React Effect Patterns

Effects are an escape hatch from React for synchronizing with external systems. Removing unnecessary Effects makes code easier to follow, faster to run, and less error-prone.

When to Use Effects

• Synchronizing with external systems (non-React widgets, network, browser DOM)
• Analytics on component display
• Data fetching (with cleanup for race conditions)

When NOT to Use Effects

• Transforming data for rendering
• Handling user events
• Updating state based on other state

Decision Tree

Why does this code run?

• Component displayed → Effect
• User action → Event handler
• Calculable from props/state → Calculate during render

Anti-Patterns and Solutions

1. Derived State

// ❌ Bad
const [fullName, setFullName] = useState("");
useEffect(() => {
   setFullName(firstName + " " + lastName);
}, [firstName, lastName]);

// ✅ Good - calculate during render
const fullName = firstName + " " + lastName;

2. Expensive Calculations

// ❌ Bad
const [visibleTodos, setVisibleTodos] = useState([]);
useEffect(() => {
   setVisibleTodos(getFilteredTodos(todos, filter));
}, [todos, filter]);

// ✅ Good - useMemo for expensive operations
const visibleTodos = useMemo(
   () => getFilteredTodos(todos, filter),
   [todos, filter],
);

Use console.time()/console.timeEnd() to measure. Memoize if ≥1ms.

3. Reset State on Prop Change

// ❌ Bad
useEffect(() => {
   setComment("");
}, [userId]);

// ✅ Good - use key to reset
<Profile userId={userId} key={userId} />;

4. Adjust Part of State on Prop Change

// ❌ Bad
useEffect(() => {
   setSelection(null);
}, [items]);

// ✅ Good - derive from state
const selection = items.find((item) => item.id === selectedId) ?? null;

5. Event-Specific Logic

// ❌ Bad
useEffect(() => {
   if (product.isInCart) {
      showNotification("Added " + product.name + "!");
   }
}, [product]);

// ✅ Good - in event handler
function handleBuyClick() {
   addToCart(product);
   showNotification("Added " + product.name + "!");
}

6. Form Submission

// ❌ Bad
useEffect(() => {
   if (jsonToSubmit !== null) {
      post("/api/register", jsonToSubmit);
   }
}, [jsonToSubmit]);

// ✅ Good - in event handler
function handleSubmit(e) {
   e.preventDefault();
   post("/api/register", { firstName, lastName });
}

7. Effect Chains

// ❌ Bad - cascading Effects
useEffect(() => {
   setGoldCardCount((c) => c + 1);
}, [card]);
useEffect(() => {
   setRound((r) => r + 1);
}, [goldCardCount]);
useEffect(() => {
   setIsGameOver(true);
}, [round]);

// ✅ Good - calculate + update in handler
const isGameOver = round > 5;

function handlePlaceCard(nextCard) {
   setCard(nextCard);
   if (nextCard.gold) {
      if (goldCardCount < 3) {
         setGoldCardCount(goldCardCount + 1);
      } else {
         setGoldCardCount(0);
         setRound(round + 1);
      }
   }
}

8. App Initialization

// ❌ Bad - runs twice in dev
useEffect(() => {
   loadDataFromLocalStorage();
   checkAuthToken();
}, []);

// ✅ Good - module level or guard
let didInit = false;
function App() {
   useEffect(() => {
      if (!didInit) {
         didInit = true;
         loadDataFromLocalStorage();
         checkAuthToken();
      }
   }, []);
}

9. Notify Parent of State Changes

// ❌ Bad - extra render pass
useEffect(() => {
   onChange(isOn);
}, [isOn, onChange]);

// ✅ Good - update both in handler
function updateToggle(nextIsOn) {
   setIsOn(nextIsOn);
   onChange(nextIsOn);
}

// ✅ Also good - lift state (controlled component)
function Toggle({ isOn, onChange }) {
   function handleClick() {
      onChange(!isOn);
   }
}

10. Pass Data to Parent

// ❌ Bad - child fetches, passes up
useEffect(() => {
   if (data) onFetched(data);
}, [data]);

// ✅ Good - parent fetches, passes down
function Parent() {
   const data = useSomeAPI();
   return <Child data={data} />;
}

11. External Store Subscription

// ❌ Bad - manual subscription
useEffect(() => {
   const handler = () => setIsOnline(navigator.onLine);
   window.addEventListener("online", handler);
   window.addEventListener("offline", handler);
   return () => {
      window.removeEventListener("online", handler);
      window.removeEventListener("offline", handler);
   };
}, []);

// ✅ Good - useSyncExternalStore
return useSyncExternalStore(
   subscribe,
   () => navigator.onLine,
   () => true,
);

12. Data Fetching with Race Conditions

// ✅ Correct - cleanup ignores stale responses
useEffect(() => {
   let ignore = false;
   fetchResults(query).then((json) => {
      if (!ignore) setResults(json);
   });
   return () => {
      ignore = true;
   };
}, [query]);

Quick Reference