storage quota probe

navigator.storage.estimate() returns how much disk space the browser has allocated to this origin and how much is used. no permission required. from the quota value, it's possible to estimate total free disk space on the device (browsers typically allocate 60% of available free space as the quota ceiling).

this tells any page approximately how full your hard drive is — potentially narrowing down your device type, usage patterns, and whether disk space is a concern.

reading storage estimate…

• no permission, instant disk read. storage.estimate() requires no user interaction and fires synchronously in the first microtask. it runs before the user has clicked anything, and the quota value correlates strongly with free disk space.
• device tier fingerprinting. a device with a 50 gb quota ceiling likely has 80–100 gb free; a 5 gb quota likely has 8 gb free — typical of an older phone or a nearly full laptop. combined with screen resolution and dpr, this narrows hardware segment.
• eviction is real. browsers use a least-recently-used eviction policy for origins with best-effort storage. if a user's device fills up, the browser can silently delete your origin's indexeddb, caches, and service worker registrations — without warning.
• opfs is not accounted by default. origin private file system (opfs) data may appear in usageDetails on chromium only. safari and firefox may return incomplete breakdowns.