/frontend-react-best-practices

Use Lazy State Initialization

Pass a function to useState for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once.

Incorrect (runs on every render):

function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs on EVERY render, even after initialization
  const [searchIndex, setSearchIndex] = useState(buildSearchIndex(items));
  const [query, setQuery] = useState("");

  // When query changes, buildSearchIndex runs again unnecessarily
  return <SearchResults index={searchIndex} query={query} />;
}

function UserProfile() {
  // JSON.parse runs on every render
  const [settings, setSettings] = useState(
    JSON.parse(localStorage.getItem("settings") || "{}"),
  );

  return <SettingsForm settings={settings} onChange={setSettings} />;
}

Correct (runs only once):

function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs ONLY on initial render
  const [searchIndex, setSearchIndex] = useState(() => buildSearchIndex(items));
  const [query, setQuery] = useState("");

  return <SearchResults index={searchIndex} query={query} />;
}

function UserProfile() {
  // JSON.parse runs only on initial render
  let [settings, setSettings] = useState(() => {
    let stored = localStorage.getItem("settings");
    return stored ? JSON.parse(stored) : {};
  });

  return <SettingsForm settings={settings} onChange={setSettings} />;
}

Use lazy initialization when computing initial values from localStorage/sessionStorage, building data structures (indexes, maps), reading from the DOM, or performing heavy transformations.

For simple primitives (useState(0)), direct references (useState(props.value)), or cheap literals (useState({})), the function form is unnecessary.