JavaScript Comparison and Logical Operators

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Comparison and Logical operators are used to test for true or false.

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Comparison Operators

Comparison operators are used in logical statements to determine equality or difference between variables or values.
Given that x=5, the table below explains the comparison operators:

Operator	Description	Example
==	is equal to	x==8 is false
===	is exactly equal to (value and type)	x===5 is true x==="5" is false
!=	is not equal	x!=8 is true
>	is greater than	x>8 is false
<	is less than	x<8 is true
>=	is greater than or equal to	x>=8 is false
<=	is less than or equal to	x<=8 is true

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How Can it be Used

Comparison operators can be used in conditional statements to compare values and take action depending on the result:

if (age<18) document.write("Too young");

You will learn more about the use of conditional statements in the next chapter of this tutorial.

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Logical Operators

Logical operators are used to determine the logic between variables or values.
Given that x=6 and y=3, the table below explains the logical operators:

Operator	Description	Example
&&	and	(x < 10 && y > 1) is true
||	or	(x==5 || y==5) is false
!	not	!(x==y) is true

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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 has the value of "PRES", then the variable greeting will be assigned the value "Dear President " else it will be assigned "Dear".

JavaScript If...Else Statements

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

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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 - use this statement to execute some code only if a specified condition is true
• if...else statement - use this statement to execute some code if the condition is true and another code if the condition is false
• if...else if....else statement - use this statement to select one of many blocks of code to be executed
• switch statement - use this statement to select one of many blocks of code to be executed

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If Statement

Use the if statement 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 a "Good morning" greeting if //the time is less than 10 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 tell the browser to execute some code only if the specified condition is true.