Design principle

Ensures predictability, testability, and scalability by eliminating hidden state.

Writing pure functions; designing microservices or API endpoints; creating frontend components that rely only on props.

Functional programming, microservices architecture, web APIs, frontend frameworks like React.

def process(data): return result

def ... (...): return ...

data: input parameter, result: output expression that depends only on data

• pure functions
• idempotency
• dependency injection
• immutability

• Using static-method classes
• functional components in React
• stateless HTTP handlers

Accidentally modifying external variables; relying on global state; using mutable default arguments

Stateful objects (classes with instance variables); closures that capture outer scope

Coming from object-oriented programming: assuming methods can modify instance attributes without consequence

• Idempotency
• Referential transparency
• Pure function

Pure functions must avoid I/O and argument mutation; isolated logging or metrics may be acceptable if they do not affect core logic

Makes code easier to test, reason about, and scale horizontally

Think of a vending machine: same input yields same output every time

Stateless components simplify reasoning about concurrency because they avoid race conditions caused by shared mutable state, but they may require explicit passing of context or dependencies for tasks like logging, caching, or external I/O.

Introduce dependency injection for external services while keeping core logic stateless