HTML5 Rocks

HTML5 Rocks

Blob support for IndexedDB landed on Chrome Dev

By Eiji Kitamura at

Chrome Dev has landed support for Blob on IndexedDB.

This is a long awaited feature for Chrome that allows IndexedDB API to be able to store and retrieve a Blob without converting it to a Base64 string.

IndexedDB provides large scale key-value type persistent storage that is available on most of modern browsers (Safari apparently will land support in iOS8 and Mac OS X 10.10). Check out its implementation status.

Blob is a file-like binary object modern JavaScript engines can handle. File objects inherits from Blob. You can also fetch images and files as Blob via XMLHttpRequest. Check out its implementation status.

Storing a Blob on IndexedDB

There is no way to feature detect the Blob availability in IndexedDB. You basically have to try-catch then use string instead of Blob if it is not available. Here's some sample code:

// Create an example Blob object
var blob = new Blob(['blob object'], {type: 'text/plain'});

try {
  var store = db.transaction(['entries'], 'readwrite').objectStore('entries');

  // Store the object  
  var req = store.put(blob, 'blob');
  req.onerror = function(e) {
  req.onsuccess = function(event) {
    console.log('Successfully stored a blob as Blob.');
} catch (e) {
  var reader = new FileReader();
  reader.onload = function(event) {
    // After exception, you have to start over from getting transaction.
    var store = db.transaction(['entries'], 'readwrite').objectStore('entries');

    // Obtain DataURL string
    var data =;
    var req = store.put(data, 'blob');
    req.onerror = function(e) {
    req.onsuccess = function(event) {
      console.log('Successfully stored a blob as String.');
  // Convert Blob into DataURL string

Blob support for IndexedDB is already available on Firefox and Internet Explorer as well. Safari support needs to be investigated.


Defining Scope In IndexedDB Transactions

By Ido Green at

What is IndexedDB? IndexedDB is an evolving web standard for storage of significant amounts of structured data in the browser and for high performance searches on this data using indexes. In other words, IndexedDB is an object store. It is not the same as a relational database, which has tables with collections rows and columns. It is an important and fundamental difference that affects the way that you design and build your applications (more on the - basic concepts).

So what is new? Changes my friends... we have some changes that are going to throw some errors if we don't handle them with simple syntax change.

From version 17 onwards, Chrome will now throw an error if an IndexedDB transaction is not scoped to an object store. Since all reading and writing of data are done within transactions, we need to create a transaction on a database, specify the scope (such as which object stores you want to access) and determine the kind of access (read only or write).

What does it means in code? Well, instead of passing an empty array to our database.transaction:

var transaction = db.transaction([], IDBTransaction.READ_ONLY);

You should scope to a particular object store, or list of object stores:

// all stores (equivalent to what use to be marked as empty array. )
var transaction = db.transaction(db.objectStoreNames, IDBTransaction.READ_ONLY);

// multiple stores:
var transaction = db.transaction(['ObjectStoreName1', 'ObjectStoreName2'],

// single store - these are equivalent
var transaction = db.transaction(['ObjectStoreName'], IDBTransaction.READ_ONLY);
var transaction = db.transaction('ObjectStoreName', IDBTransaction.READ_ONLY);

You can speed up data access by using the right scope and mode in the transaction. Here's a couple of tips: When defining the scope, specify only the object stores you need. This way, you can run multiple transactions with non-overlapping scopes concurrently. Only specify a READ_WRITE transaction mode when necessary. You can concurrently run multiple READ_ONLY transactions with overlapping scopes, but you can have only one READ_WRITE transaction for an object store.

Other sources:

So until next time... keep pushing the web to near territories.