JavaScript: The Definitive Guide, 6th Edition

The Filesystem API

In Reading Blobs, you saw the FileReader class used to read the content of user-selected files, or of any Blob. The File and Blob types are defined by a draft specification known as the File API. Another draft specification, even newer than the File API, gives web applications controlled access to a private local filesystem “sandbox” in which they can write files, read files, create directories, list directories, and so on. At the time of this writing, this Filesystem API is implemented only by Google’s Chrome browser, but it is a powerful and important form of local storage, so it is covered here, even though the API is even less stable than most of the other APIs described in this chapter. This section covers basic filesystem tasks but does not demonstrate all features of the API. Because the API is new and unstable, it is not documented in the reference section of this book.

Working with files in the local filesystem is a multistep process. First, you must obtain an object that represents the filesystem itself. There is a synchronous API for doing this in worker threads and an asynchronous API for use on the main thread:

// Obtaining a filesystem synchronously.  Pass filesystem lifetime and size.
// Returns a filesystem object or raises an exception.
var fs = requestFileSystemSync(PERSISTENT, 1024*1024);

// The asynchronous version uses callback functions for success and error
requestFileSystem(TEMPORARY,       // lifetime
                  50*1024*1024,    // size: 50Mb
                  function(fs)