Data compression and decompression support, packing base64 into UTF8 high and low bytes that we use to 'compress' base 64 encoded to maximise the amount of binary data we can store offline on described by @triblondon.
If you want to store binary data for offline use browsers today don't officially support it. (localStorage only stores strings, WebSQL stores a few different types - but not binary, and IndexedDB doesn't support it on all browsers)
Also IndexedDB is unlikely to ever support it properly across all browsers as File API should one day fulfill the storing binary files for offline use case.
But you can work around it. If you base 64 the binary data it becomes a string and that string can be stored with the storage technology of your choosing.
Though, in reality it is only really practical to use IndexedDB (falling back to WebSQL when IndexedDB is not available as localStorage has limited capacity and base 64 encoded binary files tend to be quite large.
When data in a web browser's offline storage is stored (for example IndexedDB, WebSQL or localStorage) it stores that data as UTF16.
Because UTF-16 is capable of encoding over a million different characters and Base 64 only uses 64 of them it turns out that UTF-16 is not a very efficient format for storing base 64 encoded data.
For a more detailed explanation see: http://labs.ft.com/2012/06/text-re-encoding-for-optimising-storage-capacity-in-the-browser/
What do we do
We squash the characters together (if you look at them in dev tools the string often comes out as a mixture of characters from East Asian languages. This algorithm allows us to store more than twice as much base 64 encoded data than without it.
npm install ftdatasquasher
Credits and collaboration
The lead developer of FastClick is Rowan Beentje at FT Labs. This fork is currently maintained by Matt Andrews. All open source code released by FT Labs is licenced under the MIT licence. We welcome comments, feedback and suggestions. Please feel free to raise an issue or pull request. Enjoy.