JavaScript Prototype Chain — How Property Lookup Works
Fast recall
Quick revision
Core answer
Prototype chain traversal for property lookup, Object.prototype at the top, null as the terminator, performance implications
Confusing prototype with __proto__ — prototype is a property of constructor functions; __proto__ is the reference each object uses to chain
Discuss property shadowing, performance cost of deep chains, hidden classes in V8, and how Object.create(null) makes a prototype-less object
Interview-ready guide
Interview answer
Open (15–20 seconds): Restate the question and give the one-sentence definition.
Body (45–60 seconds): Walk through Prototype chain traversal for property lookup, Object.prototype at the top, null as the terminator, performance implications as a story: what runs, what breaks, what you log/measure.
Pitfall (15 seconds): Call out Confusing prototype with __proto__ — prototype is a property of constructor functions; __proto__ is the reference each object uses to chain — interviewers reward honesty about real bugs.
Finish (15–20 seconds): Add Discuss property shadowing, performance cost of deep chains, hidden classes in V8, and how Object.create(null) makes a prototype-less object, then offer a trade-off question back (“do you optimize for TTFB or TTI here?”).
JavaScript Prototype Chain — How Property Lookup Works
Direct Answer
What is the prototype chain in JavaScript? How does property lookup work?
In one line: name the mechanism, where it applies (the type checker, emit pipeline, and how types line up with runtime JS), and the user-visible outcome.
Interviewers dig into: Prototype chain traversal for property lookup, Object.prototype at the top, null as the terminator, performance implications.
Classic weak answer: Confusing prototype with proto — prototype is a property of constructor functions; proto is the reference each object uses to chain.
Senior finish: Discuss property shadowing, performance cost of deep chains, hidden classes in V8, and how Object.create(null) makes a prototype-less object.
Common Mistakes & Pitfalls
Confusing prototype with proto — prototype is a property of constructor functions; proto is the reference each object uses to chain
Check your understanding
Answer these without looking back at the article.
- What is the difference between __proto__ and prototype?
- How do you set the prototype of an existing object?
- Why is modifying Object.prototype dangerous?
- How does the prototype chain relate to ES6 class inheritance?
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