Firefox OS: the Device Storage API

Working with the Device Storage API of Firefox OS can be a bit complicated if you don't have a real device at hand. But if you use Firefox OS Simulator, the following advices can help you.

SD Card

The SD Card location used by Firefox OS Simulator is mapped to a physical folder on your computer.

On Windows:


On Ubuntu:


On Mac:

Let me know via a comment below where is this folder on Mac.

Of course, USERNAME and PROFILEKEY are specific to your configuration, so make sure to look into the right folder.

Place files in this folder and you’ll be able to access them from the Simulator using a privileged app.

If this folder doesn't exist, I believe it's safe to create it yourself first.

The File objects

When you call get or enumerate on navigator.getDeviceStorage('sdcard') you get File objects.

For reference, these objects look like this (slightly different than the File API page on MDN):

  name: 'file.txt',
  size: 32768, // In bytes
  type: 'text/plain', // MIME type
  lastModifiedDate: 'Fri Nov 22 2013 12:00:00 GMT+0000 (GMT Standard Time)', // Date object,
  mozFullPath: 'path/to/the/file' // Relative to the SD Card folder

They also have 2 methods: slice() and mozSlice().

Then if you want to have access to the content of these files in JavaScript, you can just pass them to the FileReader API:

var reader = new FileReader();
reader.onload = function(e) {
  // is the content of the file.
reader.readAsText(file); // Or whatever method suits you best.

I found this API complex to work with. There are different way to access files depending on:

  • what you do with (read only or write)
  • whether you know the file location or not

I hope to write a more complete post or tutorial on the Device Storage API in the future, as accessing files is a common use case for web apps in Firefox OS.