How does JavaScript handle circular references in object prototypes?

0-2 years experience

1. 1If you create an object that sets its prototype to itself, what will happen when you try to access a property that isn’t on the object?
2. 2Write a small snippet that creates two objects whose prototypes reference each other. What does `objA.__proto__ === objB` evaluate to, and why?

2-5 years experience

1. 1We noticed a memory leak after adding a feature that builds a prototype chain dynamically. The chain accidentally creates a circular prototype reference. How would you detect and fix it?
2. 2During a refactor, a colleague changed a class’s `extends` logic and now `instanceof` checks are failing. Explain how a circular prototype could cause that and what steps you’d take to debug.

5-8 years experience

1. 1Our UI library builds component prototypes at runtime based on plugins. Some plugins can be loaded in any order, and we’ve seen occasional stack overflows when traversing the prototype chain. How would you redesign the prototype handling to avoid circular references while keeping the plugin system flexible?
2. 2In a large codebase we need to serialize objects that may contain prototype cycles for caching. Discuss the trade‑offs of using `JSON.stringify` with a replacer versus a custom serializer that respects prototype links.

8+ years experience

1. 1We are migrating a legacy monolith that heavily relies on prototype inheritance to a modern ES6 class architecture across multiple teams. Some modules have hidden circular prototype links that cause subtle bugs in production. How would you plan a migration strategy that identifies and eliminates these cycles without breaking existing contracts?
2. 2Our company wants to expose a sandboxed scripting environment to third‑party developers. The sandbox must prevent prototype‑pollution attacks, including circular prototype chains that could cause infinite lookups. What architectural safeguards would you put in place at the runtime and API level?