solid-principleslisted

# SOLID Principles Apply when writing new code; check for violations when reviewing. Each principle below carries the smells that betray it in a diff. ## S — Single Responsibility **One reason to change** — one actor or stakeholder whose requirements could change the code. Smells: a unit with more than one clear purpose (`UserService` that validates, persists, *and* sends email); `and` in a name (`validateAndSave()`); functions past ~30 lines; multiple unrelated tests for one unit. If you cannot name a function without "and", it has too many jobs. ## O — Open/Closed **Open for extension, closed for modification.** Adding behavior should not require changing existing tested code. Smells: a `switch`/`if-else` chain on a type field that must be edited for every new type; hardcoded variant lists; tests that break whenever a new variant is added. Fix by defining an interface for the varying behavior and adding implementations rather than branches. ## L — Liskov Substitution **Subtypes are substitutable for their base types** without the caller knowing which it got. Smells: an override that throws `NotImplementedError` or does nothing; callers checking `instanceof` before calling; subclasses weakening preconditions or strengthening postconditions; tests that cannot run against both base and subtype. Design hierarchies on behavior, not taxonomy. Prefer composition when "is-a" does not hold behaviorally. A subtype may restrict behavior (`ReadOnlyList`) but must fulfill e