IF-ELSE
While writing a
program, there may be a situation when you need to adopt one out of a given set
of paths. In such cases, you need to use conditional statements that allow your
program to make correct decisions and perform right actions.
JavaScript supports
conditional statements which are used to perform different actions based on
different conditions. Here we will explain the if..else statement.
Flow Chart of if-else
The following flow chart shows how the if-else statement works.
JavaScript supports the following forms of if..else statement:
·
if
statement
·
if...else
statement
·
if...else
if... statement
if Statement
The ‘if’ statement is the fundamental
control statement that allows JavaScript to make decisions and execute
statements conditionally.
Syntax
The syntax for a basic if statement is as follows:
if (expression){
Statement(s) to be
executed if expression is true
}
Here a JavaScript
expression is evaluated. If the resulting value is true, the given statement(s)
are executed. If the expression is false, then no statement would be not
executed. Most of the times, you will use comparison operators while making
decisions.
Example
Try the following example to understand how the if statement works.
<html>
<body>
<script
type="text/javascript">
<!--
var age = 20;
if( age > 18 ){
document.write("<b>Qualifies
for driving</b>");
}
//-->
</script>
<p>Set the
variable to different value and then try...</p>
</body>
</html>
Output
Qualifies for
driving
Set the variable
to different value and then try...
if...else Statement
The ‘if...else’ statement is the next form
of control statement that allows JavaScript to execute statements in a more
controlled way.
Syntax
The syntax of an if-else
statement is as follows:
if (expression){
Statement(s) to be
executed if expression is true
}else{
Statement(s) to be
executed if expression is false
}
Here JavaScript
expression is evaluated. If the resulting value is true, the given statement(s)
in the ‘if’ block, are executed. If the expression is false, then the given
statement(s) in the else block are executed.
Example
Try the following code
to learn how to implement an if-else statement in JavaScript.
<html>
<body>
<script
type="text/javascript">
<!--
var age = 15;
if( age > 18 ){
document.write("<b>Qualifies
for driving</b>");
}else{
document.write("<b>Does
not qualify for driving</b>");
}
//-->
</script>
<p>Set the
variable to different value and then try...</p>
</body>
</html>
Output
Does not qualify for driving
Set the variable
to different value and then try...
if...else if... Statement
The ‘if...else if...’ statement is an
advanced form of if…else that allows
JavaScript to make a correct decision out of several conditions.
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){
Statement(s) to be executed if expression 2 is true }else if (expression
3){
Statement(s) to be executed if expression 3 is true }else{
Statement(s) to be
executed if no expression is true
}
There is nothing
special about this code. It is just a series of if statements, where each if
is a part of the else clause of the
previous statement. Statement(s) are executed based on the true condition, if
none of the conditions is true, then the else
block is executed.
Example
Try the following code to learn how to
implement an if-else-if statement in JavaScript.
<html>
<body>
<script
type="text/javascript">
<!--
var book =
"maths";
if( book == "history" ){ document.write("<b>History
Book</b>");
}else if( book == "maths" ){
document.write("<b>Maths Book</b>");
}else if( book == "economics" ){
document.write("<b>Economics Book</b>");
}else{
document.write("<b>Unknown
Book</b>");
}
//-->
</script>
<p>Set the
variable to different value and then try...</p>
</body>
</html>
Output
Maths Book
Set the variable
to different value and then try...
0 comments:
Post a Comment