Local Storage

Local storage is part of the Web Storage API. It lets JavaScript save key-value pairs, as strings, in the browser for a specific origin with no expiry date. Typical capacity is about 5 MB per origin, though it varies.

Why it matters

It is a simple way to remember preferences, drafts or UI state without server round trips. It is not secure storage: any script running on the page can read it, so a cross-site scripting flaw exposes everything inside. Unlike cookies, its contents are never sent automatically with requests.

Example

localStorage.setItem('theme', 'dark');
const theme = localStorage.getItem('theme'); // 'dark'
localStorage.setItem('prefs', JSON.stringify({ lang: 'en' }));
localStorage.removeItem('theme');

Objects must be converted to strings with JSON.stringify.

Common mistakes

• Storing session tokens or personal data there. Prefer HttpOnly cookies for authentication.
• Assuming it always works: private browsing or blocked storage can throw errors.
• Treating it as a database. It is synchronous and blocks the main thread.

Related

• Service Worker
• AJAX
• CSP (Content Security Policy)
• Fetch API
• टूल: Cookie checker

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