How-To Use HTML5 File System Access API

One of the major roadblocks to using the browser as a true thin client for applications has been the lack of access to the file system. In the past a developer could access the file System using JScript (the Microsoft Javascript variant) in Internet Explorer by accessing the FileSystemObject API. However this was off limits to the other browsers until now.

Along with offline capabilities for web applications, HTML5 API offers the capability to access the file system to read and write to files. This is possible through the new File API in HTML5 ( http://www.w3.org/TR/FileAPI/).

Through the File API the following operations are possible:

FileList represents a list of files located on a file system;
Blob provides an interface to binary files;
File, another interface, includes information on a file’s attributes;
FileReader allows for reading from a file;
FileError and FileException provides the means to handle errors and exceptions, and finally
URI Scheme which provides an interface to binary files from a web application.

A second specification, File:Writer is also under consideraton at the w3c to allow web applications access to write to files in a sandbox.

The File: Writer API offers the following operations:

BlobBuilder, an interface to build a blob from a string;
FileSaver, provides a means to write to a file and and event model to interact with files;
FileWriter expands on the FileSaver interface with a richer set of operations;
FileWriterSync, provides operations to interact asynchronuously with files through a Web Worker.

As you might imagine, not all the browsers are implementing this APIs simultaneously but all the major vendors have these specifications on their development roadmap. At the time of this writing only Google Chrome 13 fully supports both specifications. Firefox 7 will support the File:Writer specification and it already supports the File API. Internet Explorer will support the APIs with in version 10 and Opera will implements these APIs in version 12.

These APIs are designed to be used in conjunction with the XMLHttpRequest2 object and other HTML5 APIs and technologies.

For example:

// To access the sandboxed file system (meaning two or more apps canât access each others files) you need to initialize the FileSystem object:

if('webkitRequestFileSystem' in window)
{
window.requestFileSystem = window.webkitRequestFileSystem;
window.requestFileSystem(window.PERSISTENT, 10*1024*1024, onSuccess, onError);
}else if('moz_requestFileSystem' in window)
{
	//firefox
	window.requestFileSystem = window.moz_requestFileSystem;
	window.requestFileSystem(window.PERSISTENT, 10*1024*1024, onSuccess, onError);
}
{
	//this browser is not suported
}
function onSuccess(fs) { //do something};
function onError(e) {// handle the error};

In the above example, the first parameter of the requestFileSystem can either window.PERSISTENT or window.TEMPORARY. The temporary access may remove the files that are created or read after the session is finished, persistent, as the name implies doesn’t remove any files.

Also in the above example I am assuming that this is a web application and not an offline app and since Google Chrome and Firefox have started prefixing their APIs, you will need to check the user agent in your initialization. Otherwise if its an offline app you would simply initialize the the file system for either browser.

Read from a File

Next to read from a file

You get access to a file through the FileEntry interace. So after getting a handle on the file system, in the onSuccess callback function you can get a file and open it for reading like so:

function onSuccess(fs)
{
	fs.root.getFile('helloWorld.txt', null, function(fileEntry){
		fileEntry.file(function(f)
			{
				var reader = new FileReader();
}
)
}

}

Write to a File

To write to a file you need a create a BlobBuilder. In Chrome as of version 12 you need to prefix the interface name with “WebKit” like WebKitBlobBuilder. This is also true for Safari. For Firefox, you would need to add the prefix “Moz” like so : MozBlobBuilder. I guess browser vendor want to keep us developers from getting bored.

function onSuccess(fs)
{
	fs.root.getFile('storage.txt', {create:true}, function(fileEntry){
		fileEntry.createWriter(function(f)
			{
				f.onwriteend = function(event) {};
				f.onerror = function(err) {};
				BlobBuilder = window.BlobBuilder || window.WebKitBlobBuilder || window.MozBlobBuilder;
				var writer = new BlobBuilder();
				writer.append('some text');
				f.write(writer.getBlob('text/plain'));
}
)
}

}

The HTML5 File API holds a lot of promise for developers, especially for offline or installable apps. For web applications that has a persistent data store, I don’t really see the point. Anyway the API has a lot of interesting features that I invite to explore further at w3c.

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Comments 4 comments

Manjunath 4 years ago

what's fs in onSuccess function


klanguedoc profile image

klanguedoc 4 years ago from Canada Author

The fs is the file system object


ingvare 3 years ago

The function "onSuccess"

Normal right hand bracket missing. Suppose it should be just before the last curly bracket. At least putting one get rid of the error message


klanguedoc profile image

klanguedoc 3 years ago from Canada Author

I'll check it out

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