01. It is type of Primitive Data Structure.
02. The undefined data type is a variable that has been declared but has not been assigned a value.
03. It indicates the absence of a value, whether intentional or unintentional.
04. Also if the variable is not declared, that will also come under "undefined" data structures.
05. Example
let age;
console.log(age);
// Output: undefined
06. Explanation
The variable 'age' is declared but not assigned a value, so it has the default value of undefined.
