Is modifying or extending built-in object prototypes recommended (e.g., adding a method to `Array.prototype`)? Why or why not?

0-2 years experience

1. 1How would you add a custom method to all arrays in a small script? Show the code you’d write.
2. 2If you added a method named `flatten` to `Array.prototype` but later discovered JavaScript added its own `flatten` method, what would happen to existing code?

2-5 years experience

1. 1We have a legacy codebase where someone patched `Array.prototype` with a `sum` method. A new library you’re using now throws an error. How would you debug and fix the issue?
2. 2When you need a reusable array operation for a new feature, would you extend `Array.prototype` or create a helper function? Explain the trade‑offs.

5-8 years experience

1. 1In a large web app, performance regressions appeared after several teams added methods to built‑in prototypes. How would you measure the impact and decide whether to refactor those extensions?
2. 2Design a strategy to prevent prototype pollution across multiple micro‑frontends that share the same global JavaScript environment.

8+ years experience

1. 1Your organization is moving from a monolithic front‑end to a modular architecture with isolated runtimes. How would you handle existing code that monkey‑patches built‑in prototypes to avoid breaking other teams?
2. 2What company‑wide policies or tooling would you introduce to enforce safe usage of prototype extensions across many projects?