---
title: Minimize State Variables and Derive Values
impact: MEDIUM
impactDescription: fewer re-renders, less state drift
tags: state, derived-state, hooks, optimization
---

## Minimize State Variables and Derive Values

Use the fewest state variables possible. If a value can be computed from existing state or props, derive it during render instead of storing it in state. Redundant state causes unnecessary re-renders and can drift out of sync.

**Incorrect (redundant state):**

```tsx
function Cart({ items }: { items: Item[] }) {
  const [total, setTotal] = useState(0);
  const [itemCount, setItemCount] = useState(0);

  useEffect(() => {
    setTotal(items.reduce((sum, item) => sum + item.price, 0));
    setItemCount(items.length);
  }, [items]);

  return (
    <View>
      <Text>{itemCount} items</Text>
      <Text>Total: ${total}</Text>
    </View>
  );
}
```

**Correct (derived values):**

```tsx
function Cart({ items }: { items: Item[] }) {
  const total = items.reduce((sum, item) => sum + item.price, 0);
  const itemCount = items.length;

  return (
    <View>
      <Text>{itemCount} items</Text>
      <Text>Total: ${total}</Text>
    </View>
  );
}
```

**Another example:**

```tsx
// Incorrect: storing both firstName, lastName, AND fullName
const [firstName, setFirstName] = useState("");
const [lastName, setLastName] = useState("");
const [fullName, setFullName] = useState("");

// Correct: derive fullName
const [firstName, setFirstName] = useState("");
const [lastName, setLastName] = useState("");
const fullName = `${firstName} ${lastName}`;
```

State should be the minimal source of truth. Everything else is derived.

Reference: [Choosing the State Structure](https://react.dev/learn/choosing-the-state-structure)
