undefined#
undefined is the default value of an uninitialised binding,
a missing function argument, a returnless function, and a
property access on an absent key. typeof undefined is
"undefined".
let x;
typeof x; // "undefined"
A function that never returns yields undefined.
function noop() {}
noop(); // undefined
Missing properties read as undefined (no exception).
({}).missing; // undefined
Prefer === undefined for explicit checks; == null covers
both null and undefined in one comparison.