Javascript basics

G.ShreeDevi M.E (CSE)
Erode
JAVASCRIPT BASICS

VARIABLES External js file
SYNTAX &STATEMENTS
IF STATEMENT,
WHILE & DO LOOP,
SWITCH CASE,FOR
BREAK AND CONTINUe
OVERVIEW
FUNCTION
PAGE PRINTING STRING & ARRAY
ERROR HANDLING

A program is a list of "instructions" to be "executed" by a computer.
JavaScript is used to create client-side dynamic pages.
JavaScript is an object-based scripting language which is lightweight and cross-platform.JavaScript is not a compiled
language, but it is a translated language.
The JavaScript Translator (embedded in the browser) is responsible for translating the JavaScript code for the web
browser.
The programming instructions are called statements.

JavaScript - Syntax
JavaScript can be implemented using JavaScript statements that are placed within the <script>... </script> HTML tags in a web page.
Place the <script> tags, containing your JavaScript, anywhere within your web page, but it is normally recommended that you should
keep it within the <head> tags.
The <script> tag alerts the browser program to start interpreting all the text between these tags as a script.
A simple syntax of your JavaScript will appear as follows.
<script ...> JavaScript code </script>

• The script tag takes two important attributes −
• Language − This attribute specifies what scripting language you are using. Typically, its value will
be javascript. Although recent versions of HTML (and XHTML, its successor) have phased out the
use of this attribute.
• Type − This attribute is what is now recommended to indicate the scripting language in use and its
value should be set to "text/javascript".
<script language = "javascript" type = "text/javascript">
JavaScript code
</script>

JavaScript in <body> and <head> Sections
• You can put your JavaScript code in <head> and <body> section altogether as follows −
<html>
<head>
<script type = "text/javascript">
function sayHello()
{ alert("Hello World") }
</script>
</head>
<body>
document.write("Hello World“)
</script>
<input type = "button" onclick = "sayHello()“ value = "Say Hello" />
</body>
</html>

STATEMENTS
JavaScript Statements
JavaScript statements are composed of: Values, Operators, Expressions, Keywords, and Comments.
The statements are executed, one by one, in the same order as they a
re written.
Semicolons separate JavaScript statements.
JavaScript White Space
JavaScript ignores multiple spaces. You can add white space to your script to
make it more readable.
The following lines are equivalent:
var person = “ANJU";
var person=“ANJU";

VARIABLES
EXAMPLES
var v1, v2, v3; // Declare 3 variables
v1= 5; // Assign the value 5 to v1
v2= 6; // Assign the value 6 to v2
v3 = a + b; // Assign the sum of v1and v2 to v3
a = 5; b = 6; c = a + b;
var sname= "Anju";
var sname="Anju";
Declaring or Creating JavaScript Variables
Creating a variable in JavaScript is called "declaring" a variable.
Declare a JavaScript variable with the var keyword:
var Studname;
To assign a value to the variable, use the equal sign:
studname = "Anju";
We can assign a valu to the variable when you declare it:
var Studname = “Anju”;

Operators,Expression,Keywords,Comments
JavaScript Keywor
ds
JavaScript keywords are used to identify
actions to be performed.
JavaScript Comme
nts
Code after double slashes // or between /* and */ is tre
ated as a comment.
Comments are ignored, and will not be executed:
Operators
Arithmetic Operators
Comparison Operators
Logical (or Relational) Operators
Assignment Operators
Conditional (or ternary) Operators
Expression
An expression is a combination of values, variables, and ope
rators, which computes to a value.
The computation is called an evaluation.
For example, 5 * 10 evaluates to 50

Operator Description Example
+ Addition 10+20 = 30
- Subtraction 20-10 = 10
* Multiplication 10*20 = 200
/ Division 20/10 = 2
% Modulus (Remainder) 20%10 = 0
++ Increment var a=10; a++; Now a = 11
-- Decrement var a=10; a--; Now a = 9
Arithmetic Operators

Operators Description
== Compares the equality of two
operands without considering
type.
=== Compares equality of two
operands with type.
!= Compares inequality of two
operands.
> Checks whether left side value is
greater than right side value. If
yes then returns true otherwise
false.
< Checks whether left operand is
less than right operand. If yes
then returns true otherwise
false.
>= Checks whether left operand is
greater than or equal to right
operand. If yes then returns true
otherwise false.
<= Checks whether left operand is
less than or equal to right
operand. If yes then returns true
otherwise false.
Comparison Operators

Operator Description
&& && is known as AND operator.
It checks whether two operands
are non-zero (0, false,
undefined, null or "" are
considered as zero), if yes then
returns 1 otherwise 0.
|| || is known as OR operator. It
checks whether any one of the
two operands is non-zero (0,
false, undefined, null or "" is
considered as zero).
! ! is known as NOT operator. It
reverses the boolean result of
the operand (or condition)
Logical Operators

Assignment operators Description
= Assigns right operand value to
left operand.
+= Sums up left and right operand
values and assign the result to
the left operand.
-= Subtract right operand value
from left operand value and
assign the result to the left
operand.
*= Multiply left and right operand
values and assign the result to
the left operand.
/= Divide left operand value by
right operand value and assign
the result to the left operand.
%= Get the modulus of left operand
Assignment Operators

Ternary Operator
JavaScript includes special operator called ternary operator :? that assigns a value to a variable based on some
condition. This is like short form of if-else condition.
Syntax:
<condition> ? <value1> :<value2>;
Example: Ternary operator
var a = 10, b = 5;
var c = a > b? a : b; // value of c would be 10
var d = a > b? b : a; // value of d would be 5

JAVASCRIPT IN EXTERNAL FILE
• The script tag provides a mechanism to allow you to
store JavaScript in an external file and then include it
into your HTML files.
<html>
<head>
<script type = "text/javascript" src = "filename.js" >
</script>
</head>
<body> ....... </body>
</html>
• the following content is saved in filename.js file and then you
can use sayHello function in your HTML file after including the
filename.js file.
function sayHello()
{ alert("Hello World"); }

CONDITIONAL STATEMENTS
For loop
if..else
statement.
switch
statement
The while Loop The do...while Loop
•To use conditional statements that allow your program to make correct
decisions and perform right actions.
• While writing a program, you may encounter a situation where you
need to perform an action over and over again. In such situations, you
would need to write loop statements to reduce the number of lines.

if...else Statement
.
• JavaScript supports conditional statements which
are used to perform different actions based on
different conditions. Here we will explain the
if..else statement. JavaScript supports the
following forms of if..else statement −
• if statement
• if...else statement
• if...else if... statement
Syntax
The syntax for a basic if statement is as follows −
if (expression)
{ Statement(s) to be executed if expression is true }
Syntax
if (expression)
{ Statement(s) to be executed if expression is true }
else
{ Statement(s) to be executed if expression is false }
Syntax
The syntax of an if-else-if statement is as follows −
if (expression 1)
{ Statement(s) to be executed if expression 1 is true }
else if (expression 2)
else if (expression 3)
else
{ Statement(s) to be executed if no expression is true }

JavaScript - Switch Case
Syntax
The objective of a switch statement is to give an expression to evaluate and several different statements to execute based on the
value of the expression. The interpreter checks each case against the value of the expression until a match is found. If nothing
matches, a default condition will be used.
switch (expression)
{ case condition 1:
statement(s)
break;
case condition 2:
statement(s)
break;
... case condition n:
statement(s)
break;
default: statement(s)
}
The break statements indicate the end of a particular case

JavaScript - While Loops
The purpose of a while loop is to execute a statement or code block repeatedly as long as an expression is true. Once the expression becomes false, the
loop terminates.
Syntax
The syntax of while loop in JavaScript is as follows −
while (expression)
{
Statement(s) to be executed if expression is true
}
<html>
<body>
var count = 0;
document.write("Starting Loop ");
while (count < 10)
{
document.write("Current Count : " + count + "<br />");
count++;
} document.write("Loop stopped!");
</script>
</body>
</html>

The do...while Loop
The do...while loop is similar to the while loop except that the condition check happens at the end of the
loop
PART 0
Syntax
The syntax for do-while loop in JavaScript is as follows −
do
{ Statement(s) to be executed; }
while (expression);
<html>
<body>
var count = 0;
document.write("Starting Loop" + "<br />");
do
{
document.write("Current Count : " + count + "<br />");
count++;
} while (count < 5);
document.write ("Loop stopped!");
</script>
</body>
</html>

JavaScript for loop
if you want to run the same code over and over again, each time with a different value.
Statement 1 is executed (one time) before the execution of the code block.
Statement 2 defines the condition for executing the code block.
Statement 3 is executed (every time) after the code block has been executed.
<html>
<body>
var count;
document.write("Starting Loop" + "<br />");
for(count = 0; count < 10; count++)
{ document.write("Current Count : " + count );
document.write("<br />");
}
document.write("Loop stopped!");
</script>
</body>
</html>

JavaScript - Loop Control
JavaScript provides break and continue statements. These statements are used to
immediately come out of any loop or to start the next iteration of any loop respectively.
.
The break statementStatement
The break statement, which was briefly introduced with the switch
statement, is used to exit a loop early, breaking out of the enclosing curly
braces.
<html>
<body>
var x = 1;
document.write("Entering the loop<br /> ");
while (x < 20)
{ if (x == 5)
break;
}
x = x + 1;
document.write( x + "<br />"); }
document.write("Exiting the loop!<br /> ");
</script>
</body>
</html>
The continue Statement
When a continue statement is encountered, the program
flow moves to the loop check expression immediately and
if the condition remains true, then it starts the next
iteration, otherwise the control comes out of the loop.
<html>
<body>
var x = 1;
document.write("Entering the loop<br /> ");
while (x < 10)
{ x = x + 1;
if (x == 5)
{
continue; // skip rest of the loop body }
document.write( x + "<br />"); }
document.write("Exiting the loop!<br /> ");
</script>
</body>
</html>

JAVASCRIPT - FUNCTIONS
Calling a Function
EXAMPLE
The most common way to define a function in JavaScript is by using the function keyword, followed by a unique
function name, a list of parameters (that might be empty), and a statement block surrounded by curly braces.
Syntax
function functionname(parameter-list)
{ statements }
</script>
<script type ="text/javascript">
function sayHello()
{alert("Hello there"); }
</script>
<html> <head>
function sayHello()
{ document.write ("Hello there!"); }
</script> </head>
<body>
<p>Click the following button to call the function</p>
<form> <input type = "button" onclick = "sayHello()" value = "Say Hello"> </form>
<p>Use different text in write method and then try...</p>
</body>
</html>

The return Statement
This is required to return a value from a function. This statement should be the last statement in a function.
<html>
<head>
function concatenate(first, last)
{
var full;
full = first + last;
return full;
}
function secondFunction()
{ var result;
result = concatenate(‘snehal', ‘smruti');
document.write (result );
}
</script>
</head>
<body>
<p>Click the following button to call the function</p>
<form>
<input type = "button" onclick = "secondFunction()" value = "Call Function">
</form>
</body>
</html>

JAVASCRIPT - PAGE PRINTING
The JavaScript print function window.print() prints the current web page when executed
PART 05
<html>
<head>
<script type="text/javascript">
</script>
</head>
<body>
<form>
<input type="button" value="Print" onclick="window.print()" />
</form>
</body>
<html>

JAVASCRIPT - THE STRINGS
OBJECT
The String parameter is a series of characters that has been properly encode
syntax to create a String object −
var val = new String(string);
Syntax
String Methods
Methods Explanation
length Returns the number of characters in a string
indexOf() Returns the index of the first time the specified
character occurs, or -1 if it never occurs, so
with that index you can determine if the string
contains the specified character.
lastIndexOf() Same as indexOf, only it starts from the right
and moves left.
match() Behaves similar to indexOf and lastIndexOf,
but the match method returns the specified
characters, or "null", instead of a numeric
value.
substr() Returns the characters you specified: (14,7)
returns 7 characters, from the 14th character.
substring() Returns the characters you specified: (7,14)
returns all characters between the 7th and the
14th.
toLowerCase() Converts a string to lower case
toUpperCase() Converts a string to upper case

<html>
<body>
var str=“welcome!"
document.write("<p>" + str + "</p>")
document.write("str.length")
</script>
</body>
</html>
<html>
<body>
var str=("Hello JavaScripters!")
document.write(str.toLowerCase())
document.write("<br>")
document.write(str.toUpperCase())
</script>
</body>
</html>

It stores a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data
JAVASCRIPT - THE ARRAYS
OBJECT
JavaScript array is an object that represents a collection of similar type of elements.
There are 3 ways to construct array in JavaScript
By array literal
By creating instance of Array directly (using new keyword)
By using an Array constructor (using new keyword)
Syntax Creating an Array
Using an array literal is the easiest way to create a JavaScript Array.
var array_name = [item1, item2, ...];
var fruits = new Array( "apple", "orange", "mango" );
JavaScript Array directly (new keyword)
The syntax of creating array directly is given below:
var arrayname=new Array();
JavaScript array constructor (new keyword)
Here, you need to create instance of array by passing arguments in constructor so that we don't have to provide value
explicitly.
The example of creating object by array constructor is given below.
<script>
var emp=new Array("Jai","Vijay","Smith");
for (i=0;i<emp.length;i++){
document.write(emp[i] + "<br>");
}
</SCRIPT>

JavaScript - Errors & Exceptions Handling
Runtime Errors
Runtime errors, also called exceptions, occur during execution
There are three types of errors in programming: (a) Syntax Errors, (b) Runtime Errors, and (c) Logical Errors.
Syntax Errors
Syntax errors, also called parsing errors, occur at compile time in traditional programming languages and at interpret time in JavaScript.
Runtime Errors
Runtime errors, also called exceptions, occur during execution
Logical Errors
Logic errors can be the most difficult type of errors to track down. These errors are not the result of a syntax or runtime error. Instead, they occur when you make a
mistake in the logic that drives your script and you do not get the result you expected.
JavaScript implements the try...catch...finally construct as well as the throw operator to handle exceptions.
The try...catch...finally block syntax −
try
{ // Code to run [break;] }
catch ( e )
{ // Code to run if an exception occurs [break;] }
[ finally { // Code that is always executed regardless of // an exception occurring }]
</script>

<html>
<head>
function disp()
{ var a = 100;
alert("Value of variable a is : " + a );
} </script>
</head>
<body>
<p>Click the following to see the result:</p>
<form> <input type = "button" value = "Click Me" onclick = “disp();" />
</form>
</body>
</html>

Javascript basics

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