Web Storage

localStorage and sessionStorage store string data per origin with simple synchronous APIs. localStorage persists across sessions; sessionStorage is scoped to a top-level browsing session. They are convenient for small non-sensitive preferences but unsuitable for large data, high-frequency writes, or secrets exposed to XSS.

					const key = 'theme';
localStorage.setItem(key, JSON.stringify({ mode: 'dark' }));
const theme = JSON.parse(localStorage.getItem(key) ?? '{"mode":"light"}');
				

The storage event fires in other documents from the same origin when localStorage changes — useful for cross-tab theme sync, not a durable message bus.

On interviews: compare Web Storage with cookies, IndexedDB, memory, and server state by sensitivity and lifetime.

Common pitfalls: synchronous reads can block the main thread. Storing tokens in Web Storage makes XSS consequences worse.

The trade-off is convenience versus control — pick the mechanism that matches your coupling and performance budget.

Checklist:

• Store only small, non-secret data.
• Serialize with explicit schema/version.
• Avoid high-frequency writes on input events.
• Know lifetime: local vs session.