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Home Interviews What are indexes in IndexedDB and how do you use them? – MindStick
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1 Answers

Manish Sharma
Manish Sharma 02 Jul 2025
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In IndexedDB, an index is a special data structure that lets you quickly search and retrieve records by a property other than the primary key (keyPath).

What is an Index?

  • An index maps values of a property to their corresponding records, allowing efficient queries and lookups.
  • Think of it like SQL CREATE INDEX on a column.
  • Great for filtering, searching, or sorting on non-primary-key fields.

Example: When to Use Indexes

Suppose your object store is:

{ id: 1, name: "Alice", email: "alice@example.com" }

You can:

  • Use "id" as the primary key (keyPath)
  • Create an index on "email" for quick lookup

How to Create an Index

Indexes are created in the onupgradeneeded event when the database schema is defined or upgraded.

const request = indexedDB.open("UserDB", 2);

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

    // Create object store if not already exists
    const userStore = db.objectStoreNames.contains("users")
        ? event.target.transaction.objectStore("users")
        : db.createObjectStore("users", { keyPath: "id" });

    // Create index on 'email' field
    userStore.createIndex("emailIndex", "email", { unique: true });
};
  • "emailIndex" = name of the index
  • "email" = path to the field being indexed
  • { unique: true } = prevents duplicate values for that index

How to Use an Index (Query)

function getUserByEmail(db, email) {
    return new Promise((resolve, reject) => {
        const tx = db.transaction("users", "readonly");
        const store = tx.objectStore("users");
        const index = store.index("emailIndex");

        const request = index.get(email); // fast lookup by email
        request.onsuccess = () => resolve(request.result);
        request.onerror = () => reject(request.error);
    });
}

You can also use:

  • index.getAll(value)
  • index.openCursor() for iteration
  • index.count() to count matches

Example: Get All Users by Country

// In onupgradeneeded
store.createIndex("countryIndex", "country");

// Later
const index = db.transaction("users").objectStore("users").index("countryIndex");
index.getAll("India").onsuccess = (e) => {
    console.log("All users from India:", e.target.result);
};

Benefits of Using Indexes

Feature Benefit
Fast search Lookup by fields other than primary key
Filtering Get records that match specific criteria
Sorting Use .openCursor() with direction
Uniqueness Enforce unique values on fields

Summary

  • Indexes improve read performance by allowing efficient queries on object properties.
  • Define them in onupgradeneeded using createIndex(name, keyPath, options).
  • Query using store.index('indexName').get(...), getAll(), or openCursor().
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