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Home Interviews How do you handle schema migrations in IndexedDB (e.g., adding a new field or object store)? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 06 Jul 2025
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Schema migrations in IndexedDB are handled using the onupgradeneeded event, which is triggered only when the database version number increases.

This is how you can safely update the schema (e.g., add a new field, index, or object store) without losing existing data.

What Triggers a Migration?

const request = indexedDB.open("MyDatabase", 2); // Bump version from 1 → 2

When the version number changes:

  • The onupgradeneeded event fires.
  • You have full access to modify the schema: create/delete stores, add indexes, etc.

Example: Add New Object Store & Index

Upgrade from version 1 → 2

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

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

    // Add new store if upgrading to version 2
    if (oldVersion < 2) {
        const logStore = db.createObjectStore("logs", { keyPath: "id" });
        logStore.createIndex("timestamp", "timestamp");
    }

    // Example: Add a new index to an existing store
    if (oldVersion < 3) {
        const noteStore = event.target.transaction.objectStore("notes");
        noteStore.createIndex("updatedAt", "updatedAt");
    }
};

Important: Existing Data Is Preserved

IndexedDB doesn't delete any data by default during upgrades.

If you change a keyPath or structure, you may need to:

  • Read old data
  • Create a new store
  • Migrate data manually
  • Delete old store

Example: Manual Data Migration

Suppose you need to convert an old store to a new structure:

if (oldVersion < 4) {
    const oldStore = db.objectStoreNames.contains("notes") && db.deleteObjectStore("notes");

    const newStore = db.createObjectStore("notes", { keyPath: "id" });
    newStore.createIndex("title", "title");
    newStore.createIndex("userId", "userId");

    // You could read from old store before deleting it and write to new one
}

Avoid Common Mistakes

Mistake Fix
Forgetting to bump the version Must increase version number
Calling createObjectStore() twice Always check if it already exists
Changing keyPath on existing store Delete and recreate store to change it
Not handling onblocked event Inform user to close other tabs 
request.onblocked = function () {
    alert("Please close other tabs to complete the database upgrade.");
};

Best Practices for Migrations

Tip Why
Use version guards (if (oldVersion < X)) Allows multi-step upgrades from any version
Preserve existing data during migration Avoids data loss
Test upgrades from older versions Ensure backward compatibility
Use onblocked and onerror Ensure upgrade isn't silently blocked

Summary

  • Schema changes are made in onupgradeneeded.
  • You must increment the version to trigger it.
  • Safely add/remove stores, indexes, or migrate structure inside this event.
  • Always handle onblocked, onerror, and version checks.
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