# 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:
        
        ```plaintext
        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:
        
        ```plaintext
        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**.
        

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### **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:
    
    ```plaintext
    var a = "";
    console.log(a); // Output: ""
    ```
    
    So, undefined is not the same as an empty value or null.
    

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### **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:

```plaintext
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
```

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### **Bad Practice**:

Assigning undefined manually to a variable is not recommended:

```plaintext
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.

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