What are object wrappers for primitive type?

0-2 years experience

1. 1If you receive a JSON payload where a numeric field is sometimes a string, how would you use Number or the Number wrapper to ensure you have a proper number before calculations?
2. 2What will console.log(typeof new Boolean(false)) output and why? How does it differ from typeof false?
3. 3When you call 'hello'.length, JavaScript temporarily creates a wrapper object. Explain what would happen if you tried to add a new property to that string, e.g., 'hello'.custom = 5; and then read it back.

2-5 years experience

1. 1Your team noticed that a function sometimes throws 'TypeError: Cannot read property "toFixed" of undefined' when processing user input. The input may be a string or a number. Walk through how primitive wrappers could be causing this and how you'd fix it.
2. 2In a codebase you see code like `if (new Number(value) > 10)`. Discuss the pros and cons of using the Number wrapper here versus a plain numeric conversion, and what subtle bugs might arise.
3. 3During a performance profiling session you see many allocations of Number objects. Explain why that might be happening and how you would refactor to reduce GC pressure.

5-8 years experience

1. 1You are designing a logging library that must accept any primitive value and preserve its type when serializing. How would you leverage object wrappers to detect the original primitive type without losing information?
2. 2A large microservice processes billions of numeric calculations per day. The current implementation uses `new Number()` in several hot loops. Evaluate the impact on latency and memory, and propose a migration plan.
3. 3When integrating with a third‑party API that expects objects with methods like `toString` and `valueOf`, you need to wrap primitive values. Describe how you'd design a utility that safely creates wrapper objects only when needed, avoiding accidental prototype pollution.

8+ years experience

1. 1Your organization is moving from a legacy codebase that heavily relies on primitive wrapper objects (e.g., `new Boolean`, `new String`) to a modern TypeScript codebase with strict typing. Outline an architectural migration strategy, including tooling, lint rules, and runtime compatibility concerns.
2. 2Several teams share a common utility library that provides functions like `isNumber(value)` which internally uses `Object.prototype.toString.call(value)`. Discuss the trade‑offs of using wrapper detection versus `typeof` checks in a polyglot environment where some services are written in Node.js and others in Deno.
3. 3Imagine you need to design a serialization format that can round‑trip both primitive values and their wrapper objects, preserving the distinction (e.g., differentiate between `5` and `new Number(5)`). How would you approach this at the system level, considering backward compatibility and performance?