Conditional Statements

It is possible to respond to the user's input in different ways.

In JavaScript there are 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 Statements

You should use the if-then statement if you want to execute some code when the condition is true.

Syntax

if (condition){ 
    statement;
    statement 
}

Example


<script type="text/javascript">
var d=new Date();
var time=d.getHours();

if (time<10) {
    document.write("<b>Good morning</b>");
}
</script>

This executes some code if the condition is true.

if....else

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){
     statement; 
}else{ 
    statement; 
    }

Example


<script type="text/javascript">
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: 
  statement; 
   break;
case label2: 
  statement 
    break;
default: 
  statement; 
  }

This is how it works: First you 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">

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

o
variable_name=(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.