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[[Implementations/NarwhalRhino|]] (draft 4 in 0.1), [[Implementations/NarwhalNode|]] (0.5), [[Implementations/RingoJS|]] (minus globbing), [[Implementations/common-node|]]

Proposal A, Draft 6


The "fs" module provides a file system API for the manipulation of paths, directories, files, links, and the construction of file streams. The IO stream API is beyond the scope of this specification.

Note: Other objects, apart from the "fs" module exports, may claim to conform to this specification.

This specification builds on the normal primitives exported by "fs-base" specified in Filesystem/A/0. All methods implemented by "fs-base" must be exported by and override methods of this module.


Objects implementing the File System API, including the "file" module object, are capability bearing objects that carry and mediate authority to read and write to the underlying storage. As such, the "fs" module can return other objects that implement and attenuate the File System API for sandboxing. Furthermore, streams returned by the file system object are implicitly attenuated to only give the receiver authority to manipulate the given file, without knowledge of the path on which it resides or access to references that would permit it to manipulate other parts of the file system.

Regarding Paths

Many methods accept paths as arguments. In every case, the path may be any object that is coercible to a String with the String constructor, that defers to toString for most objects. This includes Path objects.



open(path, mode|options)
returns an IO stream that supports an appropriate interface for the given options and mode, which include reading, writing, updating, byte, character, unbuffered, buffered, and line buffered streams.
  • path: any value coercible to a String, including a Path, that can be interpreted as a fully-qualified path, or a path relative to the current working directory.
  • mode String: any subtring of "rwa+bxc" meaning "read", "write", "append", "update", "binary", and "exclusive" flags respectively, in any order.
  • options
    • mode String: conforming to the above mentioned mode string pattern
    • charset String: an IANA, case insensitive, charset name. open must throw a (TODO) error if the charset is not supported. The ascii and utf-8 charsets must be supported.
    • read Boolean: open for reading, do not create, set position to beginning of the file, throw an error if the file does not exist.
    • write Boolean: open for writing, create or truncate, set position to the beginning of the file.
    • append Boolean: open for appending, create if it doesn't exist, do not truncate, set position to end of the file.
    • update Boolean: open for updating, create if it doesn't exist, do not truncate, set position to the beginning of the file.
    • binary Boolean return a raw stream instead of a buffered, charset encoded, text stream.
    • exclusive Boolean: open for write only if it does not already exist, otherwise throw an error.
The "open" function mediates the construction of various kinds of streams. As "open" is the only method with the authority to manipulate files, it constructs these types on behalf of a potentially unpriviledged caller. Stream constructors are not directly callable in a secure sandbox, so where and how these stream types are implemented is beyond the necessary scope of this specification. The "open" function always creates a byte stream, and by default wraps that in a textual IO wrapper. "open" returns a stream resulting from the following algorithm:
  1. create a "raw" byte stream. If "x" mode (with either "w" or "a" mode), only open if the file does not already exist. Create the file and open the stream atomically.
    • if "r" mode, make "raw" a ByteReader
    • if "w" mode, make "raw" a ByteWriter
    • if "u" mode, make "raw" a ByteUpdater
  2. if "+", seek to end.
  3. if not "b" mode, return "raw".
  4. return a wrapper encoded/decoded string stream, wrapped around a buffered stream, wrapped around "raw".
    • if "r" mode, wrap "raw" in a TextReader
    • if "w" mode, wrap "raw" in a TextWriter
    • if "u" mode, wrap "raw" in a TextUpdater
read(path, (mode|options)_opt)
opens, reads, and closes a file, returning its content. Equivalent to open(source, mode).read().
write(path, content String|Binary, (mode|options)_opt)
opens, writes, flushes, and closes a file with the given content. If the content is a ByteArray or ByteString, the binary mode is implied. Equivalent to open(source, mode + "w" + (content instanceof Binary ? "b" : "")).write(content).flush(), for mode Strings.
copy(source, target)
reads one file and writes another in byte mode. Equivalent to open(source, "b").copy(target, "b")
rename(path, name String)
Changes the name of a file at a given path. This differs from move in that the target is relative to the source instead of the current working directory. This method in particular should be implemented by the native "fs-base" module overriding any pure JavaScript implementation, if possible.


Creates the directory specified by "path" including any missing parent directories.
Removes whatever exists at the given path, regardless of whether it is a file, direcotory, or otherwise. If the path refers to a directory, but not a symbolic link to a directory, calls removeTree on each member of the directory.
copyTree(source, target)
copies files from a source path to a target path, copying the files of the source tree to the corresponding locations relative to the target, copying but not traversing into symbolic links to directories.


list(path) Array * String
listTree(path) Array * String
returns an Array that starts with the given path, and all of the paths relative to the given path, discovered by a depth first traversal of every directory in any visited directory, reporting but not traversing symbolic links to directories, in lexically sorted order within directories. The first path is always "", the path relative to itself.
listDirectoryTree(path) Array * String
returns an Array that starts with the given directory, and all the directories relative to the given path, discovered by a depth first traversal of every directory in any visited directory, not traversing symbolic links to directories, in lexically sorted order within directories.


returns an Array of all paths that match the given glob pattern from the current working directory.
glob patterns may include asterisks for 0 or more of any character except directory separator matches, question marks for exactly one of any character except directory separator matches, non-delimited square bracket notation for unions and escapes, comma delimited curly brace notation for nestable unions of substrings, and double asterisk for 0 or more of any character including directory separator matches (by way of a depth first traversal of intervening directories, visiting but not following symbolic links).
match(path, pattern)
returns whether a path matches a given glob pattern.
returns a path or path component as a glob pattern that would only match the given path component.

Path Objects

For each of the previous "list" and "glob" methods, there is a corresponding "-Paths" method that returns Path object instances instead of Strings.

listPaths(path) Array * Path
listTreePaths(paths) Array * Path
listDirectoryTreePaths(paths) Array * Path
globPaths(path) Array * Path

Iterator Objects

iterate(path) Iterator * String
returns an iterator that lazily browses a directory, backward and forward, for the base names of entries in that directory.
iteratePaths(path) Iterator * Path
returns an iterator that lazily browses a directory, backward and forward, for the full Paths of entries in that directory, joined on the given path.

Iterator objects have the following members:

next() String or Path
returns the next path in the iteration or throws a StopIteration if there is none.
prev() String or Path
returns the previous path in the iteration or throws a StopIteration if there is none.
returns itself
closes the iteration. After calling close, all calls to next and prev must throw StopIteration.

Directory iterators also support the following Array methods:

  • forEach
  • map
  • every
  • some
  • reduce
  • reduceRight


Symbolic and hard links must not be emulated with Windows Shortcuts.

link(source, target)
hardLink(source, target)
readLink(path) String


isReadable(path) Boolean
isWritable(path) Boolean
same(source, target) Boolean


lastModified(path) Date

Extended Attributes (optional)

Extended attribute methods may be defined on systems that may support the feature on some volumes.

getAttribute(path, key String, default ByteString) ByteString
setAttribute(path, key String, value ByteString)
removeAttribute(path, key String)
discardAttribute(path, key String)
Removes the extended attribute for a given key, if there is a corresponding attribute.
getAttributes(path) Object
Returns an Object that maps all of the extended attribute keys to their corresponding values.
listAttributeNames(path) Array * String


owner(path) String
changeOwner(path, name String)
permissions(path) Permissions
changePermissions(path, permissions Permissions)


workingDirectory() String
workingDirectoryPath() Path
returns the current working directory as an absolute Path object

Paths as Text

join(...) String
takes a variadic list of path Strings, joins them on the file system's path separator, and normalizes the result. details
split(path) Array * String
returns an array of path components. If the path is absolute, the first component will be an indicator of the root of the file system; for file systems with drives (such as Windows), this is the drive identifier with a colon, like "c:"; on Unix, this is an empty string "". The intent is that calling "join.apply" with the result of "split" as arguments will reconstruct the path.
normal(path) String
removes '.' path components and simplifies '..' paths, if possible, for a given path.
absolute(path) String
returns the absolute path, starting with the root of this file system object, for the given path, resolved from the current working directory. If the file system supports home directory aliases, absolute resolves those from the root of the file system. The resulting path is in normal form. On most systems, this is equivalent to expanding any user directory alias, joining the path to the current working directory, and normalizing the result. "absolute" can be implemented in terms of "workingDirectory", "join", and "normal".
canonical(path) String
readLink(path) String
returns the text of a symbolic link at a given path. Throws a TODO error if there is no accessible symbolic link at the given path.
directory(path) String
returns the path of a file's containing directory, albeit the parent directory if the file is a directory. A terminal directory separator is ignored.
base(path, extension_opt String) String
returns the part of the path that is after the last directory separator. If an extension is provided and is equal to the file's extension, the extension is removed from the result.
extension(path) String
returns the extension of a file. The extension of a file is the last dot (excluding any number of initial dots) followed by one or more non-dot characters. Returns an empty string if no valid extension exists. unit test.
a function like "join" except that it treats each argument as as either an absolute or relative path and, as is the convention with URL's, treats everything up to the final directory separator as a location, and everything afterward as an entry in that directory, even if the entry refers to a directory in the underlying storage. Resolve starts at the location "" and walks to the locations referenced by each path, and returns the path of the last file. Thus, resolve(file, "") idempotently refers to the location containing a file or directory entry, and resolve(file, neighbor) always gives the path of a file in the same directory. "resolve" is useful for finding paths in the "neighborhood" of a given file, while gracefully accepting both absolute and relative paths at each stage. unit test.
relative(source, target_opt)
returns the relative path from one path to another using only ".." to traverse up to the two paths' common ancestor. If the target is omitted, returns the path to the source from the current working directory.

Path Type

Path instances inherit from the String prototype.

[new] Path(path...) Path
returns a Path object. Path is a chainable shorthand for working with paths. Path objects have no more or less authority to manipulate the file system that produces them. If multiple arguments are passed to the Path constructor, they are joined to construct the corresponding path. If an argument is an Array, as ascertained by Array.isArray, it must conform to normal output of split, meaning that if the first value is a drive or root, the components are joined absolutely, and otherwise they are joined relatively.
path(path...) Path
a shorthand for creating a new Path that does not use the "new" keyword, and also can be more easily applied variadically with the apply and call methods.

The path constructor accepts as its first argument either a String, Path, or Array. If the path is an Array (as tested by Array.isArray, not merely typeof path == "array"), it must conform to the specification for values returned by "fs.split".

Every path object has the members absolute, base, canonical, directory, normal, and relative. All of these return new Path objects constructed by converting the path to a string, passing it through the likewise named method of the file system API, and converting it back to a Path. Thus, all of these methods are chainable. In addition, join and resolve are variadic, so additional paths can be passed as arguments in either String, Path, or Array form.

Every path object has the members listDirectoryTreePaths, listPaths, and listTreePaths. All of these return objects that provide Path objects constructed by converting the path to a string, passing it through the likewise named method of the file system API, and converting it back to a Path.

Every path object has the members copy, copy, copyTree, discardAttribute, exists, extension, getAttribute, getAttributes, isDirectory, isFile, isLink, isReadable, isWritable, iterate, iterateDirectoryTree, iterateTree, lastModified, link, linkExists, list, listDirectoryTree, listTree, makeDirectory, makeTree, move, open, read, remove, removeAttribute, removeDirectory, removeTree, removeTree, rename, setAttribute, size, split, symbolicLink, touch, and write. All of these functions convert themselves to strings and pass the results through the likewise named method of the file system API.

In addition, paths implement:

converts the path to its String representation
returns the path from this path to the target. Equivalent to Path(relative(this, target)).
returns the path from the source to this path. Equivalent to Path(relative(source, this)).
returns an Array of path Strings that match the given pattern at this path.
returns an Array of Path objects that match a given pattern at this path.


  • Path separator constants are deliberately omitted from this specification to encourage the use of the provided cross-platform methods.
  • Full Posix stat functionality, including stat, has been left for an exercise for a separate specification.

For Future Versions

  • locks
  • comprehensive stat access and modification
  • ACLs
  • temporary files and directories
  • more open options: permissions, owner, groupOwner
  • copy with metadata
  • other proposed names.

Relevant Discussions