ความแตกต่างระหว่าง null และ undefined คืออะไร?

let a;                 // undefined — declared, not assigned
const obj = {};
obj.missing;           // undefined — property doesn't exist
function f() {}
f();                   // undefined — no return

const b = null;        // null — you chose to empty it

null == undefined;   // true  — loose equality treats them as equal
null === undefined;  // false — different types
typeof undefined;    // "undefined"
typeof null;         // "object"  (historical bug)

const value = input ?? "default"; // defaults on EITHER null or undefined
user?.address?.city;               // optional chaining — undefined if any link is null/undefined