Choose by whether the server needs the data and how long it should live. Cookies can be sent automatically with matching HTTP requests, which makes them suitable for server-managed sessions. localStorage keeps origin-scoped data across ordinary browser restarts, while sessionStorage keeps origin-and-tab-scoped data for the tab’s lifetime. Neither Web Storage API sends its contents to the server automatically.
How the three storage options differ
| Mechanism | Scope and lifetime | Sent automatically with requests? | Good fit | Key caution |
|---|---|---|---|---|
| Cookie | Configurable domain, path and expiry; a session cookie ends according to the browser’s session behavior. | Yes, when the request matches the cookie’s scope and attributes. | Server-managed session IDs and small values the server needs on requests. | Cookies add request overhead and have constrained capacity. Configure scope, expiry, Secure, HttpOnly and SameSite deliberately; cookie-based authentication still needs CSRF protections. |
localStorage |
Origin-scoped; persists across ordinary browser restarts. | No. | Non-sensitive client preferences or state reused across visits. | JavaScript can read it, and the API is synchronous. Do not treat it as a protected place for session secrets. |
sessionStorage |
Origin-and-tab-scoped; the associated data is cleared when that tab closes. | No. | Temporary state isolated to one tab, such as a tab-specific draft or workflow. | JavaScript can read it, and the API is synchronous. Separate tabs have separate storage areas. |
For a more detailed guide to the Web Storage API’s origin and tab behavior, see MDN’s Web Storage API documentation.
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Why request behavior matters most
The browser includes applicable cookies in the Cookie request header. The server can therefore receive a cookie without page code copying its value into each request. Scope and cookie attributes determine whether it applies to a particular request.
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How to choose
Use a cookie for a server-managed signed-in session
If the server must recognize a request as belonging to a signed-in user, use a server-managed session identifier in a cookie. Configure Secure, HttpOnly and an appropriate SameSite value, keep the cookie’s domain and path as narrow as practical, and define expiry and invalidation on the server. MDN recommends cookies for session IDs because attributes such as HttpOnly can limit direct JavaScript access; its session management guide also explains the remaining risks.
Use localStorage for non-sensitive state reused across visits
Choose localStorage when a client-side preference or other non-sensitive value should remain available after an ordinary browser restart and does not need to accompany requests. For example, a site might retain a display preference. Do not put a session secret there on the assumption that it is hidden from scripts running on the page.
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Use sessionStorage for temporary state within a tab
Choose sessionStorage when state should be available within one tab but separated from other tabs and discarded when that tab closes. Its storage is partitioned by origin and tab, making it useful for temporary per-tab workflows.
Use another client-storage API for larger data
Cookies are a poor fit for arbitrary bulk client data because they travel with matching requests and have limited capacity. MDN describes cookie capacity as usually around 4 KB per cookie, with the number allowed per domain depending on the browser and generally in the hundreds; these are approximate guidance, not universal fixed limits. For more client-side data, MDN points developers toward Web Storage or IndexedDB. Web Storage quotas also vary by browser and conditions, so there is no single universal quota to assume. See MDN’s guide to HTTP cookies.
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What the security attributes do—and do not do
HttpOnlyprevents JavaScript from reading the cookie value directly. It does not stop malicious injected script from making authenticated requests through the user’s browser.Securerestricts the cookie to encrypted HTTPS requests.SameSitecontrols some cross-site cookie sending and can contribute to CSRF defense, but it is not a complete CSRF defense by itself.- Both Web Storage APIs are readable by JavaScript running in the origin. A script injected into the page can generally read their values, so they are not protected vaults for credentials.
Cookie-based sessions still need ordinary XSS prevention, server-side expiry and invalidation, and CSRF protections. An HttpOnly cookie reduces direct credential theft by script, but does not neutralize script that can act through an authenticated session.
Cookie expiry is not the same as a reliable browser clock
A cookie with Expires or Max-Age is persistent according to the configured expiry. Without either, it is a session cookie, but the end of a browser session is defined by the browser rather than by a universally predictable clock. Session restore can preserve session cookies across a browser restart. Set and enforce the intended session lifetime server-side rather than assuming that closing the browser always clears a session cookie.
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Embedded pages and privacy partitioning
Storage behavior in third-party embeds can vary with browser privacy features. Firefox documents partitioning state by resource origin and top-level site; that detail should not be generalized to every browser. Test embedded integrations in the browsers and privacy modes the application supports. Mozilla’s State Partitioning documentation describes Firefox’s behavior and cautions against relying on transitional access heuristics.
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