React Use State

React: useState Hook Best Practices

Core Concept

useState is a React Hook that adds a state variable to your component, triggering re-renders when the state changes.

const [state, setState] = useState(initialState);

When to Use useState

Ideal Use Cases

Use useState When

• State is local to the component
• State transitions are simple (direct value replacement)
• Changes should trigger re-renders
• You need to persist values between renders

When NOT to Use useState

Use useRef Instead

When you need mutable values that don't trigger re-renders:

// Interval IDs, DOM references, previous values
const intervalRef = useRef(null);
const inputRef = useRef(null);

Use useReducer Instead

When state logic is complex:

// Multiple related values, complex transitions
const [state, dispatch] = useReducer(reducer, initialState);

Use useReducer when:

• State has multiple sub-values
• Next state depends on previous state in complex ways
• You want to centralize state logic

Avoid Redundant State

If a value can be computed from props or other state, don't store it:

// BAD: Redundant state
const [fullName, setFullName] = useState('');
useEffect(() => {
  setFullName(`${firstName} ${lastName}`);
}, [firstName, lastName]);

// GOOD: Compute during render
const fullName = `${firstName} ${lastName}`;

// If expensive, use useMemo
const sortedItems = useMemo(() => 
  items.sort((a, b) => a.name.localeCompare(b.name)), 
  [items]
);

Don't Use for Global/Shared State

For state shared across multiple components:

• React Context for moderate sharing
• External stores (Zustand, Jotai) for complex apps
• Server state libraries (TanStack Query) for async data

Critical Rules

1. Never Mutate State Directly

// BAD: Mutation
obj.x = 10;
setObj(obj); // React ignores this!

// GOOD: Create new object
setObj({ ...obj, x: 10 });

// BAD: Array mutation
arr.push(item);
setArr(arr); // React ignores this!

// GOOD: Create new array
setArr([...arr, item]);

2. State Updates Are Asynchronous

function handleClick() {
  setCount(count + 1);
  console.log(count); // Still old value!
  
  // If you need the new value:
  const nextCount = count + 1;
  setCount(nextCount);
  console.log(nextCount); // New value
}

3. Use Updater Function for Sequential Updates

// BAD: Only increments by 1
function handleClick() {
  setCount(count + 1); // 0 + 1 = 1
  setCount(count + 1); // 0 + 1 = 1 (same stale value!)
  setCount(count + 1); // 0 + 1 = 1
}

// GOOD: Increments by 3
function handleClick() {
  setCount(c => c + 1); // 0 -> 1
  setCount(c => c + 1); // 1 -> 2
  setCount(c => c + 1); // 2 -> 3
}

4. Use Initializer Function for Expensive Initial Values

// BAD: createTodos() runs every render
const [todos, setTodos] = useState(createTodos());

// GOOD: createTodos runs only once
const [todos, setTodos] = useState(createTodos);

// Or with arrow function for arguments
const [todos, setTodos] = useState(() => createTodos(userId));

5. Call Hooks at Top Level Only

// BAD: Conditional hook
if (condition) {
  const [state, setState] = useState(0); // Error!
}

// GOOD: Always call, conditionally use
const [state, setState] = useState(0);
if (condition) {
  // use state here
}

Common Patterns

Resetting State with Key

// Parent controls reset via key
<Form key={version} />

// When version changes, Form remounts with fresh state

Storing Functions in State

// BAD: Function gets called
const [fn, setFn] = useState(someFunction);

// GOOD: Wrap in arrow function
const [fn, setFn] = useState(() => someFunction);
setFn(() => newFunction);

Updating Objects/Arrays

// Object: spread and override
setForm({ ...form, email: newEmail });

// Nested object
setUser({
  ...user,
  address: { ...user.address, city: newCity }
});

// Array: filter, map, spread
setItems(items.filter(i => i.id !== id));        // Remove
setItems([...items, newItem]);                    // Add
setItems(items.map(i => i.id === id ? {...i, done: true} : i)); // Update

Quick Reference

DO

• Use for simple, local component state
• Create new objects/arrays when updating
• Use updater function when depending on previous state
• Use initializer function for expensive initial values

DON'T

• Store computed/derived values
• Mutate existing state objects/arrays
• Read state immediately after setting (it's a snapshot)
• Call setState unconditionally during render

Alternative Hooks Comparison

References

• React Docs - useState
• React Docs - State as a Snapshot
• React Docs - Choosing the State Structure

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