What are the properties of an object?

0-2 years experience

1. 1If you have const user = {name: 'Alice', age: 30}, how would you add a new property 'email' to this object without using the dot notation?
2. 2What will console.log(Object.keys(user)) output after you delete the 'age' property? Explain why.
3. 3How does JavaScript treat a property that you set to undefined versus a property that doesn't exist on the object?

2-5 years experience

1. 1We have a function that copies properties from one object to another using a for...in loop, but it's unexpectedly also copying prototype properties. How would you fix it?
2. 2During a bug hunt, a property you assumed was a number is actually a string, breaking a calculation. How would you verify and enforce the correct type when the object comes from an API response?
3. 3Explain why Object.defineProperty can be used to make a property read‑only, and show how you'd apply it to protect a configuration object from accidental mutation.

5-8 years experience

1. 1Our caching layer stores objects with many nested properties. We need to serialize them for Redis but also preserve non‑enumerable properties. How would you design a serialization strategy that handles enumerable vs non‑enumerable properties?
2. 2A performance hotspot was traced to repeatedly checking if a property exists on a large object. What are the trade‑offs between using 'in', 'hasOwnProperty', and 'Object.hasOwn' in this scenario, and which would you choose?
3. 3We are refactoring a legacy codebase that relies on mutating object properties directly. How would you introduce immutable patterns while still allowing selective property updates, considering prototype inheritance?

8+ years experience

1. 1Our organization is moving from a monolithic JavaScript service to a micro‑frontend architecture. How would you standardize object property definitions (enumerable, configurable, writable) across teams to avoid subtle bugs when sharing data between services?
2. 2We need to design a library that exposes configuration objects to third‑party developers. What guidelines would you set for property naming, descriptor defaults, and deprecation handling to ensure long‑term maintainability?
3. 3When migrating a large codebase from ES5 to ES6+ modules, how would you audit and refactor usage of dynamic property addition (e.g., obj[prop] = value) to improve tree‑shaking and bundle size?