Don't try to store Blobs directly in IndexedDB, unless you want to cry. Browsers still suck at it.
PouchDB and blob-util have workarounds to avoid the browser bugs.
I know it's 2015, and Blobs/IndexedDB should be universally supported already. But sadly they're not, so here's the sorry state of things.
Browsers have three ways of storing data: LocalStorage, WebSQL, and IndexedDB. They all suck for different reasons, which is why there are so many abstraction layers out there: PouchDB, LocalForage, Lawnchair, YDN-DB, MakeDrive, etc.
Browsers don't consistently handle Blobs either. The caniuse.com page for Blobs is a bit disingenuous; really IE should be yellowy-green, because it doesn't consistently support all the canvas and FileReader methods. Blobs in Chrome also have severe bugs before v43.
So let's see all the different browsers and storage engines, and how they stack up:
Supported by most browsers, althought not Chrome extensions, Chrome apps, web workers, or service workers.
You can store Blobs in LocalStorage as base64 strings, which is really inefficient. Plus, many LocalStorage implementations only let you store up to 5MB, so you hit the limit pretty fast.
WebSQL and IndexedDB have much higher limits. So let's see how the different browsers work with those two.
Supports both IndexedDB and WebSQL. Chrome originally got IndexedDB in v23.
WebSQL doesn't support storing Blobs themselves, only strings. You can store binary strings directly, which is the most efficient, but then the '\u0000' byte causes data to get lost. PouchDB works around this by eliminating the '\u0000' in a safe and very efficient way.
IndexedDB has many Blob bugs in Chrome. Here's the history:
- pre-v36: Chrome didn't support IndexedDB Blobs at all, so PouchDB (and most other libs like LocalForage) work around this by storing data as base64-encoded strings. Note this also includes Android up to Lollipop 5.0. (Chromium issue)
- v37: Chrome introduced broken support for Blobs (issue). It was broken because the mimetype wasn't correctly returned.
- v38: The mimetype bug was fixed in v38, but Chrome had two more Blob/IndexedDB bugs: this one and this one. The second one in particular was a race condition causing data to be permanently unreadable, which was a big enough blocker that PouchDB continued downgrading Chrome to base64-only.
- v43: Chrome finally fixed all the Blob bugs, so PouchDB auto-detects it and upgrades to Blob support (test it out here).
Android didn't support IndexedDB until 4.4 Kitkat, and as of this writing, more than half of all Android devices are still pre-Kitkat. Some Samsung/HTC Android 4.3 devices have a broken implementation of IndexedDB based on an older version of the spec. PouchDB detects this and falls back to WebSQL.
Additionally, many pre-4.4 devices don't support Blobs correctly - either they're using vendor prefixes like window.webkitURL or they use the deprecated BlobBuilder API. blob-util works around these issues.
4.4 Kitkat devices will either have Chrome 30 or Chrome 33, depending on whether it's 4.4.0-4.4.1 or 4.4.2+. Lollipop is auto-updating; it debuted with Chrome v37 and is up to v42 as of this writing.
Note this applies to WebViews (i.e. Cordova/PhoneGap apps), the stock browser, and most of the non-Chrome/non-Firefox browsers you'll find in the Play Store, since they just wrap a WebView (e.g. CM Browser, Dolphin Browser, and Link Bubble).
WebSQL: Safari WebSQL has the same '\u0000' bug as Chrome (on both iOS and desktop), as well as another bug that affects Safari pre-v7.1 and iOS pre-8.0 where all data is coerced to UTF-16 instead of UTF-8, meaning it takes up twice the space. PouchDB detects UTF-16 vs UTF-8 encoding and reacts accordingly.
IndexedDB: The less said about Safari IndexedDB, the better. It is so buggy that PouchDB, LocalForage, and YDN-DB all ignore it. For what it's worth, though, it doesn't support binary Blobs according to HTML5Test.com.
Neither one supports WebSQL, but they're actually both great about storing Blobs in IndexedDB. IE has supported Blobs since it introduced IndexedDB in v10, and Firefox has had them since 2011.
That being said, these two have bugs related to the Blob/FileReader APIs themselves:
IE doesn't have FileReader.prototype.readAsBinaryString (only readAsArrayBuffer), so if you want to convert a Blob to a binary string or a base64 string most efficiently, you want to use readAsBinaryString everywhere but IE. PouchDB and blob-util both do this.
Firefox, conversely, doesn't have the canvas.toBlob() method, so if you want to convert a canvas to a Blob, you need to use canvas.toDataURL() and convert the dataURL to a Blob instead. blob-util does this under the hood.
Edit: My bad; it's actually Chrome, Safari, and IE that don't support canvas.toBlob(). So that leaves Firefox as the one browser with a spotless record!
Edit: IE (from version 10 on) has a vendor-prefixed canvas.msToBlob() method (https://msdn.microsoft.com/library/hh771732%28v=vs.85%29.aspx) so it does support the toBlob method.
A lot of this is documented in the PouchDB FAQs, the PouchDB 3.0.6 release notes, and "10 things I learned from reading and writing the PouchDB source". More research on browser storage can be found in this gist.
I'm not aware of any database library that stores Blobs as efficiently or in as many browsers as PouchDB (if I'm wrong, though, then let me know on Twitter 😃). You can even use the localstorage adapter to store Blobs that way (in which case they will be inefficiently base64-encoded). And the proof is in the pudding: the PouchDB test suite is insane.