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Index Queries

Indexes are core to fast queries. Create in onupgradeneeded:

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

  // Create multiple indexes
  store.createIndex('category', 'category');
  store.createIndex('price', 'price');
  store.createIndex('name', 'name');
  store.createIndex('inStock', 'inStock', { unique: false });
};

Index Query Methods

Method	Description
index.get(value)	Exact query by index value, returns first match
index.getAll([query])	Returns all matching records
index.getAllKeys([query])	Returns primary keys only (saves memory)
index.count([query])	Returns count of matching records

const tx = db.transaction('products', 'readonly');
const store = tx.objectStore('products');

// Query all electronics
const index = store.index('category');
const req = index.getAll('electronics');

// Price range query (100 ~ 500)
const range = IDBKeyRange.bound(100, 500);
const priceIndex = store.index('price');
const priceReq = priceIndex.getAll(range);

Cursor

Cursors are more memory-efficient than getAll() for large datasets:

const tx = db.transaction('products', 'readonly');
const store = tx.objectStore('products');
const index = store.index('price');

// Open cursor, traverse by price ascending
const cursor = index.openCursor(IDBKeyRange.lowerBound(100));

cursor.onsuccess = (event) => {
  const cur = event.target.result;
  if (!cur) return; // Traversal ended
  console.log('Product:', cur.value.name, 'Price:', cur.value.price);
  cur.continue();
};

Cursor Direction

index.openCursor(null, 'next');      // Default: ascending
index.openCursor(null, 'prev');      // Descending
index.openCursor(null, 'nextunique'); // Skip duplicate values

Key Range (IDBKeyRange)

Exact values, ranges, open/closed intervals freely combined:

const { upperBound, lowerBound, bound, only } = IDBKeyRange;

only(100);                           // Only 100
lowerBound(100);                     // >= 100
upperBound(500);                     // < 500
bound(100, 500);                     // >= 100 && < 500 (closed interval)
bound(100, 500, false, false);     // > 100 && <= 500 (open interval)

Version Migration

onupgradeneeded is the only place for schema migration. Here's a multi-version migration example:

const request = indexedDB.open('MyAppDB', 3); // Upgrade to version 3

request.onupgradeneeded = (event) => {
  const db = event.target.result;
  const oldVersion = event.oldVersion;

  if (oldVersion < 2) {
    db.createObjectStore('notes', { keyPath: 'id', autoIncrement: true });
  }

  if (oldVersion < 3) {
    if (!db.objectStoreNames.contains('users')) {
      const store = db.createObjectStore('users', { keyPath: 'id' });
      store.createIndex('email', 'email', { unique: true });
    }
    const store = db.objectStore('users');
    if (!store.indexNames.contains('age')) {
      store.createIndex('age', 'age');
    }
  }
};

Cannot await in onupgradeneeded

onupgradeneeded is a synchronous callback, cannot await inside. All schema changes must complete synchronously.

Performance Tips

Tip	Reason
Batch writes use single transaction	Each transaction has fixed overhead
Large fields stored as Blob	Blob stored in IndexedDB's separate area, won't block main database
Use indexes for queries, not full table scans	store.getAll() traverses entire table, index query is O(log n)
Close transactions promptly	Long-running transactions block schema upgrades

Notes

• IDBKeyRange.only() only matches unique values: If index is not unique, get() returns only the first match
• Index updates are not real-time: If you modify an indexed field in a record, the index auto-updates; no manual rebuild needed
• Don't modify store structure during cursor traversal: Will cause cursor to unexpectedly end