io/file

We are planning to deprecate the use by Firefox add-ons of the techniques described in this document.

Don't use these techniques to develop new add-ons. Use WebExtensions instead.

If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions instead.

Add-ons developed using these techniques might not work with multiprocess Firefox (e10s), which is already the default in Firefox Nightly and Firefox Developer Edition, and will soon be the default in Beta and Release versions of Firefox. We have documentation on making your add-ons multiprocess-compatible, but it will be more future-proof for you to migrate to WebExtensions.

A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.

Experimental

Provides access to the local filesystem.

Usage

Paths

Path specifications in this API are platform-specific. This means that on Windows paths are specified using the backslash path separator (\), and on Unix-like systems like Linux and OS X paths are specified using the forward slash path separator (/).

If your add-on uses literal Windows-style path specifications with this API, your add-on likely won't work when users run it on Unix-like systems. Likewise, if your add-on uses literal Unix-style path specifications, it won't work for users on Windows.

To ensure your add-on works for everyone, generate paths using the join function. Unfortunately this API does not currently provide a way to obtain an absolute base path which you could then use with join. For now, you need to require("chrome") and use the XPCOM directory service as described in this article about File I/O.

Note that if you do decide to hardcode Windows-style paths, be sure to escape backslashes in strings. For example, to specify the file at C:\Users\Myk, you need to use the string "C:\\Users\\Myk", not "C:\Users\Myk".  Read more about escaping characters in strings.

Globals

Functions

basename(path)

The path parameter must be an absolute path, relative paths will cause an error.

Use the fs/path module for relative path support.

Returns the last component of the given path. For example, basename("/foo/bar/baz") returns "baz". If the path has no components, the empty string is returned.

Parameters

path : string
The path of a file.

Returns

string : The last component of the given path.

dirname(path)

Returns the path of the directory containing the given file. If the file is at the top of the volume, the empty string is returned.

Parameters

path : string
The path of a file.

Returns

string : The path of the directory containing the file.

exists(path)

Returns true if a file exists at the given path and false otherwise.

Parameters

path : string
The path of a file.

Returns

boolean : True if the file exists and false otherwise.

join(...)

Takes a variable number of strings, joins them on the file system's path separator, and returns the result.

Parameters

... : strings
A variable number of strings to join. The first string must be an absolute path.

Returns

string : A single string formed by joining the strings on the file system's path separator.

list(path)

Returns an array of file names in the given directory.

Parameters

path : string
The path of the directory.

Returns

array : An array of file names. Each is a basename, not a full path.

mkpath(path)

Makes a new directory named by the given path. Any subdirectories that do not exist are also created. mkpath can be called multiple times on the same path.

Parameters

path : string
The path to create.

open(path, mode)

Returns a stream providing access to the contents of a file.

Parameters

path : string
The path of the file to open.

mode : string
An optional string, each character of which describes a characteristic of the returned stream.

  • If the string contains "r", the file is opened in read-only mode.
  • "w" opens the file in write-only mode.
  • "b" opens the file in binary mode. If "b" is not present, the file is opened in text mode, and its contents are assumed to be UTF-8.

If mode is not given, "r" is assumed, and the file is opened in read-only text mode.

Apart from these options, this API always passes the following options:  CREATE_FILE, TRUNCATE (see https://dxr.mozilla.org/mozilla-central/source/nsprpub/pr/include/prio.h#550). This means that:

  • if the file does not exist it is created
  • if the file exists, its length is truncated to zero
  • it is not possible to open the file in append mode.
Returns

stream : If the file is opened in text read-only mode, a TextReader is returned, and if text write-only mode, a TextWriter is returned. See text-streams for information on these text stream objects. If the file is opened in binary read-only mode, a ByteReader is returned, and if binary write-only mode, a ByteWriter is returned. See byte-streams for more information on these byte stream objects. Opened files should always be closed after use by calling close on the returned stream.

read(path, mode)

Opens a file and returns a string containing its entire contents.

Parameters

path : string
The path of the file to read.

mode : string
An optional string, each character of which describes a characteristic of the returned stream. If the string contains "b", the contents will be returned in binary mode. If "b" is not present or mode is not given, the file contents will be returned in text mode.

Returns

string : A string containing the file's entire contents.

remove(path)

Removes a file from the file system. To remove directories, use rmdir.

Parameters

path : string
The path of the file to remove.

rmdir(path)

Removes a directory from the file system. If the directory is not empty, an exception is thrown.

Parameters

path : string
The path of the directory to remove.

isFile(path)

Returns true only if this path specifies a file.

const fileIO = require("sdk/io/file");

let path = "/Users/Work/";
let list = fileIO.list(path);

for (i = 0; i < list.length; i++) {
  let item = fileIO.join(path, list[i]);
  if (fileIO.isFile(item)) {
    console.log(item + " is a file");
  }
  else {
    console.log(item + " is a directory");
  }
}
Parameters

path : string
The path of the object.

Document Tags and Contributors

 Contributors to this page: wbamberg, evold, Delapouite
 Last updated by: wbamberg,