What is the difference between a variable that is: null, undefined or undeclared?

Q: What is the difference between a variable that is: null, undefined or undeclared?

Direct distinction undefined : declared, no assigned value yet. null : explicitly set to no value. undeclared : name does not exist in current scope. Where undefined appears naturally Practical rule Use null when you intentionally clear/reset a value. Treat undefined as "not provided yet". Treat undeclared as a bug (scope/typo/load-order). Common pitfalls Using truthy checks and accidentally treating 0/false/'' as missing. Assuming typeof null should be 'null'. Catching undeclared errors instead of fixing declaration/scope.

HighEasyJavascript

By FrontendAtlas Team · Updated Mar 8, 2026

Quick Answer

undefined means declared but currently no value, null means intentionally empty, and undeclared means the identifier does not exist in scope. Treat them differently in logic and debugging.

Answer

Direct distinction

undefined: declared, no assigned value yet.
null: explicitly set to no value.
undeclared: name does not exist in current scope.

State	Example	Read result
undefined	`let x; console.log(x)`	`undefined`
null	`let user = null; console.log(user)`	`null`
undeclared	`console.log(missingVar)`	ReferenceError

Same "empty-ish" feeling, very different runtime meaning.

JAVASCRIPT

let a;
console.log(a); // undefined

let b = null;
console.log(b); // null

// console.log(c); // ReferenceError: c is not defined

Where undefined appears naturally

JAVASCRIPT

const obj = {};
console.log(obj.age); // undefined (missing property)

function greet(name) {
  console.log(name); // undefined when omitted
}
greet();

JAVASCRIPT

console.log(undefined == null);  // true
console.log(undefined === null); // false
console.log(typeof null);        // 'object' (historical JS quirk)

Practical rule
Use null when you intentionally clear/reset a value. Treat undefined as "not provided yet". Treat undeclared as a bug (scope/typo/load-order).

Common pitfalls

Using truthy checks and accidentally treating 0/false/'' as missing.
Assuming typeof null should be 'null'.
Catching undeclared errors instead of fixing declaration/scope.

Still so complicated?

undefined = empty box that exists. null = box intentionally marked "empty". undeclared = no box at all.

Summary

undefined is a value.
null is an intentional value choice.
undeclared is usually a coding error.
Use strict equality when comparing these states.