set-state-in-render.md

title	set-state-in-render

Validates against unconditionally setting state during render, which can trigger additional renders and potential infinite render loops.

Rule Details {/rule-details/}

Calling setState during render unconditionally triggers another render before the current one finishes. This creates an infinite loop that crashes your app.

Common Violations {/common-violations/}

Invalid {/invalid/}

// ❌ Unconditional setState directly in render
function Component({value}) {
  const [count, setCount] = useState(0);
  setCount(value); // Infinite loop!
  return <div>{count}</div>;
}

Valid {/valid/}

// ✅ Derive during render
function Component({items}) {
  const sorted = [...items].sort(); // Just calculate it in render
  return <ul>{sorted.map(/*...*/)}</ul>;
}

// ✅ Set state in event handler
function Component() {
  const [count, setCount] = useState(0);
  return (
    <button onClick={() => setCount(count + 1)}>
      {count}
    </button>
  );
}

// ✅ Derive from props instead of setting state
function Component({user}) {
  const name = user?.name || '';
  const email = user?.email || '';
  return <div>{name}</div>;
}

// ✅ Conditionally derive state from props and state from previous renders
function Component({ items }) {
  const [isReverse, setIsReverse] = useState(false);
  const [selection, setSelection] = useState(null);

  const [prevItems, setPrevItems] = useState(items);
  if (items !== prevItems) { // This condition makes it valid
    setPrevItems(items);
    setSelection(null);
  }
  // ...
}

Troubleshooting {/troubleshooting/}

I want to sync state to a prop {/clamp-state-to-prop/}

A common problem is trying to "fix" state after it renders. Suppose you want to keep a counter from exceeding a max prop:

// ❌ Wrong: clamps during render
function Counter({max}) {
  const [count, setCount] = useState(0);

  if (count > max) {
    setCount(max);
  }

  return (
    <button onClick={() => setCount(count + 1)}>
      {count}
    </button>
  );
}

When count exceeds max, this schedules a state update during render. This is still an anti-pattern because rendering should stay pure, and state updates should happen in an event handler or effect instead.

Instead, it's often better to move this logic to the event (the place where the state is first set). For example, you can enforce the maximum at the moment you update state:

// ✅ Clamp when updating
function Counter({max}) {
  const [count, setCount] = useState(0);

  const increment = () => {
    setCount(current => Math.min(current + 1, max));
  };

  return <button onClick={increment}>{count}</button>;
}

Now the setter only runs in response to the click, React finishes the render normally, and count never crosses max.

In rare cases, you may need to adjust state based on information from previous renders. For those, follow this pattern of setting state conditionally.