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What is indexedDB (and how do you use it)?

One of my readers asked me to write about indexedDB. It’s a big topic, so we’ll be looking at bits and pieces of it over the next few days.

Let’s dig in!

What is indexedDB?

indexedDB is a type of database storage built right into the browser. It has a few advantages over cookies and local or session storage.

  • You can store almost any kind of value as a simple key/value pair, without needing to parse arrays and objects into strings and back again.
  • indexedDB databases have much higher storage limits. While it varies by browser and operating system, indexedDB can store GBs of data instead of KBs or MBs.
  • Unlike cookies and local storage, indexedDB databases can be accessed in service workers, which makes passing data between a worker and the browser a lot easier.
  • indexedDB lets you group several tasks into a transaction, and will only save and complete that group of operations if all of them succeed. This helps prevent accidental data loss.

How indexedDB works

Before digging into the specific methods and techniques, it’s helpful to understand how indexedDB works at a high-level.

You create and access a Database. You can have multiple databases per site, but as a best practice, a site or app typically only has one. Databases are scoped to a specific domain, so other sites can’t access them.

Each database has one or more Stores. A store is a collection of data of a similar topic or structure. It’s similar to a table in SQL or a collection in mongoDB, if you’re familiar with either of them.

For example, if you were building a digital library app, you might have one store that contained all of your books, with data about the title, author, published date, and so on. You might have a second store for your authors, with their bio and a list of books they’ve written.

Each item in a store is a key/value pair. The value can be almost any type of data, and it does not need to be converted to a string before saving. The indexedDB API stores it as-is.

Now that we’ve got some core concepts out of the way, let’s look at how to actually work with indexedDB.

Opening a database

Let’s imagine that we want to build a “Spellbook” app that we can use to create a directory of wizards and the spells that they know.

The first thing we need to do is open a new database using the method. Pass in two arguments: the name of the database, and the version number.

// Open a database
let openDB ='spellbook', 1);

For performance reasons, any indexedDB method that involves getting or setting data is asynchronous.

Unfortunately, instead of using Promises, the API is event-driven, with onsuccess and onerror events that fire whenever the method succeeds or fails.

We can attach onsuccess and onerror events to our openDB variable that will run when the database successfully opens or fails, respectively.

In the onsuccess callback function, the openDB.result property is the returned data, in this case the actual database itself. This is pattern used in all of the onsuccess callbacks for this API. In the onerror callback, openDB.error is the error message.

// Open a database
let openDB ='spellbook', 1);

// If the database was successfully opened
openDB.onsuccess = function () {
	let db = openDB.result;
	console.log('success', db);

// If there was an error
openDB.onerror = function () {

Google developer Jake Archibald has created a tiny library that uses the same API as the browser-native indexedDB API, but returns Promises instead of events. We’ll be using vanilla JS for these articles, but Jake’s library is worth a look once you understand the basics of indexedDB.

Closing a database

It’s generally not a good idea to keep a database open for the lifetime of an app.

Open the database when you need to get or write data to it, then close it again when you’re done. You can close a database using the IDBDatabase.close() method on your database.

// If the database was successfully opened
openDB.onsuccess = function () {

	// Get the database
	let db = openDB.result;

	// Do some stuff...

	// Close the database


Actually using a database

Over the new few days, we’ll dig into how to create database stores, and how to add, update, and remove data from them. If there’s time, we’ll also look at how to handle database upgrades, and changes to your data structure that might result.

If you don’t want to wait, or want to dig into even more advanced topics than we’ll cover in this series, I have a guide and ebook on this topic that you might enjoy.