localStorage keeps data forever. That is a problem for things that should go stale, like a cached API response, a one-time code or a "dismissed this banner today" flag. Cookies have Max-Age, but localStorage has nothing like it.
Write a function createExpiringStorage(storage, options) that wraps any Storage-like object (for example window.localStorage or window.sessionStorage) and adds expiry on top of it.
const cache = createExpiringStorage(window.localStorage, { prefix: "cache:" });
cache.set("user", { id: 7, name: "Ada" }, 60_000); // keep for one minute
cache.get("user"); // { id: 7, name: "Ada" }
// ...a minute later
cache.get("user"); // null, and the entry is gone from localStorage
Options
prefix (default ""): every key is stored in the underlying storage as prefix + key.now (default: the current time from Date.now()): a function that returns the current time in milliseconds. Tests use it to move time forward.Methods
set(key, value, maxAge?) stores any JSON-serializable value. With maxAge (in ms) the value expires once maxAge ms have passed. Without it the value never expires. A maxAge of 0 or less removes the key instead of storing it. Setting a key again replaces both the value and its expiry.get(key) returns the stored value, or null if the key is missing or expired. An expired entry is removed from the underlying storage when it is read. If the raw string under that key wasn't written by your wrapper (for example plain text or broken JSON), return null instead of throwing.remove(key) deletes one key.clear() deletes every key under the prefix and leaves other keys in the storage alone.prune() deletes every expired entry under the prefix and returns how many it deleted.How you encode the value and its expiry time inside the stored string is up to you.
const cache = createExpiringStorage(window.localStorage, { prefix: "cache:" });
cache.set("user", { id: 7, name: "Ada" }, 60_000); // keep for one minute
cache.get("user"); // { id: 7, name: "Ada" }
// ...a minute later
cache.get("user"); // null, and the entry is gone from localStorage