Prevent `undefined`/`void`/falsy values from becoming accidental runtime states. Apply these rules: 1. **Eliminate transient `undefined`**: If a value can be absent (e.g., localStorage, optional query data), wire an explicit `defaultValue` so hooks/state initialize to a concrete value and consumers never render an “undefined first frame”. - Prefer setting the `useState` initial value from `defaultValue` inside the utility hook rather than letting it be `undefined` and patching later. 2. **Guard globals by environment**: Never assume browser APIs exist—use a shared “is server/client” check before touching `window`/DOM. 3. **Treat `void`/no-arg and falsy as testable cases**: When designing/using optional variables or cached data, add type and runtime tests for `void` (no variables) and for falsy cached values (e.g., `''`, `0`, `false`) so “absent” isn’t conflated with “no result”. 4. **Don’t rely on `undefined` being “missing” during key/hash matching**: If keys are serialized/hashed (e.g., via `JSON.stringify`) or displayed via hashes, `undefined` may be removed, making different logical keys collide or fail to match. - Prefer key shapes where absence is explicit (e.g., use a dedicated sentinel or omit the property at the call site) and document/align hashing with comparison semantics. Example (default to avoid initial `undefined`): ```ts function useLocalStorage(key: string, defaultValue: T) { const [value, setValue] = React.useState(() => { const raw = localStorage.getItem(key) return raw == null ? defaultValue : (JSON.parse(raw) as T) }) // ...sync back to localStorage return [value, setValue] as const } ``` If a behavior constraint exists (e.g., a function must not return `undefined`), encode it in types and tests—don’t “test” impossible states.