IndexedDB

An asynchronous database built into browsers.

Also known as: indexeddb, indexed database, browser database

IndexedDB is the browser’s built-in NoSQL store: asynchronous, transactional, holding structured data (objects, files, blobs) far beyond localStorage’s 5MB string limit. Data lives in object stores (like tables) with indexes, inside versioned databases — and it survives reloads, working offline alongside service workers.

open db (version) → transaction → object store → get/put/index query

It’s genuinely a database: transactions with modes, cursors for iteration, indexes for queries, quotas in the hundreds of MB to GB. That power comes with the clunkiest grave-born API on the web platform — event-based requests wrapped by every sane project in a promise library.

The classic mistakes:

Using localStorage for real data. Synchronous, tiny, string-only — localStorage blocks the main thread and fits tokens and prefs, not datasets. IndexedDB is the data store.
Raw API without a wrapper. The event-based API multiplies code and bugs. Use a minimal promise wrapper; the raw API is a rite of passage nobody should repeat.
Ignoring versions. Schema changes (new stores, indexes) happen in versioned upgrades only. Bumping versions and migrating is the whole schema story — plan it.
One giant transaction. Long transactions across many stores block other tabs’ access and risk aborts. Keep transactions scoped and short.
No quota handling. Storage can be evicted under pressure (especially “best effort” origins). Handle QuotaExceededError, request persistence where it matters, and sync critical data server-side.
Assuming sync across devices. IndexedDB is per-browser, per-origin. Multi-device sync is a server problem — the local DB is a cache and offline buffer, not the system of record.
Blocking on it at startup. Reads are async but still cost; don’t gate first paint on database opens that could lazy-load.

When to use it: offline-capable apps, large client datasets, caches and outbox queues. Small synchronous needs stay in localStorage; everything shaped like data belongs in IndexedDB.