A variable used in the code which is not already declared with any keyword like var, let and const is called undeclared variable.
When we use it in the code gives an error.If we use typeof operator to get the value of it then return 'undefined' value.
console.log(r); // output as an error - Uncaught ReferenceError: r is not defined
console.log(typeof d); // undefined
The undefined type has only one value, which is the special value undefined.
When a variable is declared using var or let but not initialized, It is assigned the value of undefined.
var a; let b; console.log(a); // undefined console.log(b); // undefined
The Null type is the second data type that has only one value: the special value null. Logically, a null value is an empty object pointer, which is why typeof returns “object” when it ’s passed a null value
var x = null; console.log(typeof x); //object