karton-best-practices

name: karton-best-practices description: Performance-focused guidelines for writing React code that consumes Karton state. Use when creating or reviewing components that use useKartonState, useKartonProcedure, useComparingSelector, or any Karton React hooks. Covers selector patterns, re-render prevention, structural sharing, and edge cases.

Karton React Best Practices

Performance guidelines for React components consuming Karton state. Karton uses Immer internally — understanding its structural sharing model is essential for writing efficient selectors.

Core Mental Model

Karton state is deeply frozen (Immer freeze(state, true)). Updates use produce() which creates new references only for changed paths — unchanged subtrees keep the same reference. useSyncExternalStore uses Object.is (reference equality) to decide re-renders.

The golden rule: Your selector's return value must have a stable reference when nothing relevant changed. If it doesn't, wrap it with useComparingSelector.

Quick Rules

Selectors

// GOOD: Select a primitive — stable when unchanged
const isWorking = useKartonState((s) => s.agents.instances[id]?.state.isWorking ?? false);

// GOOD: Select a frozen subtree — Immer preserves the reference
const agent = useKartonState((s) => s.agents.instances[id]);

// BAD: Creates a new object every call — always re-renders
const data = useKartonState((s) => ({
  name: s.agents.instances[id]?.name,
  status: s.agents.instances[id]?.state.isWorking,
}));

// FIX: Wrap with useComparingSelector (default: shallow equality)
const data = useKartonState(
  useComparingSelector((s) => ({
    name: s.agents.instances[id]?.name,
    status: s.agents.instances[id]?.state.isWorking,
  })),
);

// BAD: Object.keys/values/entries always creates a new array
const ids = useKartonState((s) => Object.keys(s.agents.instances));

// FIX: shallow comparison handles arrays element-by-element
const ids = useKartonState(
  useComparingSelector((s) => Object.keys(s.agents.instances)),
);

When to Use useComparingSelector

Procedures

// GOOD: Procedures are stable proxies — select once, use freely
const createAgent = useKartonProcedure((p) => p.agents.create);

// GOOD: Store in ref for use in callbacks to prevent effect re-triggers
const createAgentRef = useRef(createAgent);
createAgentRef.current = createAgent;

Derived State

// GOOD: useMemo to compute derived values from selected state
const agent = useKartonState((s) => s.agents.instances[id]);
const displayName = useMemo(() => agent?.name ?? 'Unnamed', [agent]);

// BAD: Expensive computation inside selector (runs on every state change)
const sorted = useKartonState((s) =>
  Object.values(s.agents.instances).sort((a, b) => a.name.localeCompare(b.name)),
);

// FIX: Select raw data, compute outside
const instances = useKartonState(
  useComparingSelector((s) => s.agents.instances),
);
const sorted = useMemo(
  () => Object.values(instances).sort((a, b) => a.name.localeCompare(b.name)),
  [instances],
);

Key Pitfalls

1. Every useKartonState call runs its selector on every state change — keep selectors cheap
2. Selectors that create new objects/arrays always cause re-renders unless wrapped with useComparingSelector
3. useComparingSelector with deep is expensive — prefer shallow or restructure state
4. Selecting the full state (useKartonState() with no selector) re-renders on any change
5. Inline selector functions recreate on every render — this is fine because useSyncExternalStore compares snapshot values, not function references
6. Connection changes (onOpen/onClose) trigger all listeners — components using useKartonState may re-render briefly on reconnect even if state didn't change

References

For detailed analysis, see:

• references/state-update-model.md — How Immer structural sharing affects object references
• references/selector-patterns.md — Comprehensive selector patterns with examples
• references/re-render-prevention.md — useComparingSelector internals and memoization strategies
• references/edge-cases.md — Arrays, Maps, conditional selection, and reconnection behavior