immutability

Validates against mutating props, state, and other values that are immutable.

Rule Details

A component’s props and state are immutable snapshots. Never mutate them directly. Instead, pass new props down, and use the setter function from useState.

Common Violations

Invalid

// ❌ Object property assignment
function Component() {
  const [user, setUser] = useState({name: 'Alice'});

  const updateName = () => {
    user.name = 'Bob'; // Mutating!
    setUser(user); // Same reference
  };
}

Note

The rule reports mutations it can prove, such as assigning to a property of a value from props or state. It does not infer the types of state values, so it cannot tell that a method call like items.push(4) or items.sort() mutates an array, and it does not report those calls yet. They still mutate state and are bugs, so create a new array instead, as shown below.

Valid

// ✅ Create new array
function Component() {
  const [items, setItems] = useState([1, 2, 3]);

  const addItem = () => {
    setItems([...items, 4]); // New array
  };
}

// ✅ Sort a copy
function Component() {
  const [items, setItems] = useState([3, 1, 2]);

  const sortItems = () => {
    setItems([...items].sort()); // Sorts the copy, not the state
  };
}

// ✅ Create new object
function Component() {
  const [user, setUser] = useState({name: 'Alice'});

  const updateName = () => {
    setUser({...user, name: 'Bob'}); // New object
  };
}

Troubleshooting

I need to add items to an array

Mutating arrays with methods like push() won’t trigger re-renders. The linter doesn’t currently report these method calls on state, so you need to catch them yourself:

// ❌ Wrong: Mutating the array
function TodoList() {
  const [todos, setTodos] = useState([]);

  const addTodo = (id, text) => {
    todos.push({id, text});
    setTodos(todos); // Same array reference!
  };

  return (
    <ul>
      {todos.map(todo => <li key={todo.id}>{todo.text}</li>)}
    </ul>
  );
}

Create a new array instead:

// ✅ Better: Create a new array
function TodoList() {
  const [todos, setTodos] = useState([]);

  const addTodo = (id, text) => {
    setTodos([...todos, {id, text}]);
    // Or: setTodos(todos => [...todos, {id: Date.now(), text}])
  };

  return (
    <ul>
      {todos.map(todo => <li key={todo.id}>{todo.text}</li>)}
    </ul>
  );
}

I need to update nested objects

Mutating nested properties doesn’t trigger re-renders:

// ❌ Wrong: Mutating nested object
function UserProfile() {
  const [user, setUser] = useState({
    name: 'Alice',
    settings: {
      theme: 'light',
      notifications: true
    }
  });

  const toggleTheme = () => {
    user.settings.theme = 'dark'; // Mutation!
    setUser(user); // Same object reference
  };
}

Spread at each level that needs updating:

// ✅ Better: Create new objects at each level
function UserProfile() {
  const [user, setUser] = useState({
    name: 'Alice',
    settings: {
      theme: 'light',
      notifications: true
    }
  });

  const toggleTheme = () => {
    setUser({
      ...user,
      settings: {
        ...user.settings,
        theme: 'dark'
      }
    });
  };
}

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