rspec/rules/S6440/javascript/rule.adoc

== Why is this an issue?

React relies on the order in which Hooks are called to correctly preserve the state of Hooks between multiple `useState` and `useEffect` calls. This means React Hooks should be called in the same order each time a component renders and should not be called inside loops, conditions, or nested functions.

Additionally, this rule ensures that the Hooks are called only from React function components or custom Hooks.

[source,javascript,diff-id=1,diff-type=noncompliant]
----
function Profile() {
  const [ordersCount, setOrdersCount] = useState(0);
  if (ordersCount !== 0) {
    useEffect(function() { // Noncompliant: Hook is called conditionally
      localStorage.setItem('ordersData', ordersCount);
    });
  }

  return <div>{ getName() }</div>
}

function getName() {
  const [name] = useState('John'); // Noncompliant: Hook is called from a JavaScript function, not a React component
  return name;
}

----

Instead, always use Hooks at the top of your React function, before any early returns.

[source,javascript,diff-id=1,diff-type=compliant]
----
function Profile() {
  const [ordersCount, setOrdersCount] = useState(0);
  useEffect(function() {
    if (ordersCount !== 0) {
      localStorage.setItem('ordersData', ordersCount);
    }
  });

  const [name] = useState('John');
  return <div>{ name }</div>
}
----

== Resources

=== Documentation

* React Documentation - https://react.dev/warnings/invalid-hook-call-warning#breaking-rules-of-hooks[Breaking Rules of Hooks]
* React Documentation - https://legacy.reactjs.org/docs/hooks-rules.html[Rules of Hooks]