulate cookies using the cookie property of the Document object. JavaScript can read, create, modify, and delete the cookie or cookies that apply to the current web page.

Storing Cookies:

The simplest way to create a cookie is to assign a string value to the document.cookie object, which looks like this:

Syntax:

document.cookie = "key1=value1;key2=value2;expires=date";

Here expires attribute is option. If you provide this attribute with a valid date or time then cookie will expire at the given date or time and after that cookies' value will not be accessible.

Note: Cookie values may not include semicolons, commas, or whitespace. For this reason, you may want to use the JavaScript escape() function to encode the value before storing it in the cookie. If you do this, you will also have to use the corresponding unescape() function when you read the cookie value.

Example:

Following is the example to set a customer name in input cookie.

<html>

<head>

<script type="text/javascript">

<!--

function WriteCookie()

   if( document.myform.customer.value == "" ){

      alert("Enter some value!");

      return;

   cookievalue= escape(document.myform.customer.value) + ";";

   document.cookie="name=" + cookievalue;

   alert("Setting Cookies : " + "name=" + cookievalue );

//-->

</script>

</head>

<body>

<form name="myform" action="">

Enter name: <input type="text" name="customer"/>

<input type="button" value="Set Cookie" onclick="WriteCookie();"/>

</form>

</body>

</html>

What is page redirection

When you click a URL to reach to a page X but internally you are directed to another page Y that simply happens because of page re-direction.

There could be various reasons why you would like to redirect from original page. I'm listing down few of the reasons:

· You did not like the name of your domain and you are moving to a new one. Same time you want to direct your all visitors to new site. In such case you can maintain your old domain but put a single page with a page re-direction so that your all old domain visitors can come to your new domain.

· You have build-up various pages based on browser versions or their names or may be based on different countries, then instead of using your server side page redirection you can use client side page redirection to land your users on appropriate page.

· The Search Engines may have already indexed your pages. But while moving to another domain then you would not like to lose your visitors coming through search engines. So you can use client side page redirection. But keep in mind this should not be done to make search engine a fool otherwise this could get your web site banned.

How Page Re-direction works ?

This is very simple to do a page redirect using JavaScript at client side. To redirect your site visitors to a new page, you just need to add a line in your head section as follows:

<head>

<script type="text/javascript">

<!--

   window.location="http://www.newlocation.com";

//-->

</script>

</head>

Example 2:

You can show an appropriate message to your site visitors before redirecting them to a new page. This would need a bit time delay to load a new page. Following is the simple example to implement the same:

<head>

<script type="text/javascript">

<!--

function Redirect()

    window.location="http://www.newlocation.com";

document.write("You will be redirected to main page in 10 sec.");

setTimeout('Redirect()', 10000);

//-->

</script>

</head>

Pages

Sunday 15 July 2012

Java script meterial

JavaScript

Are Java and JavaScript the Same?

What can a JavaScript Do?

Client-side JavaScript:

Advantages of JavaScript:

Limitations with JavaScript:

Where to Put the JavaScript

Using an External JavaScript

JavaScript Statements

JavaScript is Case Sensitive

JavaScript Statements

JavaScript Code

JavaScript Blocks

JavaScript Comments

JavaScript Multi-Line Comments

Using Comments to Prevent Execution

Using Comments at the End of a Line

JavaScript Variables

· Do You Remember Algebra From School?

Example

Declaring (Creating) JavaScript Variables

Assigning Values to Undeclared JavaScript Variables

Redeclaring JavaScript Variables

JavaScript Operators

JavaScript Arithmetic Operators

JavaScript Assignment Operators

The + Operator Used on Strings

Adding Strings and Numbers

JavaScript Comparison and Logical Operators

Comparison Operators

How Can it be Used

Logical Operators

Conditional Operator

Syntax

Example

Conditional Statements

If Statement

Syntax

Example 2

If...else Statement

Example

If...else if...else Statement

Syntax

JavaScript Switch Statement

Syntax

Example

JavaScript Loops

The for Loop

The while loop

The do...while Loop

JavaScript Break and Continue

JavaScript break and continue Statements

Break

Continue

JavaScript For...In Statement

JavaScript For...In Statement

Example

JavaScript Popup Boxes

Alert Box

Confirm Box

Prompt Box

JavaScript Functions

Example

How to Define a Function

The return Statement

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

JavaScript Events

onclick Event Type:

Example:

onsubmit event type:

Example:

onmouseover and onmouseout:

Example:

onload and onUnload

onFocus, onBlur and onChange

onSubmit

JavaScript - Catching Errors