Basic Syntax
of JavaScript
The essential syntax need to write in the form to implement java
script code
<script
type="text/javascript">
// Your code here
</script>
Comment Types in JavaScript
- Single line :- ( // ) indicates
that the rest of the current line is a comment
- Multiline :- ( /* ) indicatesp
the beginning of a comment that covers more than one line and end with (*/)
Important
Summary
- A
semicolon or a newline defines the end of a statement, and a statement is
a single command.
- Semicolons
are in fact optional, but it’s still a good idea to use them to make clear
where statements end, because doing so makes your code easier to read and
debug.
- Curly braces ({
and })
are used to indicate a block of code. They ensure that all the lines inside
the braces are treated as one block.
- Java
script code in script tag is executed in sequence and it is visible to
users also
- We can
check the width and resoluon of computer screen as “screen.availWidth”
- Q - Diff bw prompt and alert in
javascript??
Ans
- prompt is type of alert use to get the data from user
prompt(‘What is your name’); and
document.write is used to write something
on output stream
Datatypes
in JavaScript
The three most basic data types that store data in JavaScript
are
• String: A series of characters, for example, “some
characters”
Everything written in ‘’ or “” is treated as string in
javascript. The only difference is that’’ single quotes will always work except
if the string itself has ‘ single quote init like “paul’s cow”
• Number: A number, including floating point numbers
Javascript automatically change the data types like var a =
1+2; result is 3 means it treated it as number. Although we have Nunber(), parseInt and parseFloat
to convert the string to integer and float resp. It also has one function as
typeof which tells the datatype of a variable
The
isNaN() function determines whether a value is an illegal number
(Not-a-Number).This function returns true if the value is NaN, and false if
not.
• Boolean: Can contain a true or false value
There are two other important
datatypes as
• Null: Indicates that there is no data.
• Undefined: Indicates that something has
not been defined and given a value. This
is important when
you’re working with variables
Operator What It Does in JavaScript
+ Adds
two numbers together or concatenates two strings.
- Subtracts
the second number from the first.
* Multiplies
two numbers.
/ Divides
the first number by the second.
% Finds
the modulus—the reminder of a division. For example, 98 % 10 = 8.
-- Decreases
the number by 1: only useful with variables, which we’ll see at work later.
++ Increases the number by 1:
only useful with variables, which we’ll see at work later.
Some Important Escape Sequences Character Represented in JavaScript
\b Backspace.
\f Form
feed.
\n Newline.
\r Carriage
return.
\t Tab.
\' Single
quote.
\" Double
quote.
\\ Backslash.
\xNN NN
is a hexadecimal number that identifies a character in the
Latin-1
character set (the Latin-1 character is the norm for
English-speaking
countries).
\uDDDD DDDD is
a hexadecimal number identifying a Unicode character.
Composite data types are
1.
Object
I.
String
String object have many other method like “indexOf”, “substring” etc
II.
Math
III.
Date
Date is not a primitive datatype
in JS as in java, We have to create the object by new oprator for data as
var dt = new Date();
it has many function as getDate,
getFullYear etc
JS counting month starting from
0(Janaury)
2.
Array
Var
ar = new Array(3); OR var ar = new Array
(“de”,”ff”,”fg”);
Er.join(‘,’); it will convert all element of ar array into
string by adding comma in between all the elements
Split (‘, ’); this is opposite to join
We can sort and reverse the array
as ar.sort() and ar.reverse()
All operators have an order of precedence. Multiplication,
division, and modulus have
equal precedence, so where they all appear in an equation the
sum will be calculated from left
to right. Try this calculation:
2 * 10 / 5%3
The result is 1, because the calculation simply reads from left
to right:
2 * 10 = 20
20 / 5 = 4
4%3
= 1
Addition
and subtraction also have equal precedence.
Two ways to create a
string object are:
var myStringObject =
"abc" ;
//Implicit
var
myStringObject = new String( "abc" );
//Ecplicit
The result of checking the length property
is the same whether we create the String
object implicitly or explicitly. The only real difference
between creating String objects
explicitly or implicitly is that creating them explicitly is
marginally more efficient if you’re
going to be using the same String object
again and again. Explicitly creating String objects
also helps prevent the JavaScript interpreter getting confused
between numbers and strings,
as
it can do.
But
for Date in JS we have only explicit way like
var
todaysDate = new Date();
The Math
object
provides us with lots of mathematical functionality, like finding the square of
a
number or producing a random number. The Math
object is different from the Date
and String
objects in two ways:
• You can’t create a Math object
explicitly, you just go ahead and use it.
•
The Math object doesn’t store data,
unlike the String and Date
object.
• round(): Rounds a number up when the
decimal is . 5 or greater
• ceil() (as in ceiling):
Always rounds up, so 23.75 becomes 24, as does 23.25
•
floor(): Always rounds down, so 23.75
becomes 23, as does 23.25
Array in
JavaScript:
Array objects, like String
and Date objects, are created using the
new keyword together
with the constructor. We can initialize an Array
object when we create it:
var preInitArray = new Array( "First item",
"Second item",➥
"Third
Item" );
Using index numbers to store items is useful if you want to
loop through the array—we’ll
be looking at loops next.
You
can use keywords to access the array elements instead of a numerical index,
like this:
var anArray = new Array( );
var itemIndex = 0;
var itemKeyword = "CostOfApple";
anArray[itemIndex] = "Fruit";
anArray[itemKeyword] = 0.75;
The slice() method takes two parameters:
the index of the first element of the slice, which
will be included in the slice, and the index of the final
element, which won’t be. To access the
second,
third, and fourth values from an array holding five values in all, we use the
indexes 1
The Array object’s concat()
method allows us to concatenate arrays. We can add two or more
arrays using this method, each new array starting where the
previous one ends. Here we’re
joining
three arrays: arrayOne, arrayTwo,
and arrayThree:
The
sort() method allows us to sort the
items in an array into alphabetical or numerical order:
The Logical and Comparison Operators in JavaScript
There are two main groups of operators we’ll look at:
• Data comparison operators: Compare operands and return
Boolean values.
•
Logical operators: Test for more than one
condition.
Operator Description Example
== Checks whether the left and
right operands are equal 123 == 234 returns false.
123 == 123 returns true.
!= Checks whether the left
operand is not equal to the
right side
123 != 123 returns false.
123 != 234 returns true.
> Checks whether the left
operand is greater than the
right
123 > 234 returns false.
234 > 123 returns true.
>= Checks whether the left
operand is greater than or
equal to the right
123 >= 234 returns false.
123 >= 123 returns true.
< Checks whether the left
operand is less than the right 234 < 123 returns
false.
123 < 234 returns true.
<= Checks whether the left
operand is less than, or equal
to, the right
234 <= 123 returns false.
234
<= 234 returns true.
Symbol Operator Description Example in JavaScript
&& And Both conditions must be
true. 123 == 234 && 123 < 20 (false)
123 == 234 && 123 == 123 (false)
123 == 123 && 234 < 900 (true)
|| Or Either or both of the
conditions
must be true.
123 == 234 || 123 < 20 (false)
123 == 234 || 123 == 123 (true)
123 == 123 || 234 < 900 (true)
! Not Reverses the logic. !(123
== 234) (true)
!(123
== 123) (false)
ExMPLE
OF if….else and else if
<html>
<body>
<script
type="text/javascript">
var userNumber = Number( prompt(
"Enter a number between➥
1 and 10", "" ) );
if ( isNaN( userNumber ) ){
document.write( "Please ensure a
valid number is➥
entered" );
} else if ( userNumber > 10 ||
userNumber < 1 ) {
document.write( "The number you
entered is not➥
between 1 and 10" );
} else {
52 CHAPTER
2 ■
DATA AND DECISIONS
document.write( "The number you
entered was " + ➥
userNumber );
}
</script>
</body>
</html>
One more thing before we move on: you can break a conditional
statement or loop using the
break statement. This simply
terminates the block of code running and drops the processing
through
to the next statement.
As you’ve already seen, the break statement
is great for breaking out of any kind of loop once a
certain event has occurred. The continue
keyword works like break in
that it stops the execution
of the loop. However, instead of dropping out of the loop, continue
causes execution to
resume
with the next iteration.
Switch in
JavaScript:
The switch statement allows us to
“switch” between sections of code based on the value of a
variable or expression. This is the outline of a switch
statement:
switch( expression ) {
case someValue:
// Code to execute if expression == someValue;
break; // End execution
case someOtherValue:
// Code to execute if expression == someOtherValue;
break; // End execution
case yesAnotherValue:
// Code to execute if expression == yetAnotherValue;
break; // End execution
default:
// Code to execute if no values matched
}
FOR LOOP in JavaScript
Repeating a Set Number of Times: the for
Loop
The for loop is designed to loop
through a code block a number of times and looks like this:
for( initial-condition; loop-condition;
alter-condition ) {
//
// Code to be repeatedly executed
//
}
//
After loop completes, execution of code continues here
While Loop in JavaScript
while ( some condition true ) {
// Loop code
}