What is the prototype chain?

Q: What is the prototype chain?

**Target Audience:** Engineers preparing for precise JavaScript object-model interview questions. **Difficulty:** Medium > **How to read this doc:** Concepts are explained in plain language first, then tagged with 📌 Interview term: . Every result below was actually run on Node.js v24.19.0 — the printed output is real, captured output, not illustrative sample text. This is a focused, definitional look at the lookup mechanism itself; for the broader concept of how prototypal inheritance works and how objects get linked in the first place, see Explain prototypal inheritance. , and for how this same chain powers the instanceof operator, see How does instanceof work? ## 1. Definition 📌 **Interview term:** the **prototype chain** is the complete sequence of [[Prototype]] links, followed from a starting

The problem

Question presented to candidate: "What is the prototype chain, and what actually happens when you access a property that is not directly on an object?"

What a strong answer should cover:

Every object has an internal [[Prototype]] link to another object, or to null -- the prototype chain is the full sequence of those links, followed from an object all the way to null.
When a property is accessed and not found directly on the object, the engine automatically checks the next object up the chain, then the next, until it finds the property or reaches null.
If the chain is exhausted without finding the property, the result is undefined -- accessing a missing property never throws, it just returns undefined after walking the entire chain.
The in operator and for...in walk the chain (checking inherited properties too), while Object.prototype.hasOwnProperty and Object.keys only ever look at an object's own properties, ignoring the chain entirely -- a common source of confusion.
Different kinds of built-in objects have different chain lengths -- a plain object's chain is one hop to Object.prototype then null, while an array's chain is two hops (Array.prototype, then Object.prototype) before null.

Code / implementation expected: Yes -- a runnable snippet walking a real, multi-level chain and showing the in vs hasOwnProperty vs Object.keys distinction with real output.

prototype

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 precise JavaScript object-model interview questions. Difficulty: Medium

How to read this doc: Concepts are explained in plain language first, then tagged with 📌 Interview term:. Every result below was actually run on Node.js v24.19.0 — the printed output is real, captured output, not illustrative sample text. This is a focused, definitional look at the lookup mechanism itself; for the broader concept of how prototypal inheritance works and how objects get linked in the first place, see Explain prototypal inheritance., and for how this same chain powers the instanceof operator, see How does instanceof work?

1. Definition

📌 Interview term: the prototype chain is the complete sequence of [[Prototype]] links, followed from a starting object, object by object, until reaching null. Property lookup walks this chain automatic

The solution is hidden so you can try first

6 min read · 8 sections

Run the code

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

JavaScriptWalking a real 3-hop chain, chain-length differences, and in vs hasOwnProperty vs Object.keys (run directly)