Live Demo: Javascript Check If a Variable is Undefined or Null

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Javascript Check If a Variable is Undefined or Null</title>
</head>
<body>
 Note: Press F12 key on the keyboard and click on the "console" tab to view the console output. Refresh the page or click the "Show Output" button to run the test again.
 <hr>
	<script> 
 var firstName;
 var lastName = null;
 // Try to get non existing DOM element
 var comment = document.getElementById('comment');
 
 console.log(firstName); // Print: undefined
 console.log(lastName); // Print: null
 console.log(comment); // Print: null
 
 console.log(typeof firstName); // Print: undefined
 console.log(typeof lastName); // Print: object
 console.log(typeof comment); // Print: object
 
 console.log(null == undefined) // Print: true 
 console.log(null === undefined) // Print: false
 
 /* Since null == undefined is true, the following statements will catch both null and undefined */
 if(firstName == null){
 document.write('Variable "firstName" is undefined.');
 } 
 if(lastName == null){
 document.write('Variable "lastName" is null.');
 }
 
 /* Since null === undefined is false, the following statements will catch only null or undefined */
 if(comment === undefined) {
 document.write('Variable "comment" is undefined.');
 } else if(comment === null){
 document.write('Variable "comment" is null.');
 }
 </script>
</body>