What is Undefined in JavaScript?

In JavaScript, the undefined keyword represents the primitive value that is assigned to variables that have not been assigned a value. It is also the initial value of all global variables.

How to Check for Undefined

You can use the typeof operator to check if a variable is undefined. For example:


const myVariable;
console.log(typeof myVariable); // "undefined"


When to Use Undefined

You should use undefined when you want to indicate that a variable has not been assigned a value. For example, you might use undefined as the default value for a function parameter.

Example

The following code shows how to use undefined as the default value for a function parameter:


function myFunction(param = undefined) {
console.log(param);
}

myFunction(); // "undefined"


Conclusion

Undefined is a primitive value in JavaScript that represents a variable that has not been assigned a value. You can use the typeof operator to check if a variable is undefined. You should use undefined when you want to indicate that a variable has not been assigned a value.