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
const [settings, setSettings] = useState(() => {
const 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.