Java script questions

1. 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 is lines of executable computer code
• A JavaScript is usually embedded directly in HTML pages
• JavaScript is an interpreted language (means that scripts execute without
preliminary compilation)
• Everyone can use JavaScript without purchasing a license
• JavaScript is supported by all major browsers, like Netscape and Internet Explorer

2. 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!

3. Where to Put the JavaScript?

Scripts 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>
some statements
</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>
some statements
</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>
some statements
</script>
</head>
<body>
some statements
</script>

</body>

4. Define 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

5. How to 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

6. How to Assign a Value to a Variable?

You 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".

7. Explain Arithmetic Operators

Operator Description Example Result
+ Addition x=2 4
x+2
- Subtraction x=2 3
5-x
* Multiplication x=4 20
x*5
/ Division 15/5 3
5/2 2.5
% Modulus (division remainder) 5%2 1
10%8 2
10%2 0
++ Increment x=5 x=6
x++
-- Decrement x=5 X=4
x--

8. What are the Assignment Operators available in Javascript?

Operator Example Is The Same As
= x=y x=y

+= x+=y x=x+y
-= x-=y x=x-y
*= x*=y x=x*y
/= x/=y x=x/y
%= x%=y x=x%y

9. Comparison Operators in Javascript.

Operator Description Example
== is equal to 5==8 returns false
!= is not equal 5!=8 returns true
> is greater than 5>8 returns false
< is less than 5<8 returns true
>= is greater than or equal to 5>=8 returns false
<= is less than or equal to 5<=8 returns true

10. Logical Operators in javascript.

&& and x=6
y=3

(x < 10 && y > 1) returns true
|| or x=6
y=3

(x==5 || y==5) returns false
! not x=6
y=3

!(x==y) returns true

11. String Operator in Javascript.

A string is most often text, for example "Hello World!". To stick two or more string
variables together, use the + operator.

txt1="What a very"
txt2="nice day!"

txt3=txt1+txt2

The variable txt3 now contains "What a verynice day!".

To add a space between two string variables, insert a space into the expression, OR in one
of the strings.

txt1="What a very"
txt2="nice day!"
txt3=txt1+" "+txt2
or
txt1="What a very "
txt2="nice day!"
txt3=txt1+txt2

The variable txt3 now contains "What a very nice day!".

12. What is Functions?

A function contains some code that will be executed by an event or a call to that
function. A function is a set of statements. You can reuse functions within the same
script, or in other documents. You define functions at the beginning of a file (in the head
section), and call them later in the document. It is now time to take a lesson about the
alert-box:

This is JavaScript's method to alert the user.

Alert("This is a message")

13. How to Define a Function

To create a function you define its name, any values ("arguments"), and some statements:

function myfunction(argument1,argument2,etc)
{
some statements
}

A function with no arguments must include the parentheses:

function myfunction()
{
some statements
}

Arguments are variables used in the function. The variable values are values passed on by
the function call.

By placing functions in the head section of the document, you make sure that all the code
in the function has been loaded before the function is called.

Some functions return a value to the calling expression

function result(a,b)
{
c=a+b
return c
}

14. How to Call a Function?

A function is not executed before it is called.

You can call a function containing arguments:

myfunction(argument1,argument2,etc)

or without arguments:

myfunction()

The return Statement

Functions that will return a result must use the "return" statement. This statement
specifies the value which will be returned to where the function was called from. Say you
have a function that returns the sum of two numbers:

function total(a,b)
{
result=a+b
return result
}

When you call this function you must send two arguments with it:

sum=total(2,3)

The returned value from the function (5) will be stored in the variable called sum.

15. What is DHTML?

DHTML stands for Dynamic HTML. When implemented in full it allows
complete control of all the structures and contents of a Web page by exposing every part
of the Document Object Model (DOM) to control by JavaScript. Thus the position,
visibility, color, size, content, etc of every aspect of a web page can be dynamically
controlled.

16. List the three main components of Dynamic HTML.

1. Positioning: precisely placing blocks of content on the page and, if desired,
moving these blocks around (strictly speaking, a subset of style modifications).
2. Style modifications: on-the-fly altering the aesthetics of content on the page.
3. Event handling: how to relate user events to changes in positioning or other style
modifications.

17. Write short notes on layers.
In general, "layer" refers to elements that can be positioned at exact coordinates
on the page. These elements can be defined with the DIV, SPAN, LAYER, or ILAYER
tags. Layers created with DIV and SPAN are referred to as CSS layers because their
properties are defined by the Cascading Style Sheets specification by the World Wide
web Consortium. This specification defines style properties (e.g. font, color, padding,
margin, word-spacing) in addition to the positioning properties associated with layers
(top, left, z-index, visibility).

18. How DHTML Does it Work?

Various elements of a web page such as images and blocks of text are organized into
groups with the <div> or <layer> tags. The groups are then given a list of properties
using the cascading style sheet specification and a name to distinguish them from each
other. Then through the use of a scripting language, you can dynamically change the CSS
attribute of each group. It's really quite neat how the three elements of web design can
come together in a dynamic union.

19. What DHTML Can Do ?

DHTML gives you ability to position elements of a web page to precise (x,y,z)
coordinates and dynamically change the position with script. Every property of a web
page element can be altered with the use of a script language. Some properties include
color, size, visibility, alignment, etc.. You can achieve some awesome effects with
DHTML.

20. Write About The Technology Components Of DHTML.
The major components of Dynamic HTML technology are:-

• Style Sheets (NS) (MS) let you specify the stylistic attributes of the typographic
elements of your web page. They let you change the color, size, or style of the
text on a page without waiting for the screen to refresh.
• Content Positioning (NS) (MS) lets a web developer animate any element on a
web page, moving pictures, text, and objects at will. It lets you ensure that pieces
of content are displayed on the page exactly where you want them to appear, and
you can modify their appearance and location after the page has been displayed.
• Dynamic Content (MS) actually changes the words, pictures, or multimedia on a
page without another trip to the web Server.
• Data Binding (MS) lets you get all the information you need to ask questions,
change elements, and get results without going back to the web server.
• Downloadable Fonts (NS) let you use the fonts of your choice to enhance the
appearance of your text. Then you can package the fonts with the page so that the
text is always displayed with your chosen fonts.

Introduction to 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, 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

How to Put a JavaScript Into an HTML Page
<html>
<body>
</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>
</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>
</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>

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>
</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>
</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>
</head>
<body>
....
</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.

JavaScript If...Else Statements
Conditional statements in JavaScript are used to perform different actions based on
different conditions.

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
//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
//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
//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
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>
Conditional statements in JavaScript are used to perform different actions based on
different conditions.

The 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
//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 Operators
Arithmetic Operators

Operator Description Example Result
+ Addition x=2 4
y=2
x+y
- Subtraction x=5 3
y=2
x-y
* Multiplication x=5 20
y=4
x*y
/ Division 15/5 3
5/2 2.5
% Modulus (division remainder) 5%2 1
10%8 2
10%2 0
++ Increment x=5 x=6
x++
-- Decrement x=5 x=4
x--
Assignment Operators
Operator Example Is The Same As
= x=y x=y
+= x+=y x=x+y
-= x-=y x=x-y
*= x*=y x=x*y
/= x/=y x=x/y
%= x%=y x=x%y
Comparison Operators
== is equal to 5==8 returns false
=== is equal to (checks for both value and x=5
type) y="5"

x==y returns true
x===y returns false
!= is not equal 5!=8 returns true
> is greater than 5>8 returns false
< is less than 5<8 returns true
>= is greater than or equal to 5>=8 returns false
<= is less than or equal to 5<=8 returns true
Logical Operators
&& and x=6

y=3

(x < 10 && y > 1) returns true
|| or x=6
y=3

(x==5 || y==5) returns false
! not x=6
y=3

!(x==y) returns true
String Operator

A string is most often text, for example "Hello World!". To stick two or more string
variables together, use the + operator.

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

The variable txt3 now contains "What a verynice day!".

To add a space between two string variables, insert a space into the expression, OR in one
of the strings.

txt1="What a very"
txt2="nice day!"
txt3=txt1+" "+txt2
or
txt1="What a very "
txt2="nice day!"
txt3=txt1+txt2

The variable txt3 now contains "What a very nice day!".

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 is equal to PRES, then put the string "Dear President " in the
variable named greeting. If the variable visitor is not equal to PRES, then put the string
"Dear " into the variable named greeting.

JavaScript Popup Boxes
In JavaScript we can create three kind of popup boxes: Alert box, Confirm box, and
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")

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 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>
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.

JavaScript For Loop
Loops in JavaScript are used to execute the same block of code a specified number
of times or while a specified condition is true.

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>
var i=0
for (i=0;i<=10;i++)
{
document.write("The number is " + i)
document.write("<br />")
}
</script>
</body>
</html>

Result

The number is 0
The number is 1
The number is 2
The number is 3
The number is 4
The number is 5
The number is 6
The number is 7
The number is 8
The number is 9

The number is 10

JavaScript While Loop
Loops in JavaScript are used to execute the same block of code a specified number
of times or while a specified condition is true.

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>
var i=0
while (i<=10)
{
i=i+1
}
</script>
</body>
</html>

Result

The number is 0
The number is 1
The number is 2
The number is 3
The number is 4
The number is 5
The number is 6
The number is 7
The number is 8
The number is 9

The number is 10
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 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>
var i=0
do
{
i=i+1
}
while (i<0)
</script>
</body>
</html>

Result

The number is 0

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>

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.

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>
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 Events
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!

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="sch.gif" width="100" height="30">
</a>

JavaScript Special Characters
In JavaScript you can add special characters to a text string by using the backslash
sign.

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)

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

Here is another example:

document.write ("You & me are singing!")

The example above will produce the following output:

You & me 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 Guidelines
Some other important things to know when scripting with JavaScript.

JavaScript is Case Sensitive

A function named "myfunction" is not the same as "myFunction" and a variable named
"myVar" is not the same as "myvar".

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

White Space

JavaScript ignores extra spaces. You can add white space to your script to make it more
readable. The following lines are equivalent:

name="Hege"
name = "Hege"

Break up a Code Line

You can break up a code line within a text string with a backslash. The example below
will be displayed properly:

document.write("Hello
World!")

However, you cannot break up a code line like this:

document.write
("Hello World!")

Comments

You can add comments to your script by using two slashes //:

//this is a comment

or by using /* and */ (this creates a multi-line comment block):

/* This is a comment
block. It contains
several lines */

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.

However, creating your own objects will be explained later, in the Advanced JavaScript
section. We will start by looking at the built-in JavaScript objects, and how they are used.
The next pages will explain each built-in JavaScript object in detail.

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.

In the following example we are using the length property of the String object to return
the number of characters in a string:

var txt="Hello World!"
document.write(txt.length)
</script>

The output of the code above will be:

12

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:

var str="Hello world!"
document.write(str.toUpperCase())
</script>

The output of the code above will be:

HELLO WORLD!

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

String object

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

Examples of use:

The following example uses the length property of the String object to find the length of a
string:

var txt="Hello world!"
document.write(txt.length)

The code above will result in the following output:

12

The following example uses the toUpperCase() method of the String object to convert a
string to uppercase letters:

var txt="Hello world!"
document.write(txt.toUpperCase())


HELLO WORLD!

Complete String Object Reference

For a complete reference of all the properties and methods that can be used with the
String object, go to our complete String object reference.

The reference contains a brief description and examples of use for each property and
method!

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

Defining Dates

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

We define a Date object with the new keyword. The following code line defines a Date
object called myDate:

var myDate=new Date()

Note: The Date object will automatically hold the current date and time as its initial
value!

Manipulate 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):

myDate.setFullYear(2010,0,14)

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

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!

Comparing Dates

The Date object is also used to compare two dates.

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

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")

Complete Date Object Reference

For a complete reference of all the properties and methods that can be used with the Date
object, go to our complete Date object reference.

method!

JavaScript Array Object
The Array object is used to store a set of values in a single variable name.

Defining Arrays

The Array object is used to store a set of values in a single variable name.

We define an Array object with the new keyword. The following code line defines an
Array object called myArray:

var myArray=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).

1:

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"

2:

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.

Accessing Arrays

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

Modify Values in Existing Arrays

To modify a value in an existing array, just add a new value to the array with a specified
index number:

mycars[0]="Opel"

Now, the following code line:

document.write(mycars[0])

will result in the following output:

Opel

Complete Array Object Reference

Array object, go to our complete Array object reference.

method!

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

Boolean Object

The Boolean object is an object wrapper for a Boolean value.

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

We define a Boolean object with the new keyword. The following code line defines 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")

Complete Boolean Object Reference

Boolean object, go to our complete Boolean object reference.

method!

JavaScript Math Object
The Math object allows you to perform common mathematical tasks.

Math Object

The Math object allows you to perform common mathematical tasks.

The Math object includes several mathematical values and functions. You do not need to
define the Math object before using it.

Mathematical Values

JavaScript provides eight mathematical values (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 values 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 values that can be accessed from the Math object there
are also several functions (methods) available.

Examples of functions (methods):

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))


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:

0.8669997601016495

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))

The code above can result in the following output:

3

Complete Math Object Reference

For a complete reference of all the properties and methods that can be used with the Math
object, go to our complete Math object reference.

method!

The Navigator Object

The JavaScript Navigator object contains all information about the visitor's browser. We
are going to look at two properties of the Navigator object:

• appName - holds the name of the browser
• appVersion - holds, among other things, the version of the browser

Example
<html>
<body>
<script>
with(navigator)
{
document.writeln("Browser Code Name:"+appCodeName+"<br>")
document.writeln("Browser Name:"+appName+"<br>")
document.writeln("Browser Version:"+appVersion+"<br>")
document.writeln("Language:"+language+"<br>")
document.writeln("Operating Platform:"+platform+"<br>")
document.writeln("User Agent:"+userAgent)
}
</script>
</body>
</html>

<html>
<body>
<script>
with(navigator)
{
document.writeln("Browser Code Name:"+appCodeName+"<br>")
document.writeln("Browser Name:"+appName+"<br>")
document.writeln("Browser Version:"+appVersion+"<br>")
document.writeln("Language:"+language+"<br>")
document.writeln("Operating Platform:"+platform+"<br>")
document.writeln("User Agent:"+userAgent)
}
</script>
</body>
</html>

The variable browser in the example above holds the name of the browser, i.e.
"Netscape" or "Microsoft Internet Explorer".

The appVersion property in the example above returns a string that contains much more
information than just the version number, but for now we are only interested in the
version number. To pull the version number out of the string we are using a function
called parseFloat(), which pulls the first thing that looks like a decimal number out of a
string and returns it.

IMPORTANT! The version number is WRONG in IE 5.0 or later! Microsoft start the
appVersion string with the numbers 4.0. in IE 5.0 and IE 6.0!!! Why did they do that???
However, JavaScript is the same in IE6, IE5 and IE4, so for most scripts it is ok.

Example

The script below displays a different alert, depending on the visitor's browser:

<html>
<head>
function detectBrowser()
{
var browser=navigator.appName
var b_version=navigator.appVersion
var version=parseFloat(b_version)
if ((browser=="Netscape"||browser=="Microsoft Internet Explorer")
&& (version>=4))
{alert("Your browser is good enough!")}
else
{alert("It's time to upgrade your browser!")}
}
</script>
</head>
<body onload="detectBrowser()">
</body>
</html>

JavaScript Timing Events
With JavaScript, it is possible to execute some code NOT immediately after a
function is called, but after a specified time interval. This is called timing events.

JavaScript Timing Events

With JavaScript, it is possible to execute some code NOT immediately after a function is
called, but after a specified time interval. This is called timing events.

It's very easy to time events in JavaScript. The two key methods that are used are:

• setTimeout() - executes a code some time in the future
• clearTimeout() - cancels the setTimeout()

Note: The setTimeout() and clearTimeout() are both methods of the HTML DOM
Window object.

setTimeout()

Syntax
var t=setTimeout("javascript statement",milliseconds)

The setTimeout() method returns a value - In the statement above, the value is stored in a
variable called t. If you want to cancel this setTimeout(), you can refer to it using the
variable name.

The first parameter of setTimeout() is a string that contains a JavaScript statement. This
statement could be a statement like "alert('5 seconds!')" or a call to a function, like
"alertMsg()".

The second parameter indicates how many milliseconds from now you want to execute
the first parameter.

Note: There are 1000 milliseconds in one second.

clearTimeout()

Syntax
clearTimeout(setTimeout_variable)

Example

When the button is clicked in the example below, an alert box will be displayed after 5
seconds.

<html>
<head>
<script>
var c=setTimeout("alert('Press the OK button to continue')",5000);
function clr()
{
clearTimeout(c);
alert("The setTimeout() method was cancelled");
}
</script>
</head>
<body>
<form>
<input type="button" value="OK" onclick="clr()">
</form>
</html>

JavaScript Summary

This tutorial has taught you how to add JavaScript to your HTML pages, to make your
web site more dynamic and interactive.

You have learned how to create responses to events, validate forms and how to make
different scripts run in response to different scenarios.

You have also learned how to create and use objects, and how to use JavaScript's built-in
objects.

Java script questions

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