IndexedDB

                indexedDB.open(name, version)
              

Purpose

IndexedDB stores large amounts of structured data client-side, with indexes for efficient querying — unlike localStorage’s small, string-only, synchronous model, it’s designed for genuinely sizable, offline-capable application data.

Syntax

const request = indexedDB.open('readingListDB', 1);

request.onupgradeneeded = (event) => {
  const db = event.target.result;
  db.createObjectStore('articles', { keyPath: 'id' });
};

request.onsuccess = (event) => {
  const db = event.target.result;
  const tx = db.transaction('articles', 'readwrite');
  tx.objectStore('articles').put({ id: 1, title: 'Example', read: false });
};

Behavior notes

• The API is entirely asynchronous and event-based (older callback style) — most real code wraps it in a small promise-based helper, or uses a library, rather than working with raw events directly.
• Data is organized into object stores (roughly analogous to tables), each with a key path, and optional indexes for querying by other fields.
• A transaction groups related reads/writes; it auto-commits once its associated events finish, without an explicit “commit” call.
• Storage limits are much larger than Web Storage’s — often a meaningful percentage of available disk space — though exact limits and eviction policy are browser- and platform-dependent.

Common mistakes

• Reaching for IndexedDB for small amounts of simple data, where localStorage would be far simpler — its complexity is worth it specifically for larger, structured, or offline-critical data.
• Letting a transaction span an awaited operation unrelated to the database — some browsers auto-close a transaction if it appears idle, causing a subsequent operation on it to fail.

Runtime notes

Standard in every current browser; no built-in equivalent in Node (a compatible package exists for testing purposes, but it isn’t a runtime API there).