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Home Interviews What happens during the onupgradeneeded event? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 02 Jul 2025
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The **onupgradeneeded** event in IndexedDB is triggered when:

  • A database is created for the first time, or
  • You open the database with a higher version number than the existing one.

It’s the only place where you can safely define or update the database schema, such as:

When Does onupgradeneeded Run?

Scenario Triggered?
First time opening the DB Yes
Opening DB with higher version number Yes
Opening DB with same version number No

What Can You Do in onupgradeneeded?

Typical Tasks:

  • Create object stores:
db.createObjectStore('users', { keyPath: 'id' });
  • Create indexes:
store.createIndex('emailIndex', 'email', { unique: true });
  • Rename or migrate data (for upgrades)
  • Delete or recreate outdated object stores

Example

const request = indexedDB.open('MyAppDB', 2); // version 2 triggers upgrade

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

    if (!db.objectStoreNames.contains('users')) {
        db.createObjectStore('users', { keyPath: 'id' });
    }

    const userStore = event.target.transaction.objectStore('users');

    if (!userStore.indexNames.contains('emailIndex')) {
        userStore.createIndex('emailIndex', 'email', { unique: true });
    }

    console.log('Upgrade complete.');
};

Important Notes

  • This event is mandatory for schema upgrades — it’s the only place you can alter the structure.
  • You must not read/write outside of transactions in this event — use the event.target.transaction.

Summary

Feature Description
When it triggers On first open or version upgrade
What it does Schema creation or modification (stores, indexes)
Access object stores Via event.target.result and event.target.transaction
Transaction type Auto-created versionchange transaction (read/write)


 

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