React Development Principles

Component Model

• Components are pure functions: (props, state) → JSX. Render must be side-effect-free.
• One-way data flow: parent → child via props, child → parent via callbacks.
• Never mutate state or props in-place.

Thinking in React: Design Process

1. Hierarchy — decompose by SRP; one component, one concern. Map data model shape to component tree.
2. Static first — props only, no state, until the skeleton renders correctly.
3. Minimal state — DRY: if it's derivable, it's not state.
4. Ownership — hoist state to the closest common ancestor of all consumers.
5. Inverse flow — pass setters/handlers down; children call them on interaction.

What is NOT state

• Computable from existing state or props → derive during render
• Passed in as a prop
• Unchanged over time

Prefer storing IDs/primitives; derive full objects at render time.

Effect Discipline

useEffect is for synchronizing with external systems only — DOM APIs, network connections, third-party widgets, browser subscriptions. It is not a lifecycle hook and not a data-transformation pipeline.

Deciding when to write an Effect

Why does this code run?

• Because the user did something → event handler
• Because the component must stay in sync with something external → Effect

Unnecessary Effects — replace with:

Antipattern	Correct approach
Derived state via useEffect + useState	Compute during render; useMemo if expensive
State reset on prop change via Effect	key prop
Partial state adjustment on prop change	Store primitive ID; derive object during render
Shared event logic routed through state+Effect	Extract a plain function; call from event handlers
User-initiated POST in Effect	Move directly into the event handler
Effect chains (A → Effect → B → Effect → C)	Compute all next state in one event handler
Parent notification via Effect	Call both setters in the same event handler; or lift state
Pushing data up to parent via Effect	Parent fetches; passes data down as prop
One-time app init in component Effect	Module-level or didInit guard outside component
External store subscription via Effect	useSyncExternalStore
Data fetch without cleanup	ignore flag cleanup; prefer framework data-fetching

Effect rules

• Every reactive value the Effect reads must be in its dependency array.
• Never suppress react-hooks/exhaustive-deps — fix the code.
• Every resource acquired in setup must be released in cleanup.
• One Effect = one synchronization concern. Split unrelated concerns.
• Think start sync / stop sync, not mount / unmount.

Key Heuristics

1. If it's computable at render time — compute it. No state, no Effect.
2. If it runs because the user did X — it's an event handler.
3. If it runs because the component is visible — it's an Effect.
4. If two state variables drift out of sync — lift state or derive one from the other.
5. If the dep array is long — the Effect does too much; split it.
6. If you want to suppress the deps linter — redesign instead.

Supporting Reference Files

Load these files when deeper guidance is needed for the specific topic at hand:

• Eliminating unnecessary Effects (derived state, event logic, Effect chains, data fetching): see references/not-need.md
• Effect lifecycle, reactive values, and dependency contract: see references/lifecycle.md
• Distinguishing event handlers from Effects, and using useEffectEvent: see references/separating.md
• Strategies to reduce and stabilize Effect dependencies: see references/dependencies.md
• Extracting reusable logic into custom hooks: see references/reusing.md