DAY -5
Control statement
When we write code for a particular
program, we sometimes takes various decisions for executing different action. These
can be done through conditional / control statement.
JavaScript has the following control
statement:
1.
If statement
2.
Switch statement
JavaScript - 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
Ø The syntax for a basic if statement is as follows −
if(expression){
Statement(s) to be executed if expression istrue
}
Ø The 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.
Example
<html>
<body>
<scripttype="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
if...else statement:
Ø Syntax
if(expression){
Statement(s) to be executed if expression istrue
}
else{
Statement(s) to be executed if expression isfalse
}
Ø The 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.
Ø Example
<html>
<body>
<scripttype="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
if...else if... statement
Ø The if...else
if... statement is an advanced form of if…else that allows JavaScript to make a correct decision out
of several conditions.
Ø 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.
Ø Syntax
if(expression 1){
Statement(s) to be executed if expression 1istrue
}
elseif(expression 2){
Statement(s) to be executed if expression 2istrue
}
elseif(expression 3){
Statement(s) to be executed if expression 3istrue
}
else{
Statement(s) to be executed ifno expression istrue
}
Ø Example
<html>
<body>
<scripttype="text/javascript">
<!--
var book ="maths";
if( book =="history"){
document.write("<b>History
Book</b>");
}
elseif( book =="maths"){
document.write("<b>Maths
Book</b>");
}
elseif( 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
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