Difference between undefined and not defined in JavaScript | Code

Let’s discuss first what are the undefined and not defined in JavaScript, then we will examples of it.

A undefined is a keyword in JavaScript, that has a special meaning. Everything which gets a space in memory will contain undefined until we assign a value to that memory space.

Where not defined is one of the reference errors that JavaScript will throw when someone accesses the variable which is not inside the memory heap

Difference between undefined and not defined in JavaScript

Simple example code.

undefined

Access variable in cosnloe log before define it.

<script>

   console.log(a);
   var a = 100;
   console.log(a);

</script>

Output:

not defined

At the last line when JavaScript encounters the “console.log(b)” it searches for “b” inside the memory heap of execution context but it is not available.

<script>

   console.log(a);
   var a = 100;
   console.log(a);
   console.log(b);

</script>

Output: Uncaught ReferenceError: b is not defined

undefined	not defined
It works like when we declared a variable in the code but did not assign the value before printing the variable value	It works like when we did not declare the variable and try to call that variable.

Do comment if you have any doubts or suggestions on this JS Undefined topic.

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

OS: Windows 10

Code: HTML 5 Version