Storage Quotas

IndexedDB, the Cache API and OPFS share one quota per origin, exposed by navigator.storage (StorageManager, Baseline Widely available). estimate() returns approximate usage and quota in bytes, plus usageDetails in Chromium 4,389 . Data is best-effort: under disk pressure the browser evicts whole origins, least recently used first, and Safari 10 also clears script-written data of sites unvisited for seven days. persist() asks for exemption from that eviction; Firefox 555 prompts, while Chrome 1 and Safari decide silently from the user's interaction with the site (a fresh headless profile is refused):

await (await caches.open('big')).put('/zeros', new Response(new Uint8Array(3 * 1024 ** 2)));
const { usage, quota, usageDetails } = await navigator.storage.estimate();
const mib = (bytes) => `${(bytes / 1024 ** 2).toFixed(1)} MiB`;
console.log(mib(usage), 'of', mib(quota), JSON.stringify(usageDetails));  // details: Chromium
console.log(await navigator.storage.persisted(), await navigator.storage.persist());

3.0 MiB of 10243.0 MiB {"caches":3145815}
false false

Quotas vary: Chrome allows an origin up to 60% of the disk, Firefox the smaller of 10% of the disk or 10 GiB per site (up to 50% of the disk once persistent), and Safari about 60%. Check estimate() before large downloads, catch QuotaExceededError, and request persistence only for data the user cannot download again.