File IO

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==User input via nsIFilePicker== ==User input via nsIFilePicker==
-See [[Dev : nsIFilePicker]].+The [ file picker component (nsIFilePicker)] can be used to open standard Open / Save dialogs. The components returns user-specified file as nsIFile.
==nsIFile and path strings== ==nsIFile and path strings==

Revision as of 18:48, 17 June 2006

This page is part of the extension development documentation project.

Ask your questions in MozillaZine Forums. Also try browsing example code.

Note: development documentation is in process of being moved to Mozilla Development Center (MDC).

This article describes local file input/output in chrome JavaScript.

You access the filesystem using Mozilla XPCOM components. The list of components used for local I/O is available at


Available libraries

There are a few JavaScript wrappers for I/O XPCOM components. See JSLib and io.js (original by MonkeeSage). The io.js module is much smaller and very easy to use (simple examples are included in the module).

Creating a file object ("opening" files)

var file = Components.classes[";1"]

Note, that the path passed to initWithPath() should be in "native" form (e.g. "C:\\Windows"). If you need to use file:// URIs as initializers, see below.

Also note, that initWithPath() / initWithFile() functions don't throw an exception if specified file does not exist. An exception is thrown when methods that require the file existance are called, e.g. isDirectory(), moveTo() etc.

Getting special files

// get profile directory
var file = Components.classes[";1"]
                     .get("ProfD", Components.interfaces.nsIFile);

Here are some strings you can put in place of "ProfD" (stolen from MonkeeSage's I/O module comments):

String Meaning
ProfD profile directory
DefProfRt user (e.g., /root/.mozilla)
UChrm %profile%/chrome
DefRt %installation%/defaults
PrfDef %installation%/defaults/pref
ProfDefNoLoc %installation%/defaults/profile
APlugns %installation%/plugins
AChrom %installation%/chrome
ComsD %installation%/components
CurProcD installation (usually)
Home OS root (e.g., /root)
TmpD OS tmp (e.g., /tmp)

Look in the Source for other strings available: [1] [2].

Getting your extension's folder

This will only work in Firefox/Thunderbird 1.5, not 1.0.

To get the directory that an extension is installed in, you have to use nsIExtensionManager like this:

const id = "ID";
var ext = Components.classes[";1"]

Replace ID with the extension's ID, and this will return an nsIFile of the directory of the extension . This value is read only. For additional information, view the source [3].

Creating Folders

To create a folder, use nsIFile.create():

var file = Components.classes[";1"]
                     .get("ProfD", Components.interfaces.nsIFile);
if( !file.exists() || !file.isDirectory() ) {   // if it doesn't exist, create
   file.create(Components.interfaces.nsIFile.DIRECTORY_TYPE, 0664);

The above example creates a folder called "DIR" in the Profile folder. For more information, refer to the nsIFile Reference at XULPlanet.

Creating temporary files

To create a temporary file, use nsIFile.createUnique():

var file = Components.classes[";1"]
                     .get("TmpD", Components.interfaces.nsIFile);
file.createUnique(Components.interfaces.nsIFile.NORMAL_FILE_TYPE, 0664);
// do whatever you need to the created file

User input via nsIFilePicker

The file picker component (nsIFilePicker) can be used to open standard Open / Save dialogs. The components returns user-specified file as nsIFile.

nsIFile and path strings

You can use nsIFile.path to get platform-specific path string, e.g. "C:\Windows\System32" or "/usr/share".

If you want to get a file:// URL of a file or an nsIFile from file:// URL, you need to use nsIFileProtocolHandler:

// file is nsIFile
var ios = Components.classes[";1"]
var fileHandler = ios.getProtocolHandler("file")
var URL = fileHandler.getURLSpecFromFile(file);

To load from file://, http://, chrome://, resource:// and other URLs directly, use XMLHttpRequest or nsIChannel (example).

Also note that generally you don't need to use nsIFile::path. Use nsIFile directly wherever possible. An example below shows how you should save a path in user prefs.

Storing nsILocalFile in preferences

The following two snippets show the right way to store a file path in user preferences (more about preferences in Mozilla):

Absolute path (nsILocalFile)

To store arbitrary path in user preferences, use this code.

// |file| is nsILocalFile
// 1. Write path to prefs
var prefs = Components.classes[";1"]
prefs.setComplexValue("filename", Components.interfaces.nsILocalFile, file);

// 2. Read path from prefs
var file = prefs.getComplexValue("filename", Components.interfaces.nsILocalFile);

Relative path (nsIRelativeFilePref)

To store paths relative to one of the predefined folders listed above, for example file relative to profile folder, use the following code:

// 1. Write to prefs
var relFile = Components.classes[";1"]
relFile.relativeToKey = "ProfD"; // or any other string listed above
relFile.file = file;             // |file| is nsILocalFile
     Components.interfaces.nsIRelativeFilePref, relFile);

// 2. Read from prefs
var value = prefs.getComplexValue("filename", 
// |value.file| is the file.

Navigating with nsIFile

Get a file in given directory

Assume, file is an nsIFile pointing to some directory (f.e. a user profile directory). You can use file.append("myfile.txt"); to make file point to myfile.txt inside that directory.

Note: avoid using dir.path+"\\"+"myfile.txt", as it is not cross-platform at all. Using something like ((\\/) != -1) ? path + "\\" : path + "/") + "myfile.txt"; is possible, but nsIFile::append() is much easier to read and is guaranteed to work on all platforms Mozilla itself works.

Enumerating files in given directory

The snippet below makes an array of |nsIFile|s corresponding to sub-directories/"sub-files" of given directory. You can tell files from folders by calling nsIFile::isDirectory() and nsIFile::isFile() methods on each entry.

// file is the given directory (nsIFile)
var entries = file.directoryEntries;
var array = [];
  var entry = entries.getNext();

Reading from a file

Note: This code is not intl-aware. Reading non-ASCII characters will not work correctly. See Reading textual data for intl-aware code.


// |file| is nsIFile
var data = "";
var fstream = Components.classes[";1"]
var sstream = Components.classes[";1"]
fstream.init(file, 1, 0, false);

var str =;
while (str.length > 0) {
  data += str;
  str =;


Line by line

// open an input stream from file
var istream = Components.classes[";1"]
istream.init(file, 0x01, 0444, 0);

// read lines into array
var line = {}, lines = [], hasmore;
do {
  hasmore = istream.readLine(line);
} while(hasmore);


// do something with read data


This will allow you to read a file without locking up the UI thread.

// |file| is nsIFile
var ios = Components.classes[";1"]
var fileURI = ios.newFileURI(file);
var channel = ios.newChannelFromURI(fileURI);
var observer = {
  onStreamComplete : function(aLoader, aContext, aStatus, aLength, aResult)
var sl = Components.classes[";1"]
sl.init(channel, observer, null);

Writing to a file

Note: This code is not intl-aware. Writing non-ASCII characters will not work correctly. See Writing textual data for intl-aware code.

// file is nsIFile, data is a string
var foStream = Components.classes[";1"]

// use 0x02 | 0x10 to open file for appending.
foStream.init(file, 0x02 | 0x08 | 0x20, 0664, 0); // write, create, truncate
foStream.write(data, data.length);

Flags parameter to the nsIFileOutputStream::init() function (For more information refer to prio.h file).

flags: The file status flags. It is a bitwise OR of the following bit flags (only one of the first three flags below may be used):

Name Value Description
PR_RDONLY 0x01 Open for reading only.
PR_WRONLY 0x02 Open for writing only.
PR_RDWR 0x04 Open for reading and writing.

If the file does not exist, the file is created. If the file exists, this flag has no effect.


The file pointer is set to the end of the file prior to each write.


If the file exists, its length is truncated to 0.

PR_SYNC 0x40

If set, each write will wait for both the file data and file status to be physically updated.

PR_EXCL 0x80

With PR_CREATE_FILE, if the file does not exist, the file is created. If the file already exists, no action and NULL is returned.


There are more methods and properties on nsIFile and nsILocalFile interfaces, please refer to documentation on XUL Planet. Those methods/properties are mostly self-explanatory, so we haven't included examples of using them here.