JavaScript Conditional Statements
Conditional statements in JavaScript are used to
perform different actions based on different conditions.
Examples
If statement How to write an If statement. Use
this statement if you want to execute a set of code if a specified
condition is true.
If...else statement How to write an If...Else
statement. Use this statement if you want to execute one set of code
if the condition is true and another set of code if the condition is
false.
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Switch statement How to write a switch
statement. Use this statement if you want to select one of many
blocks of code to execute.
Conditional Statements
Very often when you write code, you want to perform different
actions for different decisions. You can use conditional statements
in your code to do this.
In JavaScript we have three conditional statements:
- if statement - use this statement if you want to
execute a set of code when a condition is true
- if...else statement - use this statement if you want to
select one of two sets of lines to execute
- switch statement - use this statement if you want to
select one of many sets of lines to execute
If and If...else Statement
You should use the if statement if you want to execute some code
if a condition is true.
Syntax
if (condition)
{
code to be executed if condition is true
} |
Example
<script type="text/javascript">
//If the time on your browser is less than 10,
//you will get a "Good morning" greeting. var d=new Date()
var time=d.getHours()
if (time<10)
{
document.write("<b>Good morning</b>")
}
</script> |
Notice that there is no ..else.. in this syntax. You just tell
the code to execute some code if the condition is true.
If you want to execute some code if a condition is true and
another code if a condition is false, use the if....else
statement.
Syntax
if (condition)
{
code to be executed if condition is true
}
else
{
code to be executed if condition is false
} |
Example
<script type="text/javascript">
//If the time on your browser is less than 10,
//you will get a "Good morning" greeting.
//Otherwise you will get a "Good day" greeting. var d = new Date()
var time = d.getHours()
if (time < 10)
{
document.write("Good morning!")
}
else
{
document.write("Good day!")
}
</script> |
Switch Statement
You should use the Switch statement if you want to select one of
many blocks of code to be executed.
Syntax
switch (expression)
{
case label1:
code to be executed if expression = label1
break
case label2:
code to be executed if expression = label2
break
default:
code to be executed
if expression is different
from both label1 and label2
} |
This is how it works: First we have a single expression (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.
Example
<script type="text/javascript">
//You will receive a different greeting based
//on what day it is. Note that Sunday=0,
//Monday=1, Tuesday=2, etc. var d=new Date()
theDay=d.getDay()
switch (theDay)
{
case 5:
document.write("Finally Friday")
break
case 6:
document.write("Super Saturday")
break
case 0:
document.write("Sleepy Sunday")
break
default:
document.write("I'm looking forward to this weekend!")
}
</script> |
Conditional Operator
JavaScript also contains a conditional operator that assigns a
value to a variable based on some condition.
Syntax
variablename=(condition)?value1:value2 |
Example
greeting=(visitor=="PRES")?"Dear President ":"Dear " |
If the variable visitor is equal to PRES, then put the string
"Dear President " in the variable named greeting. If the variable
visitor is not equal to PRES, then put the string "Dear " into the
variable named greeting.
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