💾 JavaScript

localStorage in JavaScript — Save Data Without a Backend

📅 Jun 18, 2026 ⏱ 4 min read

localStorage gives every website ~5MB of permanent key-value storage in the browser — no server, no database, no login.

The whole API

localStorage.setItem("theme", "dark");
localStorage.getItem("theme");      // "dark" (null if missing)
localStorage.removeItem("theme");
localStorage.clear();

Objects need JSON

localStorage.setItem("user", JSON.stringify({ name: "Priya", cgpa: 8.9 }));
const user = JSON.parse(localStorage.getItem("user") ?? "{}");

The three storages

localStorage: forever, ~5MB, never sent to server
sessionStorage: same API, dies with the tab
Cookies: ~4KB, sent with EVERY request — only for things the server needs (sessions)

Real example — remember dark mode

if (localStorage.getItem("theme") === "dark") document.body.classList.add("dark");

toggle.onclick = () => {
  const dark = document.body.classList.toggle("dark");
  localStorage.setItem("theme", dark ? "dark" : "light");
};

Never store passwords or tokens for sensitive apps — any script on the page can read localStorage. Our DSA judge uses it to save your progress.

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