Undefined

In JavaScript null is “nothing” . It is supposed to be something that doesn’t exist. Unfortunately, in JavaScript, the data type of null is an object. You can consider it a bug in JavaScript that typeof null is an object. It should be null .

The typeof keyword returns “object” for null , so that means a little bit more effort is required. Comparisons can be made: null === null to check strictly for null or null == undefined to check loosely for either null or undefined.

In a JavaScript program, the correct way to check if an object property is undefined is to use the `typeof` operator .26 May 2018

The undefined property indicates that a variable has not been assigned a value, or not declared at all .