It's important to distinguish between undefined and undeclared variables. Also, to is the one of most asked questions in the interview.
For Example
let message;
console.log(message) // undefined
Undefined variables are those variables which is been not initialized with a value. In the above example, we can see that the message a variable has been declared and console logged but the message has not been initialized.
Whereas, Undeclared variables are those variables which are been called without declaring a variable.
console.log(area) // Reference Error: area is not defined