Intern / FresherJavaScript
What is the difference between null and undefined in JavaScript?
Quick answer
undefined means a variable has been declared but no value was assigned; null is an explicit value meaning 'no value' that you assign on purpose.
JavaScript uses undefined automatically: for uninitialised variables, missing function arguments, missing object properties and functions with no return statement. null is something a developer sets deliberately to say that a value is intentionally empty.
typeof undefined is 'undefined', while typeof null is 'object', a long-standing bug in the language that cannot be fixed without breaking the web. Use value == null to test for both, or optional chaining and nullish coalescing (user?.name ?? 'Guest') to handle them safely.
let a; // undefined
const b = null; // null
typeof a; // "undefined"
typeof b; // "object" (historical bug)
a == b; // true
a === b; // false
const name = user?.name ?? "Guest";