Difference between null, undefined and not defined in javascript

What is "undefined"? In JavaScript, "undefined" is a primitive value automatically assigned to variables that have been declared but have not been initialized with a value. It also represents the value returned by functions that do not explicitly return anything. Essentially, it denotes the absence of a meaningful value. Example let x; console.log(x); // Output: undefined function doSomething() { // Kuch bhi return nahi kiya gaya } console.log(doSomething()); // Output: undefined ``` `` # Not defined - In JavaScript Not Defined is a ReferenceError that occurs when you try to use a variable, function, or object property that has not been declared or created in your code. This error is often caused by typographical mistakes or using the wrong variable name. `` ```javascript console.log(a); var a = 5; console.log(a); console.log(b); // // ReferenceError: b is not defined ``` `` # What is null in JavaScript? - In JavaScript, null is a special value that represents an empty or unknown value. It is often used to indicate that a variable or object property has no value or is explicitly empty. Unlike undefined, null is a deliberate assignment to a variable to indicate that it has no value.

Mar 21, 2025 - 04:39

Difference between null, undefined and not defined in javascript

What is "undefined"?

In JavaScript, "undefined" is a primitive value automatically assigned to variables that have been declared but have not been initialized with a value. It also represents the value returned by functions that do not explicitly return anything. Essentially, it denotes the absence of a meaningful value.

Example

let x;
console.log(x); // Output: undefined

function doSomething() {
    // Kuch bhi return nahi kiya gaya
}
console.log(doSomething()); // Output: undefined

```

``


# Not defined

- In JavaScript Not Defined is a ReferenceError that occurs when you try to use a variable, function, or object property that has not been declared or created in your code. This error is often caused by typographical mistakes or using the wrong variable name.


``

```javascript
console.log(a); 
var a = 5; 
console.log(a); 
console.log(b);  //  // ReferenceError: b is not defined

```

``

# What is null in JavaScript?


- In JavaScript, null is a special value that represents an empty or unknown value. It is often used to indicate that a variable or object property has no value or is explicitly empty. Unlike undefined, null is a deliberate assignment to a variable to indicate that it has no value.