Files
stack/packages/mosaic/framework/skills/vercel-react-best-practices/rules/client-localstorage-schema.md
T
fargo 1a822493ba format: apply repo prettier (3.8.1) to the folded skills tree
963 markdown files reformatted with the repository's pinned prettier so
pnpm format:check covers the folded tree like every other repo file.

The formatter's embedded-language pass also normalized code fences
(TS semicolons, closed HTML tags in examples, lowercased CSS hex colors,
one renumbered list that skipped an index). Alphanumeric token deltas vs
the fold commit were audited file-by-file; all are formatter-equivalent
markup normalizations plus the four sanitized skills.
2026-08-19 14:37:17 -05:00

1.9 KiB

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.