Skip to content

Difference between undefined and null in JavaScript | Example code

  • by

The difference between undefined and null is undefined means a variable has been declared but has not yet been assigned a value, where null is an assignment value. It can be assigned to a variable as a representation of no value.

undefined: It means that the object doesn’t have any value, therefore undefined. This occurs when you create a variable and don’t assign a value to it.

null: It means that the object is empty and isn’t pointing to any memory address.

#UndefinedNull
Valueundefinednull
Typeundefinedobject
OccurrenceOccurs when a variable has been declared but has not been assigned a value.Occurs when a developer intentionally sets a variable with no value.
UsageTypically represents a variable that has not been initialized or a non-existent property of an object.Typically represents a deliberate absence of any object value or the result of an operation that intentionally does not return a value.
Equality CheckWhen compared with double or triple equals, it returns true when compared with null or undefined.When compared with double or triple equals, it returns true only when compared with null.
Object ReferenceCannot be assigned as a property of an object, and when an attempt is made to do so, it will result in an error.Can be assigned as a property of an object, and will not result in an error.
Default ValueWhen a function parameter is not passed a value, its default value is undefined.When assigning a default value to a function parameter, null can be used to represent an intentional absence of a value.

Difference between undefined and null in JavaScript

From the simple example code, it is clear that undefined and null are two distinct types: undefined is a type itself (undefined) while null is an object.

<!DOCTYPE html>
<html>
<body>

  <script>
    var testVar;
    console.log(testVar); 
    console.log(typeof testVar); 

    var testVar = null;
    console.log(testVar); 
    console.log(typeof testVar); 

  </script>

</body>
</html> 

Output:

Difference between undefined and null in JavaScript

How to check for an undefined or null variable in JavaScript?

Answer: The most efficient way to test for “value is null or undefined” is

if ( some_variable == null ){
  // some_variable is either null or undefined
}

So these two lines are equivalent:

if ( typeof(some_variable) !== "undefined" && some_variable !== null ) {}
if ( some_variable != null ) {}

Comment if you have any doubts or suggestions on this JS undefined and null topic.

Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.

OS: Windows 10

Code: HTML 5 Version

Leave a Reply

Discover more from Tutorial

Subscribe now to keep reading and get access to the full archive.

Continue reading