What is the difference between null and undefined? | Interview Question

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

01

Understand 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.

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02

Attempt it yourself

Sketch your approach before reading the solution — that's what interviews test.

Nudge consolestandby

Stuck? Beam a request up — the console returns a conceptual nudge that guides your logic without spoiling the implementation.

03

Study the 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

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Solution ready — 2 min read

Classified // press E to declassify

04

Run the code

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

Practice

05

Join the discussion

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