discountry 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
- Hierarchy — decompose by SRP; one component, one concern. Map data model shape to component tree.
- Static first — props only, no state, until the skeleton renders correctly.
- Minimal state — DRY: if it's derivable, it's not state.
- Ownership — hoist state to the closest common ancestor of all consumers.
- 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
- If it's computable at render time — compute it. No state, no Effect.
- If it runs because the user did X — it's an event handler.
- If it runs because the component is visible — it's an Effect.
- If two state variables drift out of sync — lift state or derive one from the other.
- If the dep array is long — the Effect does too much; split it.
- 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