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Difference Between Undefined And Null In JavaScript

Here, I will explain the difference between Undefined and Null. If you are working with JavaScript then You have to know when we get a variable as undefined and when do we get it as null. 


Undefined : 
This means that the variable is not declared or variable is declared but the value is not assigned.
For example,
var abc;  
alert(abc); //undefined   
alert(typeof abc); //undefined 

In the above code, we are able to see that the variable named "abc" is not assigned any value. So, it means that JavaScript considers the variable as undefined. If we check with alert, it will give undefined. We can check the type of the same variable, which is also undefined because the "abc" variable is never assigned a value. So, the unassigned variables are considered as undefined.


null :
This means that the variable is assigned with a null value.
For example,
var test= null;  
alert(abc); //null  
alert(typeof abc); //object 

In the above code, we are able to see that the "abc" variable is assigned with the null value. When we check it with alert, it will return null and the type of that variable will give "object".

Check variable is null or undefined
  • typeof variaible === "undefined"  //check undefined  variable
  • variable === null  //check null variable
  • (null == undefined) = true //Equality operator considers both as equal.
  • (null === undefined) = false//  Identity not considered as equal.
Finally you have to remember that undefined is type and null is an object. 
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