Offline JavaScript Part 2 – Overview of Interfaces and APIs

In Part 1 we looked at the differences between partial and fully offline use cases. Part 2 provides an overview of the HTML5 Interfaces and JavaScript APIs that make it possible to go offline with web applications. Going offline involves working with multiple pieces and coding for specific patterns. I’ve tried my best to stick to technology that is widely available across the largest variety of browsers.

Offline dependencies

Offline web applications are dependent on three things.  It doesn’t matter if your application is partially or fully offline, you’ll still need to address these in your code.

  • Caching HTML, CSS and JavaScript
  • Data Storage
  • Offline/Online detection


Application Cache. The Application Cache, or AppCache, interface lets you specify and store HTML and CSS files as well as JavaScript libraries so that they are available from the browser’s native cache. Once an item is in the cache the browser will use it regardless of whether it’s online or offline. It’s almost like you never went offline!

The AppCache is an essential part of your application strategy for allowing offline browser reloads or restarts. Without this an application will simply fail to re-load while offline.

Data Storage

Browsers have a variety built-in JavaScript APIs for storing data. The data can be for maintaining the applications state such as for storing bookmarks and form data. Or, it can be used for storing information such as maps, address and phone lists, TO-DOs or points of interest for a vacation.

LocalStorage. The LocalStorage API is super-easy to use. It stores Strings in simple key/value pairs. It’s limited to about 5MBs on most browsers. The two main challenges you’ll run into with LocalStorage are hitting the storage limit and performance hits when serializing and deserializing data.

IndexedDB. IndexedDB is essentially an asynchronous noSQL database that lets you store a wide variety of datatypes so that you don’t have to deal with serialization/deserialization.  Datatypes include String, Object, Array, Blob, ArrayBuffer, Uint8Array and File. While many online sources will tell you that there isn’t a size limit, I’ll tell you that in general you should limit your storage on a mobile device around 50 – 100MBs to help prevent the browser from crashing.

WebSql. It’s widely recommended that you not build applications directly on WebSql. The World Wide Web Consortium (W3C) is letting this standard die off in favor of IndexedDB and LocalStorage. I’m really only including this here for reasons such as Safari 7 and and the Android native browsers before 4.4 only support WebSql. For more information on how to get around this read down to the section on IndexedDBShim.

3rd Party Browser Storage

If the built-in browser storage capabilities aren’t meeting your needs you still have other options.

IndexedDBShim. IndexedDBshim is a Polyfill for WebSQL-based browsers. Because IndexedDB isn’t natively supported on older versions Safari 7 and Opera you can use this 3rd party shim to transparently translate your IndexedDB code to work across Android and iOS.

PouchDB. PouchDB is an Open Source experimental library that is an attempt to smooth some of IndexedDB’s rough edges as well as provide additional functionality, such as the ability to sync with remote stores.

LocalForage (Mozilla).  LocalForage is also an attempt to bridge the gap between LocalStorage and IndexedDB. It gives you an interface that provides much wider browser coverage than IndexedDB by itself.  One of the downsides is the amount of storage you can use. If a user is on an older browser such as IE8 that’s limited to LocalStorage then that user will be limited to storing about 5MBs of data. If you requirements call for using more than that, such as downloading large address lists, then the app won’t work on that browser or you’ll have to build in some sort of paging mechanism that deletes the old data and brings in the new.

Offline/Online Detection

There are a number of ways to detect if the browser is online or offline as well as when the internet status changes.  Some browsers have a built-in detection mechanism. However, it is not always reliable and false positives are a distinct possibility. For that reason, you will have to build additional detection capabilities or lean towards a 3rd party library.

Offline.js. Offline.js is a small Open Source library (~3KB) that detects when you lose an internet connection and when it comes back up. While not perfect, it does handle a lot of cross-browser compatibility issues for you. And, if you find bugs you can always create a fix and submit pull requests.


Caniseuse – IndexedDB

Caniuse – LocalStorage

Caniuse – WebSQL

Let’s Take This Offline