What is the difference between null and undefined?

Q: What is the difference between null and undefined?

**Target Audience:** Engineers preparing for JavaScript fundamentals interviews. **Difficulty:** Easy > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term: . Every check below was actually run in Node. ## 1. Why This Even Matters — A Story First Think of a form field on a job application. A field that was never shown to the applicant at all is like `undefined` — nobody had the chance to fill it in. A field that was shown, and the applicant deliberately left it blank on purpose (say, "middle name: N/A") is like `null` — someone actively decided there is nothing there. Both look "empty," but one is an accident of omission and the other is a deliberate statement. ## 2. The Core Idea 📌 **Interview term:** `undefined` is the engine's own default for "nothing has been assigned yet." `null` is a value a developer assigns on purpose to represent an intentional absence. ## 3. Verified: typeof, coercion, and where each

The problem

Question presented to candidate: "What's the difference between null and undefined, and when would you deliberately choose to use one over the other in your own code?"

What a strong answer should cover:

📌 Interview term: undefined — the value JavaScript automatically assigns to a variable that has been declared but not yet given a value, to a missing function argument, or to accessing a property that does not exist on an object. It represents an implicit, unintentional absence.
📌 Interview term: null — a value a developer explicitly assigns to represent "no value" or "empty" on purpose. It is an intentional absence, set deliberately by code, not automatically by the engine.
📌 Interview term: the famous typeof null bug — typeof null returns "object", a historical bug preserved for backward compatibility since fixing it would break the web. Verified directly: typeof undefined correctly returns "undefined", but typeof null does not return "null".
A precise answer names null + 1 (which is 1, since Number(null) is 0) versus undefined + 1 (which is NaN, since Number(undefined) is NaN) — verified directly, a real, sharp distinction in numeric coercion.
A precise answer names JSON.stringify behavior: a property with value undefined is genuinely dropped from the output entirely, while a property with value null is kept — verified directly. This is a real, practical reason APIs often use null over undefined for "explicitly no value" fields.

Clarifying questions expected:

None — this is a definitional/comparison question; naming the intentional-vs-implicit distinction plus the coercion/serialization differences is the strong signal.

Code / implementation expected: Optional — showing the typeof and JSON.stringify contrasts directly demonstrates real understanding.

null
undefined

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Engineers preparing for JavaScript fundamentals interviews. Difficulty: Easy

How to read this doc: Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every check below was actually run in Node.

1. Why This Even Matters — A Story First

Think of a form field on a job application. A field that was never shown to the applicant at all is like undefined — nobody had the chance to fill it in. A field that was shown, and the applicant deliberately left it blank on purpose (say, "middle name: N/A") is like null — someone actively decided there is nothing there. Both look "empty," but one is an accident of omission and the other is a deliberate statement.

2. The Core Idea

📌 Interview term: undefined is the engine's own default for "nothing has been assigned yet." null is a value a developer assigns on purpose

The solution is hidden so you can try first

5 min read · 8 sections

Run the code

Edit the code and run it here. Each console line with an expected value becomes a test.

JavaScriptReal typeof, coercion, and JSON.stringify differences between null and undefined