OOP versus functional style

OOP bundles data and behavior on objects with mutable identity; functional style favors pure functions, immutable data, and explicit effect boundaries (IO at the edges). Many codebases blend both: immutable values inside entity methods, or Redux reducers (functional) driving React views (component objects).

Compare trade-offs on testability, concurrency, and where state lives—not ideology.

On interviews: discuss how you'd structure a pricing engine with discounts as pure functions versus polymorphic strategy objects.

Common pitfalls: claiming one paradigm always wins, hidden mutation in "functional" pipelines, and anemic services plus mutable globals.

The trade-off is localized mutation with clear objects versus reproducible transforms with immutable data.

Checklist:

• Place side effects at boundaries.
• Use pure functions for calculations.
• Choose style per subdomain, not globally.