Introduction
Today’s websites need to go much beyond HTML. There is a definite need to allow users, browsing through a website to interact with the website. The website must be intelligent to accept user’s input and dynamically structure the contents of the webpage according to the input. This requires a website development environment that will allow the creation interactive of web pages.
Capturing user requests is traditionally
done through a form. After a form
captures user input, the form must have built-in technique for sending the
information captured back to the web server for processing. The captured input
need to be validated before sending
it to the server. Hence the development environment must facilitate coding
which runs in a browser at client side for data validation.
JavaScript allows us to add such
capabilities to a webpage.
JavaScript is a scripting language
introduced by Netscape. JavaScript
made its first appearance in Netscape 2.0 in 1995 with a name LiveScript.
JavaScript is the world's most popular
programming language. It is the language for HTML and the web. JavaScript is a
lightweight, interpreted programming language with object-oriented capabilities
that allows you to build interactivity into otherwise static HTML pages.
JavaScript is programming code that can be inserted into HTML pages.
Advantages
Some of the other advantages of JavaScript
are:
·
An Interpreted language: JavaScript is
an interpreted language. So it does not require any compilation steps.
·
Quick Development: JavaScript does not
require any special or separate editor. It also does not require time consuming
compilation steps. This facilitates fast development.
·
Designed for simple small programs: It
is well suited to implement simple, small programs (Eg: a unit conversion).
Such programs can be written and executed at acceptable speeds.
·
Performance: JavaScript can be written
in such a way that HTML pages are compact and small. As it is embedded in the
HTML code, there is no need of additional network access to retrieve the page.
·
Procedural capabilities: JavaScript has
capabilities of any programming language like condition checking, looping and
branching.
·
Designed for programming user events:
JavaScript supports event based programming. That is it can detect when an user
event occurs (Eg: A button being pressed) and respond accordingly.
·
Easy Debugging and testing: Being an
interpreted language, scripts in JavaScript are tested line by line and the
errors are also listed as they are encountered. So its easy to locate errors.
·
Platform independence / Architecture neutral: JavaScript is completely independent of the hardware on which it
works. Browsers are platform specific. They take care of implementing
JavaScript according to the platform. This relieves the user of maintaining
multiple source code files.
JavaScript Syntax
A JavaScript consists of JavaScript
statements that are placed within the
<script>... </script> HTML tags in a web page.
You can place the <script> tag
containing your JavaScript within the <head> or <body> of you web
page, but it is preferred way to keep it within the <head> tags.
The <script> tag alert the browser
program to begin interpreting all the text between these tags as a script. So
simple syntax of your JavaScript will be as follows
<script >
JavaScript
code
</script>
The script tag takes one important
attribute language. This attribute
specifies what scripting language is being used. Typically, its value will be javascript.
So your JavaScript segment will look like:
<script
language="javascript">
JavaScript code
</script>
Data Types
One of the most fundamental characteristics
of a programming language is the set of data types it supports. These are the
type of values that can be represented and manipulated in a programming
language.
JavaScript allows you to work with three
primitive data types:
·
Number: It defines all integer and
floating point numbers and a special number NaN
(Not a Number). Integers can be represented in Decimal, Octal and Hexadecimal
format.
e.g. 123, 120.50 etc.
·
String: It defines all strings that are
enclosed in single (‘ ’) or double (“ ”) quotes.
e.g. "This
text string", ‘Krishna’ etc.
·
Boolean: Defines logical values true and false. JavaScript automatically converts true into 1 and false
into 0 when used in numerical expressions.
·
Null: Defines only a single value null which identifies null, empty or
non-existent reference.
JavaScript does not make a distinction
between integer values and floating-point values. All numbers in JavaScript are
represented as floating-point values. JavaScript represents numbers using the
64-bit floating-point format.
Variables
Like many other programming languages, JavaScript
has variables. Variables can be thought of as named containers. You can place
data into these containers and then refer to the data simply by naming the
container.
Before you use a variable in a JavaScript
program, you must declare it. Variables are declared with the var keyword as follows:
<script
language="javascript">
var
money;
var
name;
</script>
Storing a value in a variable is called
variable initialization. You can do variable initialization at the time of
variable creation or later point in time when you need that variable as
follows:
<script
language ="javascript">
var
name = "Ram";
var
money;
money
= 2000.50;
</script>
JavaScript is untyped language. This means that a JavaScript variable can hold a
value of any data type. Unlike many other languages, you don't have to tell
JavaScript during variable declaration what type of value the variable will
hold. The value type of a variable can change during the execution of a program
and JavaScript takes care of it automatically.
Arrays
Arrays are JavaScript objects capable of holding
a sequence of values. These values are stored in contiguous memory locations
and accessed by the name of the array followed by the index. An array can be
declared as below
arrayName
= new Array(length);
If an array is created without specifying
the size, JavaScript creates an array of size zero and automatically extends
the length of the array when elements are added.
Eg: students = new Array();
Arrays are extendable even if the length is
specified initially.
Dense Arrays
A dense array is an array that has been
created with each of its element being assigned a specific value. They are
declared and initialised at the same time.
Operators
JavaScript language supports following type
of operators.
·
Arithmetic Operators
·
Assignment Operators
·
Comparison Operators
·
Logical Operators
·
The Conditional (or ternary)
Operator
·
Bitwise Operators
·
Special Operators
Let us have a look on all operators one by
one.
The Arithmetic Operators
There are following arithmetic operators
supported by JavaScript language:
Assume variable A holds 10 and variable B
holds 20 then:
Operator
|
Description
|
Example
|
+
|
Adds two operands
|
A + B will give 30
|
-
|
Subtracts second operand from the first
|
A - B will give -10
|
*
|
Multiply both operands
|
A * B will give 200
|
/
|
Divide numerator by denominator
|
B / A will give 2
|
%
|
Modulus Operator and remainder of after
an integer division
|
B % A will give 0
|
++
|
Increment operator, increases integer
value by one
|
A++ will give 11
|
--
|
Decrement operator, decreases integer
value by one
|
A-- will give 9
|
Note: Addition operator (+)
works for Numeric as well as Strings. e.g. "a" + 10 will give
"a10".
The Assignment Operators
There are following assignment operators
supported by JavaScript language:
Operator
|
Description
|
Example
|
=
|
Simple assignment operator, Assigns
values from right side operands to left side operand
|
C = A + B will assign value of A + B into
C
|
+=
|
Add AND assignment operator, It adds
right operand to the left operand and assign the result to left operand
|
C += A is equivalent to C = C + A
|
-=
|
Subtract AND assignment operator, It
subtracts right operand from the left operand and assign the result to left
operand
|
C -= A is equivalent to C = C - A
|
*=
|
Multiply AND assignment operator, It multiplies
right operand with the left operand and assign the result to left operand
|
C *= A is equivalent to C = C * A
|
/=
|
Divide AND assignment operator, It
divides left operand with the right operand and assign the result to left
operand
|
C /= A is equivalent to C = C / A
|
%=
|
Modulus AND assignment operator, It takes
modulus using two operands and assign the result to left operand
|
C %= A is equivalent to C = C % A
|
The Comparison Operators
There are following comparison operators
supported by JavaScript language
Assume variable A holds 10 and variable B
holds 20 then:
Operator
|
Description
|
Example
|
==
|
Checks if the values of two operands are
equal or not, if yes then condition becomes true. Here the data type of the
operands are not considered.
|
(A == B) is not true.
|
!=
|
Checks if the values of two operands are
equal or not, if values are not equal then condition becomes true.
|
(A != B) is true.
|
>
|
Checks if the value of left operand is
greater than the value of right operand, if yes then condition becomes true.
|
(A > B) is not true.
|
<
|
Checks if the value of left operand is
less than the value of right operand, if yes then condition becomes true.
|
(A < B) is true.
|
>=
|
Checks if the value of left operand is
greater than or equal to the value of right operand, if yes then condition
becomes true.
|
(A >= B) is not true.
|
<=
|
Checks if the value of left operand is
less than or equal to the value of right operand, if yes then condition
becomes true.
|
(A <= B) is true.
|
===
|
Strictly equal. Checks the data type of
the operands also.
|
10 == “10” is true
10 === “10” is false
|
!==
|
Strictly not equal. Considers the data
type of the operands also.
|
20 != “20” is false
20 !== “20” is true.
|
The Logical Operators
There are following logical operators
supported by JavaScript language
Assume variable A holds 10 and variable B
holds 20 then:
Operator
|
Description
|
Example
|
&&
|
Called Logical AND operator. If both the
operands are non zero then the condition becomes true.
|
(A && B) is true.
|
||
|
Called Logical OR Operator. If any of the
two operands are non zero then the condition becomes true.
|
(A || B) is true.
|
!
|
Called Logical NOT Operator. Use to
reverses the logical state of its operand. If a condition is true then
Logical NOT operator will make false.
|
!(A && B) is false.
|
The Conditional Operator (? :)
Conditional operator first evaluates an
expression for a true or false value and then execute one of the two given
statements depending upon the result of the evaluation.
The conditional operator has the syntax:
Operator
|
Description
|
Example
|
? :
|
If Condition is true ? Then value X :
Otherwise value Y
|
k = (a==b) ? 10 : 2
k gets value 10 if a is equal to b and 2
if a is not equal to b
|
Bitwise Operators
There are following bitwise operators
supported by JavaScript language
Assume variable A holds 2 and variable B
holds 3 then:
Operator
|
Description
|
Example
|
&
|
Bitwise
AND operator. It performs a Boolean AND operation
on each bit of its integer arguments.
|
(A & B) is 2
|
|
|
Bitwise
OR Operator. It performs a Boolean OR operation
on each bit of its integer arguments.
|
(A | B) is 3
|
^
|
Bitwise
XOR operator. It performs a Boolean exclusive OR
operation on each bit of its integer arguments. Exclusive OR means that
either operand one is true or operand two is true, but not both.
|
(A ^ B) is 1
|
~
|
Bitwise
NOT operator. It is a unary operator and operates
by reversing all bits in the operand.
|
(~B) is -4 .
|
<<
|
Bitwise
Shift Left operator. It moves the bits in its
first operand to the left by the number of places specified in the second
operand. New bits are filled with zeros. Shifting a value left by one
position is equivalent to multiplying by 2, shifting two positions is
equivalent to multiplying by 4, etc.
|
(A << 1) is 4.
|
>>
|
Bitwise
Shift Right with Sign operator. It moves the bits
in its first operand to the right by the number of places specified in the
second operand. The bits filled in on the left depend on the sign bit of the
original operand, in order to preserve the sign of the result. Shifting a
value right one place is equivalent to dividing by 2 (discarding the
remainder).
|
(A >> 1) is 1.
|
>>>
|
Bitwise
Shift Right with Zero operator. This operator is
just like the >> operator, except that the bits shifted in on the left
are always zero (not the sign bit).
|
(A >>> 1) is 1.
|
Special operators
JavaScript supports some additional
operators other than those given above
Operator
|
Description
|
Example
|
new
|
The new operator is used to create an instance of an
object type.
|
myArray = new Array();
|
delete
|
To delete a property of an object or an element at
an array index
|
delete myArray[5]
deletes 6th element of the array
|
Placing Text in a Browser
JavaScript
uses the write() method of document object for placing text in a browser.
Methods are called combining the object name with method name.
objectName.methodName
The
write() method accepts a string value as argument, and places it in the current
browser window.
Example:
document.write(“Hello World”);
Conditional Statements
Conditional statements are used to perform different actions
based on different conditions.
In
JavaScript we have the following conditional statements:
- if
statement - use this statement
to execute some code only if a specified condition is true
- if...else
statement - use this statement
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
to select one of many blocks of code to be executed
- switch
statement - use this statement
to select one of many blocks of code to be executed
If Statement
Use the if statement to execute some code only if a specified
condition is true.
Syntax
if (condition)
{
code to be executed if condition is true
}
Example
if (a<10)
{
a++;
}
Here
the variable a gets incremented if its value is less than 10.
If...else Statement
Use the if....else statement to execute some code if a
condition is true and another code if the condition is not true.
Syntax
if (condition)
{
code to be
executed if condition is true
}else
{
code to be
executed if condition is not true
}
Example
if (a<10){
a++;
}else{
b++;
}
Here
the variable a gets incremented if its value is less than 10 and b gets
incremented if the condition is false
Switch Statement
Use the switch statement 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 1 and 2
}
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. Use the default keyword to specify what to do if
there is no match:
Example
switch (day)
{
case 0:
x="Today it's
Sunday";
break;
case 1:
x="Today
it's Monday";
break;
case
2:
x="Today
it's Tuesday";
break;
case 3:
x="Today
it's Wednesday";
break;
case 4:
x="Today
it's Thursday";
break;
case 5:
x="Today
it's Friday";
break;
case
6:
x="Today
it's Saturday";
break;
default:
x
= "Invalid
}
In
the above example suppose day has a value 4, then x gets value ="Today it's Thursday". Suppose day is not
in the range 0 to 6, then the default block is executed and x gets the value “Invalid”.
Loops
Loops can execute
a block of code a number of times.
JavaScript
supports different kinds of loops:
- for - loops through a block of code a number of times
- for/in - loops through the properties of an object
- while - loops through a block of code while a specified condition is true
- do/while - also loops through a block of code while a specified condition is true
The For Loop
The for loop is often the tool you will use
when you want to create a loop.
The
for loop has the following syntax:
for (statement
1; statement 2; statement 3)
{
the
code block to be executed
}
Statement 1 is executed before the loop (the code block) starts.
Statement 2 defines the condition for running the loop (the code
block).
Statement 3 is executed each time after the loop (the code block)
has been executed.
Example:
for (var i=0;
i<5; i++)
{
document.write("The number is "
+ i + "<br>");
}
The For/In Loop
The JavaScript for/in statement loops through the
properties of an object:
var
person={fname:"John",lname:"Doe",age:25};
for (x in
person)
{
txt=txt + person[x];
}
The While Loop
The while loop loops through a block of code as long
as a specified condition is true.
Syntax:
while (condition)
{
code block to be executed
}
Example:
while
(i<5){
document.write("The number is "
+ i + "<br>");
i++;
}
The Do/While Loop
The do/while loop is a variant of the while loop.
This loop will execute the code block once, before checking if the condition is
true, and then it will repeat the loop as long as the condition is true.
Syntax
do
{
code block to be executed
}while (condition);
Example:
do
{
document.write(The number is " + i +
"<br>");
i++;
}while (i<5);
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