Difference between JavaScript variable that is `null`, `undefined` or undeclared

null

An explicitly empty value (set intentionally to mean “nothing”).

• Primitive type.
• Usually assigned by developers to mean “empty”.
• typeof null → "object" (legacy bug in JS).

undefined

A variable that’s declared but not yet assigned a value.

• Default value for declared variables not initialized.
• Also default return from functions with no return.

Undeclared

A variable that’s never declared at all.

• Variable never declared with var, let, or const.
• Direct access → ReferenceError.
• Safe check via typeof someVar === "undefined".

let a = null;        // null → explicitly empty
let b;               // undefined → declared but not assigned
// console.log(c);   // undeclared → ReferenceError

console.log(a); // null
console.log(b); // undefined

console.log(typeof a); // "object" (quirk)
console.log(typeof b); // "undefined"
console.log(typeof c); // "undefined" (typeof avoids ReferenceError)