localStorage — read, write, remove

Persistent key/value storage

localStorage survives page reloads. Up to ~5 MB per origin. Both keys and values are strings — numbers and objects need to be serialized.

localStorage.setItem("theme", "dark");
const t = localStorage.getItem("theme"); // "dark"
localStorage.removeItem("theme");
localStorage.clear();                     // nuclear option

Common shape

• User prefs (theme, locale, font-size).
• Session resumption ("you were on lesson-42").
• Form draft autosave.

What NOT to store

• Sensitive data (anyone with devtools can read it).
• Large blobs (use IndexedDB — there's a quota).
• Anything you'd lose sleep over if the user clicked "Clear browsing data".

The string trap

localStorage.setItem("count", 7) silently stringifies — when you read it back you get "7", not 7. Use JSON.stringify / JSON.parse for non-strings (lesson C9-02).