undefined vs null — Subtle Differences

TL;DR

Explanation

undefined: default value of unassigned variables, missing object properties, missing function parameters, void function return. null: explicit absence, intentional empty value. Key differences: typeof undefined === 'undefined', typeof null === 'object' (JS bug from 1995, can't fix). null == undefined (true), null === undefined (false). JSON.stringify converts undefined to nothing, null to 'null'. Optional chaining: obj?.prop returns undefined for missing, not null. Best practice: use null for intentional absence, never assign undefined explicitly.

Common Misconception

Why It Matters

Common Mistakes

• Assigning undefined explicitly — use null for intentional absence.
• Using == instead of === which treats null and undefined as equal.
• Forgetting that JSON.stringify drops undefined values but preserves null.

Code Examples

// Both are falsy but behave differently:
const a = null;
const b = undefined;
console.log(a == b);   // true
console.log(a === b);  // false
console.log(JSON.stringify({a, b})); // {"a":null}  (b dropped!)

// Explicit null for intentional absence:
const user = fetchUser() ?? null; // null if not found

// Strict null checks:
if (value === null) { /* intentionally empty */ }
if (value === undefined) { /* not yet set */ }
if (value == null) { /* either null or undefined */ }

// TypeScript helps:
function getUser(id: string): User | null { }

References

• ↗ https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/null

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