What is undefined in JavaScript?

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.

A variable that has not been assigned a value is automatically assigned the value undefined. For example:

“`javascript
let myVariable;
console.log(myVariable); // undefined
“`

undefined can also be used to explicitly assign a value to a variable, indicating that the variable does not have a defined value. For example:

“`javascript
let myVariable = undefined;
console.log(myVariable); // undefined
“`

undefined is also returned by functions when they do not explicitly return a value. For example:

“`javascript
function myFunction() {
// Do something
}

console.log(myFunction()); // undefined
“`

undefined can be used to check if a variable has been assigned a value. For example:

“`javascript
if (myVariable === undefined) {
// The variable has not been assigned a value
}
“`

However, it is important to note that undefined is not the same as null. null is a special value that represents an intentional absence of a value, while undefined represents the absence of a value because the variable has not been assigned a value.

In general, it is good practice to avoid using undefined as a value in your code. This is because it can be difficult to track down and fix bugs caused by undefined values. If you need to represent the absence of a value, it is better to use null instead.