What is Undefined?

In programming, the value undefined is a special value that indicates that a variable has not been assigned a value yet. It is different from the value null, which is a special value that represents an intentional absence of a value.

In JavaScript, the undefined value is automatically assigned to variables that have not been declared or initialized.

Examples of Undefined

• A variable that has not been declared:

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

• A variable that has been declared but not initialized:

let y = undefined;
console.log(y); // undefined

• A property of an object that has not been defined:

const obj = {};
console.log(obj.name); // undefined

• The return value of a function that does not return a value:

function myFunction() {
  // Does not return anything
}

const result = myFunction();
console.log(result); // undefined

Difference Between Undefined and Null

| Feature | Undefined | Null |
|—|—|—|
| Value | Special value indicating that a variable has not been assigned a value | Special value representing an intentional absence of a value |
| Assignment | Automatically assigned to variables that have not been declared or initialized | Can be explicitly assigned to variables |
| Equality | undefined === null is false | undefined == null is true (in some contexts) |

Conclusion

The undefined value is a useful tool in programming that can help you identify variables that have not been assigned a value. It is important to understand the difference between undefined and null, as they are two distinct values with different meanings.