undefined vs not defined in JavaScript:

undefined:
- In JavaScript, undefined is a primitive value and a type. It is the default value assigned to variables that are declared but not yet assigned any value.
- The JavaScript engine automatically assigns undefined when memory is allocated to a variable. This happens during the memory allocation phase.
- undefined is placeholder which kept inside variable
- Example:
```
  var x; // Declaration
  console.log(x); // undefined (no value assigned yet)
```
not defined:
- This refers to a variable that has not been declared at all. When you try to access such a variable, you’ll get a ReferenceError.
- Example:
```
  console.log(y); // ReferenceError: y is not defined
```
- Here, y is not declared anywhere, so trying to access it will result in a runtime error.

Differences between undefined and an empty value:

Empty value:
- An empty value can be null or an empty string (""), but undefined is different.
- undefined indicates that a variable has been declared but hasn’t been assigned any value yet.
- An empty string ("") is a valid value that is assigned to a variable.

Example:

    var a = "";
    console.log(a); // Output: ""

So, undefined is not the same as an empty value or null.

JavaScript's Dynamic Typing:

JavaScript is a loosely typed language, meaning variables don’t have fixed types. we can assign different types of values to a variable after its declaration. Here’s an example:

var x;  // Declare x without assigning a value
console.log(x);  // Output: undefined

x = 100;  // Assign a number to x
console.log(x);  // Output: 100

x = "Sujay";  // Assign a string to x
console.log(x);  // Output: Sujay

Bad Practice:

Assigning undefined manually to a variable is not recommended:

var a = undefined;  // Not a good practice

Important: Even though undefined is a value, it should not be manually assigned to variables. It is considered bad practice because it makes code harder to understand and debug.