What happens when you try to access a property that doesn't exist on an object?

0-2 years experience

1. 1If you have const user = {name: 'Alice'}; what will console.log(user.age) output and why?
2. 2Write a one‑line snippet that safely reads a nested property like user.profile.bio without throwing an error when any part is missing.

2-5 years experience

1. 1You’re debugging a feature where a UI component crashes because it expects data.title but the API sometimes omits title. Explain why accessing data.title results in undefined and how you’d guard against it.
2. 2During a code review you notice a colleague wrote if (obj.prop) { … } assuming missing properties throw. How would you explain the actual behavior and suggest a more reliable check?

5-8 years experience

1. 1Design a utility function that logs a warning the first time a missing property is accessed on a large shared object, without impacting performance. What trade‑offs do you consider?
2. 2In a high‑traffic Node.js service, you need to validate incoming JSON payloads. How does relying on JavaScript’s undefined for missing fields affect error handling and monitoring at scale?

8+ years experience

1. 1Your organization is migrating several microservices from loosely typed JavaScript objects to a typed schema (e.g., using TypeScript or JSON Schema). How would you approach handling existing code that frequently accesses non‑existent properties, and what architectural changes would you recommend to prevent silent undefined bugs?
2. 2When designing a public SDK, you must decide how to surface missing optional fields to SDK consumers. Discuss the pros and cons of returning undefined versus throwing errors or using sentinel objects, considering backward compatibility and developer experience.