What is undefined?

In JavaScript, undefined is a primitive value that represents the absence of a value.
It is one of the two falsy values in JavaScript, the other being null.

There are a few ways to get an undefined value:

• Declaring a variable without assigning a value to it:

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

• Accessing a property of an object that doesn’t exist:

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

• Calling a function without arguments:

function foo() {
  console.log(arguments); // undefined
}
foo();
    

undefined can be a useful value to check for when you’re not sure if a value has been assigned to a variable or property.

For example, the following code checks if the username variable has been assigned a value before using it:

let username;
if (username) {
  // Do something with the username
} else {
  // The username is undefined, so do something else
}
    

It’s important to note that undefined is not the same as null. null is a falsy value that represents a null object, while undefined represents the absence of a value.

Conclusion

undefined is a primitive value in JavaScript that represents the absence of a value. It can be a useful value to check for when you’re not sure if a value has been assigned to a variable or property.