Explain the concept of 'immutability' in JavaScript

0-2 years experience

1. 1If you need to add an item to an array without mutating the original array, how would you do it in JavaScript?
2. 2What will happen if you try to reassign a property of an object declared with const? Explain why.
3. 3Given the code `let obj = {a:1}; obj = {b:2};` does this violate immutability? Why or why not?

2-5 years experience

1. 1You notice a bug where a Redux reducer is accidentally mutating the state object, causing UI glitches. How would you identify and fix the mutation?
2. 2When using Object.freeze on a nested object, some inner properties still change. Why does this happen and how can you enforce deep immutability?
3. 3A teammate suggests using Array.prototype.push inside a map function for performance. Explain the impact on immutability and what you would recommend.

5-8 years experience

1. 1Our front‑end team shares a large immutable data store across multiple micro‑frontends. What trade‑offs do you consider when choosing a library (e.g., Immer, Immutable.js) versus plain JavaScript for deep immutability at scale?
2. 2During a performance audit you see that copying large objects for every state update is causing GC pressure. How would you redesign the data handling to keep immutability while reducing overhead?
3. 3Explain how you would enforce immutability contracts across a TypeScript codebase without runtime overhead.

8+ years experience

1. 1We are migrating a legacy monolith that heavily mutates shared objects to a new architecture that relies on immutable data streams. What strategy would you propose for incremental migration while keeping the system stable?
2. 2Across several teams, you need to define a company‑wide policy for immutable state. How would you balance developer ergonomics, runtime performance, and long‑term maintainability?
3. 3If you had to design a cross‑service event bus that guarantees immutability of payloads across language boundaries, what patterns and tooling would you choose and why?