On getting the result of the following, you can find whether a variable is null or undefined. If the result is “false”, it means the variable is null and undefined.
Here, the variable results in “True” −
<html> <body> <script> var age = 10; if(age){ document.write("True"); } else{ document.write("False"); } </script> </body> </html>
Use “typeof” to check whether a variable exists or not −
<html> <body> <script> var age = 10; if( typeof age !== 'undefined' ) { document.write("True"); } else{ document.write("False"); } </script> </body> </html>