JavaScript If ... Else Statements

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Conditional statements in JavaScript are used to perform different actions based on different conditions.

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 the following conditional statements:

If Statement

You should use the if statement if you want to execute some code only if a specified condition is true.

Syntax

if (condition)
{
  code to be executed if condition is true
}

Note that if is written in lowercase letters. Using uppercase letters (IF) will generate a JavaScript error!

Example

<script type="text/javascript">
//Write "Your Number is Negative" if number is less than 0
//Write "Your Number is Null" if number is 0
//Write "Your Number is Positive" if number is greater than 0

var number=50-Math.floor(100*Math.random())

if (number < 0)
{
document.write("<b>Your Number is Negative</b>: " + number)
}

if (number == 0)
{
document.write("<b>Your Number is Null</b>: " + number)
}

if (number > 0)
{
document.write("<b>Your Number is Positive</b>: " + number)
}

</script>

Note: When comparing variables you must always use two equals signs next to each other (==)!

Notice that there is no ..else.. in this syntax. You just tell the code to execute some code only if the specified condition is true.

If ... else Statement

If you want to execute some code if a condition is true and another code if the condition is not true, use the if ... else statement.

Syntax

if (condition)
{
code to be executed if condition is true
}
else
{
code to be executed if condition is not true
}

Example

<script type="text/javascript">
//Write "Your Number is Negative" if number is less than 0
//          and
//Write "Your Number is NOT Negative" if number is Not less than 0

var number=50-Math.floor(100*Math.random())

if (number < 0)
{
document.write("<b>Your Number is Negative</b>: " + number)
}

else
{
document.write("<b>Your Number is NOT Negative</b>: " + number)
}

</script>

 

If ... else if ... else Statement

You should use the if ... else if ... else statement if you want to select one of many sets of lines to execute.

Syntax

if (condition1)
{
code to be executed if condition1 is true
}
else if (condition2)
{
code to be executed if condition2 is true
}
else
{
code to be executed if condition1 and
condition2 are not true
}

Example

<script type="text/javascript">
//Write "Your Number is Negative" if number is less than 0
//Write "Your Number is Null" if number is 0
//Write "Your Number is Positive" if number is greater than 0

var number=50-Math.floor(100*Math.random())

if (number < 0)
{
document.write("<b>Your Number is Negative</b>: " + number)
}

else if (number == 0)
{
 document.write("<b>Your Number is Null</b>: " + number)
}

else
{
document.write("<b>Your Number is Positive</b>: " + number)
}

</script>

 

Try it

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