nextjs-elysia-vote/.agents/skills/vercel-react-best-practices/rules/client-localstorage-schema.md

title, impact, impactDescription, tags

title	impact	impactDescription	tags
Version and Minimize localStorage Data	MEDIUM	prevents schema conflicts, reduces storage size	client, localStorage, storage, versioning, data-minimization

Version and Minimize localStorage Data

Add version prefix to keys and store only needed fields. Prevents schema conflicts and accidental storage of sensitive data.

Incorrect:

// No version, stores everything, no error handling
localStorage.setItem('userConfig', JSON.stringify(fullUserObject));
const data = localStorage.getItem('userConfig');

Correct:

const VERSION = 'v2';

function saveConfig(config: { theme: string; language: string }) {
  try {
    localStorage.setItem(`userConfig:${VERSION}`, JSON.stringify(config));
  } catch {
    // Throws in incognito/private browsing, quota exceeded, or disabled
  }
}

function loadConfig() {
  try {
    const data = localStorage.getItem(`userConfig:${VERSION}`);
    return data ? JSON.parse(data) : null;
  } catch {
    return null;
  }
}

// Migration from v1 to v2
function migrate() {
  try {
    const v1 = localStorage.getItem('userConfig:v1');
    if (v1) {
      const old = JSON.parse(v1);
      saveConfig({ theme: old.darkMode ? 'dark' : 'light', language: old.lang });
      localStorage.removeItem('userConfig:v1');
    }
  } catch {}
}

Store minimal fields from server responses:

// User object has 20+ fields, only store what UI needs
function cachePrefs(user: FullUser) {
  try {
    localStorage.setItem(
      'prefs:v1',
      JSON.stringify({
        theme: user.preferences.theme,
        notifications: user.preferences.notifications
      })
    );
  } catch {}
}

Always wrap in try-catch: getItem() and setItem() throw in incognito/private browsing (Safari, Firefox), when quota exceeded, or when disabled.

Benefits: Schema evolution via versioning, reduced storage size, prevents storing tokens/PII/internal flags.