## What is Undefined?
In programming, undefined is a value that has not been assigned or initialized. It is often used as a default value for variables that have not yet been defined. When a variable is declared without an initial value, it is automatically assigned the value of undefined.
Undefined is different from null, which is a value that explicitly represents the absence of a value. Null is used to indicate that a variable has been intentionally set to have no value, while undefined indicates that a variable has not yet been assigned a value.
### How to Check if a Variable is Undefined
You can use the `typeof` operator to check if a variable is undefined. The `typeof` operator returns the type of a variable, and if the variable is undefined, it will return the string “undefined”.
“`
let x;
console.log(typeof x); // “undefined”
“`
### How to Use Undefined
Undefined is most commonly used as a default value for variables that have not yet been defined. For example, the following code declares a variable `x` without an initial value:
“`
let x;
“`
If we try to access the value of `x`, we will get the error “TypeError: Cannot read property ‘value’ of undefined”. This is because `x` has not yet been assigned a value.
We can avoid this error by using the `undefined` value as a default value:
“`
let x = undefined;
“`
Now, if we try to access the value of `x`, we will get the value `undefined`. This is because `x` has been assigned the `undefined` value, which indicates that it has not yet been assigned a value.
### Conclusion
Undefined is a special value that indicates that a variable has not yet been assigned a value. It is different from null, which indicates that a variable has been intentionally set to have no value. You can use the `typeof` operator to check if a variable is undefined. Undefined is most commonly used as a default value for variables that have not yet been defined.