What is Undefined in JavaScript?

In JavaScript, undefined is a primitive value that represents the absence of a value. It is one of the six primitive values in JavaScript, along with null, boolean, number, string, and symbol.

When is Undefined Used?

undefined is used in the following scenarios:

• When a variable is declared but not assigned a value.
• When a function parameter is not passed an argument.
• When a property of an object is not set.
• When a method returns no value.

Checking for Undefined

You can check if a value is undefined using the typeof operator. The typeof operator returns the type of a value, and it will return "undefined" for undefined values.

For example:


let myVariable;
console.log(typeof myVariable); // Logs "undefined"


Comparison with Null

undefined is often confused with null, but they are not the same thing. null is a special value that represents the intentional absence of a value, while undefined represents the absence of a value due to the lack of assignment.

For example:


let myNullVariable = null;
let myUndefinedVariable;
console.log(myNullVariable === myUndefinedVariable); // Logs false


Conclusion

undefined is a primitive value in JavaScript that represents the absence of a value. It is used in a variety of scenarios, and it is important to understand how it works in order to write effective JavaScript code.