What are the three IndexedDB transaction modes?

readonly allows concurrent reads with no writes and is the default. readwrite permits reads and writes but serialises against other writers on the same store. versionchange is created automatically during upgradeneeded and is the only mode that can add or remove stores and indexes.

What is the difference between add and put?

add inserts a new record and rejects if a record with the same key already exists, enforcing uniqueness. put performs an upsert: it inserts the record or overwrites an existing one with the same key.

When can I create an object store or index?

Only inside a versionchange transaction, which the browser creates for you during the upgradeneeded event after you open the database with a higher version number. Calling createObjectStore or createIndex at any other time throws.

How do I iterate records in IndexedDB?

Open a cursor with openCursor() on a store or index. The request's onsuccess fires once per record; call cursor.continue() to advance. You can update() or delete() the record at the cursor's current position along the way.

Does this reference store any of my data?

No. It is a static, searchable reference rendered in your browser. It does not open a real database, and nothing is uploaded or saved.

IndexedDB API Reference

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A map of the IndexedDB object graph

IndexedDB is a transactional, asynchronous, key-value store built into the browser for large structured data and offline apps. Its API spans several interfaces — a factory, the database, object stores, indexes, cursors and transactions — each with its own methods. This searchable reference lists the core members with their signatures so you can find the right call quickly.

How it works

You open a database, declare stores and indexes during an upgrade, then read and write inside transactions:

const req = indexedDB.open("app", 2);
req.onupgradeneeded = (e) => {
  const db = e.target.result;
  const store = db.createObjectStore("notes", { keyPath: "id" });
  store.createIndex("by_tag", "tag", { unique: false });
};
req.onsuccess = (e) => {
  const db = e.target.result;
  const tx = db.transaction("notes", "readwrite");
  tx.objectStore("notes").put({ id: 1, tag: "todo", body: "ship it" });
};

Every operation returns an IDBRequest whose onsuccess/onerror fire asynchronously. Transactions auto-commit when control returns to the event loop with no pending requests. Schema changes are confined to the special versionchange transaction during upgradeneeded.

Tips and notes

Use keyPath for in-line keys (the key lives in the value) or autoIncrement for generated keys.
Cursors iterate in key order; pass prev for descending or nextunique to skip duplicate index keys.
getAll() is faster than a cursor when you simply need every matching value as an array.
A thrown error or tx.abort() rolls back every change made in that transaction — IndexedDB is fully atomic per transaction.