File object

Warning: This section describes the File component of the SpiderMonkey JavaScript interpreter. File is non-standard, not generally compiled into distributions, is a potential source of huge security holes, and not well tested.

Getting Started

In order to use the File object from your JavaScript programs, you must enable it by setting the make variable JS_HAS_FILE_OBJECT during the compilation of your Spidermonkey engine.

If you are building a standalone version of Spidermonkey (see: SpiderMonkey Build Documentation), this variable can be added on the make command line, like so:

cd mozilla/js/src
make -f Makefile.ref JS_HAS_FILE_OBJECT=1

Alternatively, if you are building a larger product (such as a browser) and want to include the File object, you may need to perform minor Makefile surgery.

Summary

Non-Standard Server-Side Object

This object lets you work files and directories on the local filesystem, and create OS pipelines. Creating a pipeline involves spawning arbitrary processes; this means that giving a script access to the File object is exactly equivalent to giving the script access to the UNIX shell or DOS command interpreter.

Filesystem access is implemented with NSPR I/O Functions, and as such shares many semantics. Pipelines are implemented with the popen() system call. There is currently no support for p2open()-like semantics.

Here is the original proposal for this object, and a status update from December 1998: http://www.mozilla.org/js/js-file-object.html

Created By

The File constructor:

new File();
new File(filename);

Parameters

filename
Name of the file we want to work with. Directories and pipelines are considered special files. Pipelines either begin or end with the pipe (|) symbol.

Description

Filenames are specified as strings that have an implementation defined format. Use of standard "file:" URLs is encouraged. If no argument is supplied to the constructor, the current working directory is the file object that is returned.

Examples of possible prefix strings are "/" to indicate the Unix root directory, "c:" to specify a Windows drive letter, and "file:" to indicate a file URL.

When a file is constructed, leading and trailing spaces are removed from the filename, so new File(" abc.txt      ") just creates a file called abc.txt. Filenames starting and ending with the pipe symbol (|) are interpreted as pipes. Readable pipelines (i.e. pipe to programs generating output on stdout) begin with the pipe symbol; writeable pipelines end with the pipe symbol. Bi-directional pipelines are not supported.

Properties

Static Properties

File.input
A File object that represents the standard input (stdin).
File.currentDir
A File object that represents the standard output (stdout).
File.currentDir
A File object that represents the standard error (stderr).
File.currentDir
A File object referring to the current directory; this property may be set.
File.separator
The system name separator (slash on UNIX).

Instance Properties

File.length
The length of the file in bytes, or the number of entries in the directory. Note: calculating the length of the directory is very expensive.
File.parent
The directory containing the file.
File.path
The canonical path to the file.
File.name
The name of the file.
File.isDirectory
True if the file is a directory.
File.isFile
True if the file is a file.
File.exists
True if the file exists.
File.canRead
True if the file can be read.
File.canWrite
True if the file can be written.
File.canAppend
True if the file is in append mode.
File.canReplace
True if the file is in replace mode.
File.isOpen
True if the file is open.
File.type
A string specifying the type of data or encoding contained in the file. Currently "ascii" (ASCII), "binary" (UTF-8) or "unicode" (UCS-2). (XXX Note -- ASCII might imply ASCIIZ)
File.mode
A string describing the mode used to open the file.
File.creationTime
A Date object representing the time when the file was created.
File.lastModified
A Date object representing the time when the file was last modified.
File.size
The length of the file in bytes; undefined(?) for directories and pipelines.
File.hasRandomAccess
True if random access is supported for this file. Binary (UTF-8) files and pipelines (including File.intput) do not support random access.
File.hasAutoFlush
Force a flush on each line break? (readonly)
File.position
The file position. Writing to the property moves the file pointer.
File.isNative
True if the file is backed by a native stdio FILE stream. (i.e. file.input, file.output, file.error or a pipeline)

Methods

File.open()
Opens the file, specifying file mode and type.
File.close()
Closes the file.
File.remove()
Removes the file, provided it is not open.
File.copyTo()
Creates a verbatim copy of the file at a new location.
File.renameTo()
Removes the file.
File.flush()
Flushes the operating system's write buffers for the file and blocks until any pending data has been committed to disk.
File.seek()
Moves the file position pointer forward or backwards.
File.read()
Read a fixed number of bytes from the file.
File.readln()
Read line from the file.
File.readAll()
Read the entire file in, returning an array of lines.
File.write()
Write bytes to the file.
File.writeln()
Write bytes to the file, followed by a line separator, flushing the buffer if file.hasAutoflush.
File.writeAll()
Writes an array of lines to the file, obeying the same semantics as writeln.
File.list()
Get a list of files, potentially matching a regular expression filter, from the file. Only has meaning when the file is a directory.
File.mkdir()
Create a directory. Directory will be created in the same directory as the file, unless the file is a directory, in which case, the directory will be created in that directory. XXX
File.toString()
Returns the canonical path to the file.
File.toURL()
Returns a file:// URL describing the file relative to the local file system. URL is URI-encoded.

Examples

Example: Hello, world

File.output.writeln("Hello, world");

Example: Writing a new file

var file = new File("myfile.txt");
file.open("write,create", "text");
file.writeln("The quick brown fox jumped over the lazy dogs");
file.close();

Example: Reading from a file

var data; 
var file = new File("myfile.txt");
file.open("read", "text");
data = file.readln();
file.close(); 

Example: Sending mail through a pipeline

var mail = new File("|/usr/lib/sendmail foo@bar.com");
mail.writeln("I love JavaScript.\nPipe support is especially good!");
mail.close();

Document Tags and Contributors

Last updated by: ExE-Boss,