The fundamental difference between
undefined means a variable has been declared (or not declared), but has not yet been assigned a value. On the other hand,
undefined. Another thing,
null is valid in JSON while
undefined is not.
let a; typeof a; // undefined typeof b; // undefined let c = null; typeof c; // object
null are obviously two different types.
undefined is a type itself, while
null is an
null === undefined // false