File System Access

Draft Community Group Report,

This version:
https://wicg.github.io/file-system-access/
Issue Tracking:
GitHub
Inline In Spec
Editor:
(Google)

Abstract

This document defines a web platform API that enables developers to build powerful web apps that interact with files on the user’s local device. It builds on File API for file reading capabilities, and adds new API surface to enable modifying files, as well as working with directories.

Status of this document

This specification was published by the Web Platform Incubator Community Group. It is not a W3C Standard nor is it on the W3C Standards Track. Please note that under the W3C Community Contributor License Agreement (CLA) there is a limited opt-out and other conditions apply. Learn more about W3C Community and Business Groups.

1. Introduction

This section is non-normative.

This API enables developers to build powerful apps that interact with other (non-Web) apps on the user’s device via the device’s file system. Prominent examples of applications where users expect this functionality are IDEs, photo and video editors, text editors, and more. After a user grants a web app access, this API allows the app to read or save changes directly to files and folders on the user’s device. Beyond reading and writing files, this API provides the ability to open a directory and enumerate its contents. Additionally, web apps can use this API to store references to files and directories they’ve been given access to, allowing the web apps to later regain access to the same content without requiring the user to select the same file again.

This API is similar to <input type=file> and <input type=file webkitdirectory> [entries-api] in that user interaction happens through file and directory picker dialogs. Unlike those APIs, this API is currently purely a javascript API, and does not integrate with forms and/or input elements.

Additionally this API also makes it possible for websites to get access to some directory without having to first prompt the user for access. This enables use cases where a website wants to save data to disk before a user has picked a location to save to, without forcing the website to use a completely different storage mechanism with a different API for such files. It also makes it easier to write automated tests for code using this API. The entry point for this is the navigator.storage.getDirectory() method. This is similar to the temporary file system as defined in earlier drafts of File API: Directories and System.

2. Files and Directories

2.1. Concepts

An entry is either a file entry or a directory entry.

Each entry has an associated name (a string).

A valid file name is a string that is not an empty string, is not equal to "." or "..", and does not contain '/' or any other character used as path separator on the underlying platform.

Note: This means that '\' is not allowed in names on Windows, but might be allowed on other operating systems. Additionally underlying file systems might have further restrictions on what names are or aren’t allowed, so a string merely being a valid file name is not a guarantee that creating a file or directory with that name will succeed.

A file entry additionally consists of binary data (a byte sequence) and a modification timestamp (a number representing the number of milliseconds since the Unix Epoch).

A directory entry additionally consists of a set of children, which are themselves entries. Each member is either a file or a directory.

An entry entry should be contained in the children of at most one directory entry, and that directory entry is also known as entry’s parent. An entry's parent is null if no such directory entry exists.

Note: Two different entries can represent the same file or directory on disk, in which case it is possible for both entries to have a different parent, or for one entry to have a parent while the other entry does not have a parent. Typically an entry does not have a parent if it was returned by navigator.storage.getDirectory() or one of the local file system handle factories, and an entry will have a parent in all other cases.

Entries can (but don’t have to) be backed by files on the host operating system’s local file system, so it is possible for the binary data, modification timestamp, and children of entries to be modified by applications outside of this specification. Exactly how external changes are reflected in the data structures defined by this specification, as well as how changes made to the data structures defined here are reflected externally is left up to individual user-agent implementations.

An entry a is the same as an entry b if a is equal to b, or if a and b are backed by the same file or directory on the local file system.

TODO: Explain better how entries map to files on disk (multiple entries can map to the same file or directory on disk but an entry doesn’t have to map to any file on disk).

To resolve an entry child relative to a directory entry root, run the following steps:
  1. Let result be a new promise.

  2. Run the following steps in parallel:

    1. If child is the same as root, resolve result with an empty list, and abort.

    2. Let childPromises be « ».

    3. For each entry of root’s entry's children:

      1. Let p be the result of resolving child relative to entry.

      2. Append p to childPromises.

      3. Upon fulfillment of p with value path:

        1. If path is not null:

          1. Prepend entry’s name to path.

          2. Resolve result with path.

    4. Wait for all childPromises, with the following success steps:

      1. If result hasn’t been resolved yet, resolve result with null.

  3. Return result.

2.2. Permissions

The "file-system" powerful feature's permission-related algorithms and types are defined as follows:

permission descriptor type

FileSystemPermissionDescriptor, defined as:

enum FileSystemPermissionMode {
  "read",
  "readwrite"
};

dictionary FileSystemPermissionDescriptor : PermissionDescriptor {
  required FileSystemHandle handle;
  FileSystemPermissionMode mode = "read";
};
permission state constraints
To determine permission state constraints for a FileSystemPermissionDescriptor desc, run these steps:
  1. Let entry be desc.handle's entry.

  2. If entry represents an entry in an origin private file system, this descriptor’s permission state must always be "granted".

  3. Otherwise, if entry’s parent is not null, this descriptor’s permission state must be equal to the permission state for a descriptor with the same mode, and a handle representing entry’s parent.

  4. Otherwise, if desc.mode is "readwrite":

    1. Let read state be the permission state for a descriptor with the same handle, but mode = "read".

    2. If read state is not "granted", this descriptor’s permission state must be equal to read state.

permission request algorithm
Given a FileSystemPermissionDescriptor desc and a PermissionStatus status, run these steps:
  1. Run the boolean permission query algorithm on desc and status.

  2. If status.state is not "prompt", abort.

  3. Let settings be desc.handle's relevant settings object.

  4. Let global be settings’s global object.

  5. If global is not a Window, throw a SecurityError.

  6. If global does not have transient activation, throw a SecurityError.

  7. If settings’s origin is not same origin with settings’s top-level origin, throw a SecurityError.

  8. Request permission to use desc.

  9. Run the boolean permission query algorithm on desc and status.

Ideally this user activation requirement would be defined upstream. <https://github.com/WICG/permissions-request/issues/2>

To query file system permission given a FileSystemHandle handle and a FileSystemPermissionMode mode, run these steps:
  1. Let desc be a FileSystemPermissionDescriptor.

  2. Set desc.name to "file-system".

  3. Set desc.handle to handle.

  4. Set desc.mode to mode.

  5. Return desc’s permission state.

To request file system permission given a FileSystemHandle handle and a FileSystemPermissionMode mode, run these steps:
  1. Let desc be a FileSystemPermissionDescriptor.

  2. Set desc.name to "file-system".

  3. Set desc.handle to handle.

  4. Set desc.mode to mode.

  5. Let status be the result of running create a PermissionStatus for desc.

  6. Run the permission request algorithm for the "file-system" feature, given desc and status.

  7. Return desc’s permission state.

Currently FileSystemPermissionMode can only be "read" or "readwrite". In the future we might want to add a "write" mode as well to support write-only handles. <https://github.com/wicg/file-system-access/issues/119>

2.3. The FileSystemHandle interface

dictionary FileSystemHandlePermissionDescriptor {
  FileSystemPermissionMode mode = "read";
};

enum FileSystemHandleKind {
  "file",
  "directory",
};

[Exposed=(Window,Worker), SecureContext, Serializable]
interface FileSystemHandle {
  readonly attribute FileSystemHandleKind kind;
  readonly attribute USVString name;

  Promise<boolean> isSameEntry(FileSystemHandle other);

  Promise<PermissionState> queryPermission(optional FileSystemHandlePermissionDescriptor descriptor = {});
  Promise<PermissionState> requestPermission(optional FileSystemHandlePermissionDescriptor descriptor = {});
};

A FileSystemHandle object represents an entry. Each FileSystemHandle object is associated with an entry (an entry). Multiple separate objects implementing the FileSystemHandle interface can all be associated with the same entry simultaneously.

FileSystemHandle objects are serializable objects.

In the Origin Trial as available in Chrome 78, these objects are not yet serializable. In Chrome 82 they are.

Their serialization steps, given value, serialized and forStorage are:

  1. Set serialized.[[Origin]] to value’s relevant settings object's origin.

  2. Set serialized.[[Entry]] to value’s entry.

Their deserialization steps, given serialized and value are:
  1. If serialized.[[Origin]] is not same origin with value’s relevant settings object's origin, then throw a DataCloneError.

  2. Set value’s entry to serialized.[[Entry]]

handle . kind

Returns "file" if handle is a FileSystemFileHandle, or "directory" if handle is a FileSystemDirectoryHandle.

This can be used to distinguish files from directories when iterating over the contents of a directory.

handle . name

Returns the name of the entry represented by handle.

The kind attribute must return "file" if the associated entry is a file entry, and return "directory" otherwise.

The name attribute must return the name of the associated entry.

2.3.1. The isSameEntry() method

same = await handle1 . isSameEntry( handle2 )

Returns true if handle1 and handle2 represent the same file or directory.

This method is first available in Chrome 82.

The isSameEntry(other) method, when invoked, must run these steps:
  1. Let realm be this's relevant Realm.

  2. Let p be a new promise in realm.

  3. Run the following steps in parallel:

    1. If this's entry is the same as other’s entry, resolve p with true.

    2. Else resolve p with false.

  4. Return p.

2.3.2. The queryPermission() method

status = await handle . queryPermission({ mode : "read" })
status = await handle . queryPermission()
status = (await navigator.permissions.query({ name : "file-system", handle : handle })).state

Queries the current state of the read permission of this handle. If this returns "prompt" the website will have to call requestPermission() before any operations on the handle can be done. If this returns "denied" any operations will reject.

Usually handles returned by the local file system handle factories will initially return "granted" for their read permission state, however other than through the user revoking permission, a handle retrieved from IndexedDB is also likely to return "prompt".

status = await handle . queryPermission({ mode : "readwrite" })
status = (await navigator.permissions.query({ name : "file-system", handle : handle, mode : "readwrite"}).state

Queries the current state of the write permission of this handle. If this returns "prompt", attempting to modify the file or directory this handle represents will require user activation and will result in a confirmation prompt being shown to the user. However if the state of the read permission of this handle is also "prompt" the website will need to call requestPermission(). There is no automatic prompting for read access when attempting to read from a file or directory.

The integration with the permissions API’s query() method is not yet implemented in Chrome.

The queryPermission(descriptor) method, when invoked, must run these steps:
  1. Let result be a new promise.

  2. Run the following steps in parallel:

    1. Let state be the result of querying file system permission given this and descriptor.mode.

    2. Resolve result with state.

  3. Return result.

2.3.3. The requestPermission() method

status = await handle . requestPermission({ mode = "read" })
status = await handle . requestPermission()

If the state of the read permission of this handle is anything other than "prompt", this will return that state directly. If it is "prompt" however, user activation is needed and this will show a confirmation prompt to the user. The new read permission state is then returned, depending on the user’s response to the prompt.

status = await handle . requestPermission({ mode = "readwrite" })

If the state of the write permission of this handle is anything other than "prompt", this will return that state directly. If the status of the read permission of this handle is "denied" this will return that.

Otherwise the state of the write permission is "prompt" and this will show a confirmation prompt to the user. The new write permission state is then returned, depending on what the user selected.

The requestPermission(descriptor) method, when invoked, must run these steps:
  1. Let result be a new promise.

  2. Run the following steps in parallel:

    1. Let state be the result of requesting file system permission given this and descriptor.mode. If that throws an exception, reject result with that exception and abort.

    2. Resolve result with state.

  3. Return result.

2.4. The FileSystemFileHandle interface

dictionary FileSystemCreateWritableOptions {
  boolean keepExistingData = false;
};

[Exposed=(Window,Worker), SecureContext, Serializable]
interface FileSystemFileHandle : FileSystemHandle {
  Promise<File> getFile();
  Promise<FileSystemWritableFileStream> createWritable(optional FileSystemCreateWritableOptions options = {});
};

A FileSystemFileHandle's associated entry must be a file entry.

FileSystemFileHandle objects are serializable objects. Their serialization steps and deserialization steps are the same as those for FileSystemHandle.

In the Origin Trial as available in Chrome 78, these objects are not yet serializable. In Chrome 82 they are.

2.4.1. The getFile() method

file = await fileHandle . getFile()

Returns a File representing the state on disk of the entry represented by handle. If the file on disk changes or is removed after this method is called, the returned File object will likely be no longer readable.

The getFile() method, when invoked, must run these steps:
  1. Let result be a new promise.

  2. Run the following steps in parallel:

    1. Let permissionStatus be the result of querying file system permission given this and "read".

    2. If permissionStatus is not "granted", reject result with a NotAllowedError and abort.

    3. Let entry be this’s entry.

    4. Let f be a new File.

    5. Set f’s snapshot state to the current state of entry.

    6. Set f’s underlying byte sequence to a copy of entry’s binary data.

    7. Initialize the value of f’s name attribute to entry’s name.

    8. Initialize the value of f’s lastModified attribute to entry’s modification timestamp.

    9. Initialize the value of f’s type attribute to an implementation defined value, based on for example entry’s name and/or its file extension.

      The reading and snapshotting behavior needs to be better specified in the [FILE-API] spec, for now this is kind of hand-wavy.

    10. Resolve result with f.

  3. Return result.

2.4.2. The createWritable() method

In the Origin Trial as available in Chrome 82, createWritable replaces the createWriter method.

stream = await fileHandle . createWritable()
stream = await fileHandle . createWritable({ keepExistingData: true/false })

Returns a FileSystemWritableFileStream that can be used to write to the file. Any changes made through stream won’t be reflected in the file represented by fileHandle until the stream has been closed. User agents try to ensure that no partial writes happen, i.e. the file represented by fileHandle will either contains its old contents or it will contain whatever data was written through stream up until the stream has been closed.

This is typically implemented by writing data to a temporary file, and only replacing the file represented by fileHandle with the temporary file when the writable filestream is closed.

If keepExistingData is false or not specified, the temporary file starts out empty, otherwise the existing file is first copied to this temporary file.

There has been some discussion around and desire for a "inPlace" mode for createWritable (where changes will be written to the actual underlying file as they are written to the writer, for example to support in-place modification of large files or things like databases). This is not currently implemented in Chrome. Implementing this is currently blocked on figuring out how to combine the desire to run malware checks with the desire to let websites make fast in-place modifications to existing large files. <https://github.com/wicg/file-system-access/issues/67>

The createWritable(options) method, when invoked, must run these steps:
  1. Let result be a new promise.

  2. Run the following steps in parallel:

    1. Let permissionStatus be the result of requesting file system permission given this and "readwrite". If that throws an exception, reject result with that exception and abort.

    2. If permissionStatus is not "granted", reject result with a NotAllowedError and abort.

    3. Let entry be this’s entry.

    4. Let stream be the result of creating a new FileSystemWritableFileStream for entry in this’s relevant realm.

    5. If options.keepExistingData is true:

      1. Set stream.[[buffer]] to a copy of entry’s binary data.

    6. Resolve result with stream.

  3. Return result.

2.5. The FileSystemDirectoryHandle interface

dictionary FileSystemGetFileOptions {
  boolean create = false;
};

dictionary FileSystemGetDirectoryOptions {
  boolean create = false;
};

dictionary FileSystemRemoveOptions {
  boolean recursive = false;
};

[Exposed=(Window,Worker), SecureContext, Serializable]
interface FileSystemDirectoryHandle : FileSystemHandle {
  async iterable<USVString, FileSystemHandle>;

  Promise<FileSystemFileHandle> getFileHandle(USVString name, optional FileSystemGetFileOptions options = {});
  Promise<FileSystemDirectoryHandle> getDirectoryHandle(USVString name, optional FileSystemGetDirectoryOptions options = {});

  Promise<undefined> removeEntry(USVString name, optional FileSystemRemoveOptions options = {});

  Promise<sequence<USVString>?> resolve(FileSystemHandle possibleDescendant);
};

A FileSystemDirectoryHandle's associated entry must be a directory entry.

FileSystemDirectoryHandle objects are serializable objects. Their serialization steps and deserialization steps are the same as those for FileSystemHandle.

In the Origin Trial as available in Chrome 78, these objects are not yet serializable. In Chrome 82 they are.

In Chrome versions upto Chrome 85 getFileHandle and getDirectoryHandle where called getFile and getDirectory instead.

2.5.1. Directory iteration

for await (let [name, handle] of directoryHandle) {}
for await (let [name, handle] of directoryHandle . entries()) {}
for await (let handle of directoryHandle . values()) {}
for await (let name of directoryHandle . keys()) {}

Iterates over all entries whose parent is the entry represented by directoryHandle. Entries that are created or deleted while the iteration is in progress might or might not be included. No guarantees are given either way.

In Chrome this is currently implemented as a directoryHandle.getEntries() method that can be used in a for await..of loop. This getEntries() method returns more or less the same async iterable as what is returned by values() in this specification. The proper async iterable declaration is not yet implemented.

In the future we might want to add arguments to the async iterable declaration to support for example recursive iteration. <https://github.com/wicg/file-system-access/issues/173>

The asynchronous iterator initialization steps for a FileSystemDirectoryHandle handle ant its async iterator iterator are:
  1. Let permissionStatus be the result of querying file system permission given handle and "read".

  2. If permissionStatus is not "granted", throw a NotAllowedError.

  3. Set iterator’s past results to an empty set.

To get the next iteration result for a FileSystemDirectoryHandle handle and its async iterator iterator:
  1. Let promise be a new promise.

  2. Let directory be handle’s entry.

  3. Let permissionStatus be the result of querying file system permission given handle and "read".

  4. If permissionStatus is not "granted", reject promise with a NotAllowedError and return promise.

  5. Let child be an entry in directory’s children, such that child’s name is not contained in iterator’s past results, or null if no such entry exists.

    Note: This is intentionally very vague about the iteration order. Different platforms and file systems provide different guarantees about iteration order, and we want it to be possible to efficiently implement this on all platforms. As such no guarantees are given about the exact order in which elements are returned.

  6. If child is null, then:

    1. Resolve promise with undefined.

  7. Otherwise:

    1. Append child’s name to iterator’s past results.

    2. If child is a file entry:

      1. Let result be a new FileSystemFileHandle associated with child.

    3. Otherwise:

      1. Let result be a new FileSystemDirectoryHandle associated with child.

    4. Resolve promise with (child’s name, result).

  8. Return promise.

2.5.2. The getFileHandle() method

fileHandle = await directoryHandle . getFileHandle(name)
fileHandle = await directoryHandle . getFileHandle(name, { create: false })

Returns a handle for a file named name in the directory represented by directoryHandle. If no such file exists, this rejects.

fileHandle = await directoryHandle . getFileHandle(name, { create: true })

Returns a handle for a file named name in the directory represented by directoryHandle. If no such file exists, this creates a new file. If no file with named name can be created this rejects. Creation can fail because there already is a directory with the same name, because the name uses characters that aren’t supported in file names on the underlying file system, or because the user agent for security reasons decided not to allow creation of the file.

This operation requires write permission, even if the file being returned already exists. If this handle doesn’t already have write permission, this could result in a prompt being shown to the user. To get an existing file without needing write permission, call this method with { create: false }.

The getFileHandle(name, options) method, when invoked, must run these steps:
  1. Let result be a new promise.

  2. Run the following steps in parallel:

    1. If name is not a valid file name, reject result with a TypeError and abort.

    2. Let entry be this’s entry.

    3. If options.create is true:

      1. Let permissionStatus be the result of requesting file system permission given this and "readwrite". If that throws an exception, reject result with that exception and abort.

    4. Otherwise:

      1. Let permissionStatus be the result of querying file system permission given this and "read".

    5. If permissionStatus is not "granted", reject result with a NotAllowedError and abort.

    6. For each child of entry’s children:

      1. If child’s name equals name:

        1. If child is a directory entry:

          1. Reject result with a TypeMismatchError and abort.

        2. Resolve result with a new FileSystemFileHandle whose entry is child and abort.

    7. If options.create is false:

      1. Reject result with a NotFoundError and abort.

    8. Let child be a new file entry.

    9. Set child’s name to name.

    10. Set child’s binary data to an empty byte sequence.

    11. Set child’s modification timestamp to the current time.

    12. Append child to entry’s children.

    13. If creating child in the underlying file system throws an exception, reject result with that exception and abort.

      Better specify what possible exceptions this could throw. <https://github.com/wicg/file-system-access/issues/68>

    14. Resolve result with a new FileSystemFileHandle whose entry is child.

  3. Return result.

2.5.3. The getDirectoryHandle() method

subdirHandle = await directoryHandle . getDirectoryHandle(name)
subdirHandle = await directoryHandle . getDirectoryHandle(name, { create: false })

Returns a handle for a directory named name in the directory represented by directoryHandle. If no such directory exists, this rejects.

subdirHandle = await directoryHandle . getDirectoryHandle(name, { create: true })

Returns a handle for a directory named name in the directory represented by directoryHandle. If no such directory exists, this creates a new directory. If creating the directory failed, this rejects. Creation can fail because there already is a file with the same name, or because the name uses characters that aren’t supported in file names on the underlying file system.

This operation requires write permission, even if the directory being returned already exists. If this handle doesn’t already have write permission, this could result in a prompt being shown to the user. To get an existing directory without needing write permission, call this method with { create: false }.

The getDirectoryHandle(name, options) method, when invoked, must run these steps:
  1. Let result be a new promise.

  2. Run the following steps in parallel:

    1. If name is not a valid file name, reject result with a TypeError and abort.

    2. Let entry be this’s entry.

    3. If options.create is true:

      1. Let permissionStatus be the result of requesting file system permission given this and "readwrite". If that throws an exception, reject result with that exception and abort.

    4. Otherwise:

      1. Let permissionStatus be the result of querying file system permission given this and "read".

    5. If permissionStatus is not "granted", reject result with a NotAllowedError and abort.

    6. For each child of entry’s children:

      1. If child’s name equals name:

        1. If child is a file entry:

          1. Reject result with a TypeMismatchError and abort.

        2. Resolve result with a new FileSystemDirectoryHandle whose entry is child and abort.

    7. If options.create is false:

      1. Reject result with a NotFoundError and abort.

    8. Let child be a new directory entry.

    9. Set child’s name to name.

    10. Set child’s children to an empty set.

    11. Append child to entry’s children.

    12. If creating child in the underlying file system throws an exception, reject result with that exception and abort.

      Better specify what possible exceptions this could throw. <https://github.com/wicg/file-system-access/issues/68>

    13. Resolve result with a new FileSystemDirectoryHandle whose entry is child.

  3. Return result.

2.5.4. The removeEntry() method

await directoryHandle . removeEntry(name)
await directoryHandle . removeEntry(name, { recursive: false })

If the directory represented by directoryHandle contains a file named name, or an empty directory named name, this will attempt to delete that file or directory.

Attempting to delete a file or directory that does not exist is considered success, while attempting to delete a non-empty directory will result in a promise rejection.

await directoryHandle . removeEntry(name, { recursive: true })

Removes the entry named name in the directory represented by directoryHandle. If that entry is a directory, its contents will also be deleted recursively. recursively.

Attempting to delete a file or directory that does not exist is considered success.

The removeEntry(name, options) method, when invoked, must run these steps:
  1. Let result be a new promise.

  2. Run the following steps in parallel:

    1. If name is not a valid file name, reject result with a TypeError and abort.

    2. Let entry be this’s entry.

    3. Let permissionStatus be the result of requesting file system permission given this and "readwrite". If that throws an exception, reject result with that exception and abort.

    4. If permissionStatus is not "granted", reject result with a NotAllowedError and abort.

    5. For each child of entry’s children:

      1. If child’s name equals name:

        1. If child is a directory entry:

          1. If child’s children is not empty and options.recursive is false:

            1. Reject result with an InvalidModificationError and abort.

        2. Remove child from entry’s children.

        3. If removing child in the underlying file system throws an exception, reject result with that exception and abort.

          Note: If recursive is true, the removal can fail non-atomically. Some files or directories might have been removed while other files or directories still exist.

          Better specify what possible exceptions this could throw. <https://github.com/wicg/file-system-access/issues/68>

        4. Resolve result with undefined.

    6. Reject result with a NotFoundError.

  3. Return result.

2.5.5. The resolve() method

path = await directory . resolve( child )

If child is equal to directory, path will be an empty array.

If child is a direct child of directory, path will be an array containing child’s name.

If child is a descendant of directory, path will be an array containing the names of all the intermediate directories and child’s name as last element. For example if directory represents /home/user/project and child represents /home/user/project/foo/bar, this will return ['foo', 'bar'].

Otherwise (directory and child are not related), path will be null.

This functionality can be useful if a web application shows a directory listing to highlight a file opened through a file picker in that directory listing.

This method is first available in Chrome 82.

// Assume we at some point got a valid directory handle.
const dir_ref = current_project_dir;
if (!dir_ref) return;

// Now get a file reference by showing a file picker:
const file_ref = await self.showOpenFilePicker();
if (!file_ref) {
    // User cancelled, or otherwise failed to open a file.
    return;
}

// Check if file_ref exists inside dir_ref:
const relative_path = await dir_ref.resolve(file_ref);
if (relative_path === null) {
    // Not inside dir_ref
} else {
    // relative_path is an array of names, giving the relative path
    // from dir_ref to the file that is represented by file_ref:
    assert relative_path.pop() === file_ref.name;

    let entry = dir_ref;
    for (const name of relative_path) {
        entry = await entry.getDirectory(name);
    }
    entry = await entry.getFile(file_ref.name);

    // Now |entry| will represent the same file on disk as |file_ref|.
    assert await entry.isSameEntry(file_ref) === true;
}
The resolve(possibleDescendant) method, when invoked, must return the result of resolving possibleDescendant’s entry relative to this's entry.

2.6. The FileSystemWritableFileStream interface

enum WriteCommandType {
  "write",
  "seek",
  "truncate",
};

dictionary WriteParams {
  required WriteCommandType type;
  unsigned long long? size;
  unsigned long long? position;
  (BufferSource or Blob or USVString)? data;
};

typedef (BufferSource or Blob or USVString or WriteParams) FileSystemWriteChunkType;

[Exposed=(Window,Worker), SecureContext]
interface FileSystemWritableFileStream : WritableStream {
  Promise<undefined> write(FileSystemWriteChunkType data);
  Promise<undefined> seek(unsigned long long position);
  Promise<undefined> truncate(unsigned long long size);
};

A FileSystemWritableFileStream has an associated [[file]] (a file entry).

A FileSystemWritableFileStream has an associated [[buffer]] (a byte sequence). It is initially empty.

Note: This buffer can get arbitrarily large, so it is expected that implementations will not keep this in memory, but instead use a temporary file for this. All access to [[buffer]] is done in promise returning methods and algorithms, so even though operations on it seem sync, implementations can implement them async.

A FileSystemWritableFileStream has an associated [[seekOffset]] (a number). It is initially 0.

A FileSystemWritableFileStream object is a WritableStream object with additional convenience methods, which operates on a single file on disk.

Upon creation, an underlying sink will have been created and the stream will be usable. All operations executed on the stream are queuable and producers will be able to respond to backpressure.

The underlying sink’s write method, and therefore WritableStreamDefaultWriter’s write() method, will accept byte-like data or WriteParams as input.

The FileSystemWritableFileStream has a file position cursor initialized at byte offset 0 from the top of the file. When using write() or by using WritableStream capabilities through the WritableStreamDefaultWriter’s write() method, this position will be advanced based on the number of bytes written through the stream object.

Similarly, when piping a ReadableStream into a FileSystemWritableFileStream object, this position is updated with the number of bytes that passed through the stream.

getWriter() returns an instance of WritableStreamDefaultWriter.

To create a new FileSystemWritableFileStream given a file entry file in a Realm realm, perform the following steps:
  1. Let stream be a new FileSystemWritableFileStream in realm.

  2. Set stream.[[file]] to file.

  3. Let writeAlgorithm be an algorithm which takes a chunk argument and returns the result of running the write a chunk algorithm with stream and chunk.

  4. Let closeAlgorithm be the following steps:

    1. Let closeResult be a new promise.

    2. Run the following steps in parallel:

      1. Let permissionStatus be the permission state for a FileSystemPermissionDescriptor with handle representing stream.[[file]], and mode = "readwrite".

      2. If permissionStatus is not "granted", reject closeResult with a NotAllowedError and abort.

      3. Perform user agent-specific malware scans and safe browsing checks. If these checks fail, reject closeResult with an AbortError and abort.

      4. Set stream.[[file]]'s binary data to stream.[[buffer]]. If that throws an exception, reject closeResult with that exception and abort.

        Note: It is expected that this atomically updates the contents of the file on disk being written to.

      5. Resolve closeResult with undefined.

    3. Return closeResult.

  5. Let highWaterMark be 1.

  6. Let sizeAlgorithm be an algorithm that returns 1.

  7. Set up stream with writeAlgorithm set to writeAlgorithm, closeAlgorithm set to closeAlgorithm, highWaterMark set to highWaterMark, and sizeAlgorithm set to sizeAlgorithm.

  8. Return stream.

The write a chunk algorithm, given a FileSystemWritableFileStream stream and chunk, runs these steps:
  1. Let input be the result of converting chunk to a FileSystemWriteChunkType. If this throws an exception, then return a promise rejected with that exception.

  2. Let p be a new promise.

  3. Run the following steps in parallel:

    1. Let permissionStatus be the permission state for a FileSystemPermissionDescriptor with handle representing stream.[[file]], and mode = "readwrite".

    2. If permissionStatus is not "granted", reject p with a NotAllowedError and abort.

    3. Let command be input.type if input is a WriteParams, and "write" otherwise.

    4. If command is "write":

      1. Let data be input.data if input is a WriteParams, and input otherwise.

      2. If data is undefined, reject p with a TypeError and abort.

      3. Let writePosition be stream.[[seekOffset]].

      4. If input is a WriteParams and input.position is not undefined, set writePosition to input.position.

      5. Let oldSize be stream.[[buffer]]'s length.

      6. If writePosition is larger than oldSize, reject p with a InvalidStateError and abort.

        Not clear if this should reject, and if it does, is this really the right error type? Chrome’s implementation is actually inconsistent about this rejecting or not.

      7. If data is a BufferSource, let dataBytes be a copy of data.

      8. Else if data is a Blob:

        1. Let dataBytes be the result of performing the read operation on data. If this throws an exception, reject p with that exception and abort.

      9. Else:

        1. Assert: data is a USVString.

        2. Let dataBytes be the result of UTF-8 encoding data.

      10. Let head be a byte sequence containing the first writePosition bytes of stream.[[buffer]].

      11. Let tail be an empty byte sequence.

      12. If writePosition + data.length is smaller than oldSize:

        1. Let tail be a byte sequence containing the last oldSize - (writePosition + data.length) bytes of stream.[[buffer]].

      13. Set stream.[[buffer]] to the concatenation of head, data and tail.

      14. Set stream.[[seekOffset]] to writePosition + data.length.

      15. Resolve p.

    5. Else if command is "seek":

      1. If chunk.position is undefined, reject p with a TypeError and abort.

      2. Set stream.[[seekOffset]] to chunk.position.

      3. Resolve p.

    6. Else if command is "truncate":

      1. If chunk.size is undefined, reject p with a TypeError and abort.

      2. Let newSize be chunk.size.

      3. Let oldSize be stream.[[buffer]]'s length.

      4. If newSize is larger than oldSize:

        1. Set stream.[[buffer]] to a byte sequence formed by concating stream.[[buffer]] with a byte sequence containing newSize-oldSize 0x00 bytes.

      5. Else if newSize is smaller than oldSize:

        1. Set stream.[[buffer]] to a byte sequence containing the first newSize bytes in stream.[[buffer]].

      6. If stream.[[seekOffset]] is bigger than newSize, set stream.[[seekOffset]] to newSize.

      7. Resolve p.

  4. Return p.

2.6.1. The write() method

await stream . write(data)
await stream . write({ type: "write", data: data })

Writes the content of data into the file associated with stream at the current file cursor offset.

No changes are written to the actual file on disk until the stream has been closed. Changes are typically written to a temporary file instead.

await stream . write({ type: "write", position: position, data: data })

Writes the content of data into the file associated with stream at position bytes from the top of the file. Also updates the current file cursor offset to the end of the written data.

No changes are written to the actual file on disk until the stream has been closed. Changes are typically written to a temporary file instead.

await stream . write({ type: "seek", position: position })

Updates the current file cursor offset the position bytes from the top of the file.

await stream . write({ type: "truncate", size: size })

Resizes the file associated with stream to be size bytes long. If size is larger than the current file size this pads the file with null bytes, otherwise it truncates the file.

The file cursor is updated when truncate is called. If the offset is smaller than offset, it remains unchanged. If the offset is larger than size, the offset is set to size to ensure that subsequent writes do not error.

No changes are written to the actual file until on disk until the stream has been closed. Changes are typically written to a temporary file instead.

The write(data) method, when invoked, must run these steps:
  1. Let writer be the result of getting a writer for this.

  2. Let result be the result of writing a chunk to writer given data.

  3. Release writer.

  4. Return result.

2.6.2. The seek() method

await stream . seek(position)

Updates the current file cursor offset the position bytes from the top of the file.

The seek(position) method, when invoked, must run these steps:
  1. Let writer be the result of getting a writer for this.

  2. Let result be the result of writing a chunk to writer given «[ "type" → "seek", "position" → position ]».

  3. Release writer.

  4. Return result.

2.6.3. The truncate() method

await stream . truncate(size)

Resizes the file associated with stream to be size bytes long. If size is larger than the current file size this pads the file with null bytes, otherwise it truncates the file.

The file cursor is updated when truncate is called. If the offset is smaller than offset, it remains unchanged. If the offset is larger than size, the offset is set to size to ensure that subsequent writes do not error.

No changes are written to the actual file until on disk until the stream has been closed. Changes are typically written to a temporary file instead.

The truncate(size) method, when invoked, must run these steps:
  1. Let writer be the result of getting a writer for this.

  2. Let result be the result of writing a chunk to writer given «[ "type" → "truncate", "size" → size ]».

  3. Release writer.

  4. Return result.

3. Accessing Local File System

dictionary FilePickerAcceptType {
    USVString description;
    record<USVString, (USVString or sequence<USVString>)> accept;
};

dictionary FilePickerOptions {
    sequence<FilePickerAcceptType> types;
    boolean excludeAcceptAllOption = false;
};

dictionary OpenFilePickerOptions : FilePickerOptions {
    boolean multiple = false;
};

dictionary SaveFilePickerOptions : FilePickerOptions {
};

dictionary DirectoryPickerOptions {
};

[SecureContext]
partial interface Window {
    Promise<sequence<FileSystemFileHandle>> showOpenFilePicker(optional OpenFilePickerOptions options = {});
    Promise<FileSystemFileHandle> showSaveFilePicker(optional SaveFilePickerOptions options = {});
    Promise<FileSystemDirectoryHandle> showDirectoryPicker(optional DirectoryPickerOptions options = {});
};

The showOpenFilePicker(), showSaveFilePicker() and showDirectoryPicker() methods are together known as the local file system handle factories.

Note: What is referred to as the "local file system" in this spec, does not have to strictly refer to the file system on the local device. What we call the local file system could just as well be backed by a cloud provider. For example on Chrome OS these file pickers will also let you pick files and directories on Google Drive.

In Chrome versions earlier than 85, this was implemented as a generic chooseFileSystemEntries method.

3.1. Local File System Permissions

The fact that the user picked the specific files returned by the local file system handle factories in a prompt should be treated by the user agent as the user intending to grant read access to the website for the returned files. As such, at the time the promise returned by one of the local file system handle factories resolves, permission state for a descriptor with handle set to the returned handle, and mode set to "read" should be "granted".

Additionally for calls to showSaveFilePicker the permission state for a descriptor with handle set to the returned handle, and mode set to readwrite should be "granted".

To verify that an environment is allowed to show a file picker, run these steps:
  1. If environment’s origin is an opaque origin, return a promise rejected with a SecurityError.

  2. If environment’s origin is not same origin with environment’s top-level origin, return a promise rejected with a SecurityError.

  3. Let global be environment’s global object.

  4. If global does not have transient activation, throw a SecurityError.

3.2. The showOpenFilePicker() method

[ handle ] = await window . showOpenFilePicker()
[ handle ] = await window . showOpenFilePicker({ multiple: false })

Shows a file picker that lets a user select a single existing file, returning a handle for the selected file.

handles = await window . showOpenFilePicker({ multiple: true })

Shows a file picker that lets a user select multiple existing files, returning handles for the selected files.

Additional options can be passed to showOpenFilePicker() to indicate the types of files the website wants the user to select. See § 3.4 FilePickerOptions.types for details.

The showOpenFilePicker(options) method, when invoked, must run these steps:
  1. Let accepts options be the result of processing accept types given options.

  2. Let environment be this’s relevant settings object.

  3. Let global be environment’s global object.

  4. Verify that environment is allowed to show a file picker.

  5. Let p be a new promise.

  6. Run the following steps in parallel:

    1. Optionally, wait until any prior execution of this algorithm has terminated.

    2. Display a prompt to the user requesting that the user pick some files. If options.multiple is false, there must be no more than one file selected; otherwise any number may be selected.

      The displayed prompt should let the user pick one of the accepts options to filter the list of displayed files. Exactly how this is implemented, and what this prompt looks like is left up to the user agent.

    3. Wait for the user to have made their selection.

    4. If the user dismissed the prompt without making a selection, reject p with an AbortError and abort.

    5. Let entries be a list of file entries representing the selected files or directories.

    6. Let result be a empty list.

    7. For each entry of entries:

      1. If entry is deemed too sensitive or dangerous to be exposed to this website by the user agent:

        1. Inform the user that the selected files or directories can’t be exposed to this website.

        2. At the discretion of the user agent, either go back to the beginning of these in parallel steps, or reject p with an AbortError and abort.

      2. Add a new FileSystemFileHandle associated with entry to result.

    8. Perform the activation notification steps in global’s browsing context.

      Note: This lets a website immediately perform operations on the returned handles that might require user activation, such as requesting more permissions.

    9. Resolve p with result.

  7. Return p.

3.3. The showSaveFilePicker() method

handle = await window . showSaveFilePicker( options )

Shows a file picker that lets a user select a single file, returning a handle for the selected file. The selected file does not have to exist already. If the selected file does not exist a new empty file is created before this method returns, otherwise the existing file is cleared before this method returned.

Additional options can be passed to showSaveFilePicker() to indicate the types of files the website wants the user to select. See § 3.4 FilePickerOptions.types for details.

The showSaveFilePicker(options) method, when invoked, must run these steps:
  1. Let accepts options be the result of processing accept types given options.

  2. Let environment be this’s relevant settings object.

  3. Let global be environment’s global object.

  4. Verify that environment is allowed to show a file picker.

  5. Let p be a new promise.

  6. Run the following steps in parallel:

    1. Optionally, wait until any prior execution of this algorithm has terminated.

    2. Display a prompt to the user requesting that the user pick exactly one file.

      The displayed prompt should let the user pick one of the accepts options to filter the list of displayed files. Exactly how this is implemented, and what this prompt looks like is left up to the user agent.

    3. Wait for the user to have made their selection.

    4. If the user dismissed the prompt without making a selection, reject p with an AbortError and abort.

    5. Let entry be a file entry representing the selected file.

    6. If entry is deemed too sensitive or dangerous to be exposed to this website by the user agent:

      1. Inform the user that the selected files or directories can’t be exposed to this website.

      2. At the discretion of the user agent, either go back to the beginning of these in parallel steps, or reject p with an AbortError and abort.

    7. Set entry’s binary data to an empty byte sequence.

    8. Set result to a new FileSystemFileHandle associated with entry.

    9. Perform the activation notification steps in global’s browsing context.

      Note: This lets a website immediately perform operations on the returned handles that might require user activation, such as requesting more permissions.

    10. Resolve p with result.

  7. Return p.

3.4. FilePickerOptions.types

The showOpenFilePicker(options) and showSaveFilePicker(options) methods accept a FilePickerOptions argument, which lets the website specify the types of files the file picker will let the user select.

Each entry in types specifies a single user selectable option for filtering the files displayed in the file picker.

Each option consists of an optional description and a number of MIME types and extensions (specified as a mapping of MIME type to a list of extensions). If no description is provided one will be generated. Extensions have to be strings that start with a ".".

In addition to complete MIME types, "" can be used as the subtype of a MIME type to match for example all image formats with "image/".

Websites should always provide both MIME types and file extensions for each option. On platforms that only use file extensions to describe file types user agents can match on the extensions, while on platforms that don’t use extensions, user agents can match on MIME type.

By default the file picker will also include an option to not apply any filter, letting the user select any file. Set excludeAcceptAllOption to true to not include this option in the file picker.

For example , the following options will let the user pick one of three different filters. One for text files (either plain text or HTML), one for images, and a third one that doesn’t apply any filter and lets the user select any file.

const options = {
  types: [
    {
      description: 'Text Files',
      accept: {
        'text/plain': ['.txt', '.text'],
        'text/html': ['.html', '.htm']
      }
    },
    {
      description: 'Images',
      accept: {
        'image/*': ['.png', '.gif', '.jpeg', '.jpg']
      }
    }
  ],
};

On the other hand, the following example will only let the user select SVG files. The dialog will not show an option to not apply any filters.

const options = {
  types: [
    {
      accept: {
        'image/svg+xml': '.svg'
      }
    },
  ],
  excludeAcceptAllOption: true
};
To process accept types, given FilePickerOptions options, run these steps:
  1. Let accepts options be a empty list of pairs.

  2. For each type of options.types:

    1. Let description be type.description.

    2. For each typeStringsuffixes of type.accept:

      1. Let parsedType be the result of parse a MIME type with typeString.

      2. If parsedType is failure, throw a TypeError.

      3. If parsedType’s parameters are not empty, throw a TypeError.

      4. If suffixes is a string:

        1. If suffixes does not start with ".", throw a TypeError.

      5. Otherwise, for each suffix of suffixes:

        1. If suffix does not start with ".", throw a TypeError.

    3. Let filter be the following steps, given a filename (a string), and a type (a MIME type):

      1. For each typeStringsuffixes of type.accept:

      2. Let parsedType be the result of parse a MIME type with typeString.

        1. If parsedType’s subtype is "*":

          1. If parsedType’s type is "*", return true.

          2. If parsedType’s type is type’s type, return true.

        2. parsedType’s essence is type’s essence, return true.

        3. If suffixes is a string, set suffixes to « suffixes ».

        4. For each suffix of suffixes:

          1. If filename ends with suffix, return true.

      3. Return false.

    4. If description is an empty string, set description to some user understandable string describing filter.

    5. Append description/filter to accepts options.

  3. If either accepts options is empty, or options.excludeAcceptAllOption is false:

    1. Let description be a user understandable string describing "all files".

      1. Let filter be an algorithm that returns true.

      2. Append description/filter to accepts options.

  4. If accepts options is empty, throw a TypeError.

  5. Return accepts options.

3.5. The showDirectoryPicker() method

handle = await window . showDirectoryPicker()

Shows a directory picker that lets the user select a single directory, returning a handle for the selected directory.

The showDirectoryPicker(options) method, when invoked, must run these steps:
  1. Let environment be this’s relevant settings object.

  2. Let global be environment’s global object.

  3. Verify that environment is allowed to show a file picker.

  4. Let p be a new promise.

  5. Run the following steps in parallel:

    1. Optionally, wait until any prior execution of this algorithm has terminated.

    2. Display a prompt to the user requesting that the user pick a directory.

    3. Wait for the user to have made their selection.

    4. If the user dismissed the prompt without making a selection, reject p with an AbortError and abort.

    5. Let entry be a directory entry representing the selected directory.

    6. If entry is deemed too sensitive or dangerous to be exposed to this website by the user agent:

      1. Inform the user that the selected files or directories can’t be exposed to this website.

      2. At the discretion of the user agent, either go back to the beginning of these in parallel steps, or reject p with an AbortError and abort.

    7. Set result to a new FileSystemDirectoryHandle associated with entry.

    8. Perform the activation notification steps in global’s browsing context.

      Note: This lets a website immediately perform operations on the returned handles that might require user activation, such as requesting more permissions.

      Rather than requiring the website to prompt separately for a writable directory, we should provide some kind of API to request a writable directory in one step. <https://github.com/wicg/file-system-access/issues/89>

    9. Resolve p with result.

  6. Return p.

3.6. Drag and Drop

partial interface DataTransferItem {
    Promise<FileSystemHandle?> getAsFileSystemHandle();
};

During a drag-and-drop operation, dragged file and directory items are associated with file entries and directory entries respectively.

handle = await item . getAsFileSystemHandle()

Returns a FileSystemFileHandle object if the dragged item is a file and a FileSystemDirectoryHandle object if the dragged item is a directory.

The getAsFileSystemHandle() method steps are:

  1. If the DataTransferItem object is not in the read/write mode or the read-only mode, return a promise resolved with null.

  2. If the the drag data item kind is not File, then return a promise resolved with null.

  3. Let p be a new promise.

  4. Run the following steps in parallel:

    1. Let entry be the entry representing the dragged file or directory.

    2. If entry is a file entry:

      1. Let handle be a FileSystemFileHandle associated with entry.

    3. Else if entry is a directory entry:

      1. Let handle be a FileSystemDirectoryHandle associated with entry.

    4. Resolve p with entry.

  5. Return p.

Handling drag and drop of files and directories:
elem.addEventListener('dragover', (e) => {
  // Prevent navigation.
  e.preventDefault();
});
elem.addEventListener('drop', async (e) => {
  // Prevent navigation.
  e.preventDefault();

  // Process all of the items.
  for (const item of e.dataTransfer.items) {
    // kind will be 'file' for file/directory entries.
    if (item.kind === 'file') {
      const entry = await item.getAsFileSystemHandle();
      if (entry.kind === 'file') {
        handleFileEntry(entry);
      } else if (entry.kind === 'directory') {
        handleDirectoryEntry(entry);
      }
    }
  }
});

This currently does not block access to too sensitive or dangerous directories, to be consistent with other APIs that give access to dropped files and directories. This is inconsistent with the local file system handle factories though, so we might want to reconsider this.

4. Accessing the Origin Private File System

The origin private file system is a storage endpoint whose identifier is "fileSystem", types are « "local" », and quota is null.

Storage endpoints should be defined in [storage] itself, rather than being defined here. So merge this into the table there.

Note: While user agents will typically implement this by persisting the contents of this origin private file system to disk, it is not intended that the contents are easily user accessible. Similarly there is no expectation that files or directories with names matching the names of children of the origin private file system exist.

[SecureContext]
partial interface StorageManager {
  Promise<FileSystemDirectoryHandle> getDirectory();
};

In Chrome this functionality was previously exposed as FileSystemDirectoryHandle.getSystemDirectory({type: "sandbox"}). This new method is available as of Chrome 85.

directoryHandle = await navigator . storage . getDirectory()

Returns the root directory of the origin private file system.

The getDirectory() method, when invoked, must run these steps:
  1. Let environment be the current settings object.

  2. Let map be the result of running obtain a local storage bottle map with environment and "fileSystem". If this returns failure, return a promise rejected with a SecurityError.

  3. If map["root"] does not exist:

    1. Let dir be a new directory entry.

    2. Set dir’s name to "".

    3. Set dir’s children to an empty set.

    4. Set map["root"] to dir.

  4. Return a promise resolved with a new FileSystemDirectoryHandle, whose associated entry is map["root"].

Note: In Chrome the directory entry returned by the above algorithm refers to the same storage as the temporary file system as used to be defined in File API: Directories and System.

5. Accessibility Considerations

This section is non-normative.

When this specification is used to present information in the user interface, implementors will want to follow the OS level accessibility guidelines for the platform.

6. Privacy Considerations

This section is non-normative.

This API does not give websites any more read access to data than the existing <input type=file> and <input type=file webkitdirectory> APIs already do. Furthermore similarly to those APIs, all access to files and directories is explicitly gated behind a file or directory picker.

There are however several major privacy risks with this new API:

6.1. Users giving access to more, or more sensitive files than they intended.

This isn’t a new risk with this API, but user agents should try to make sure that users are aware of what exactly they’re giving websites access to. This is particularly important when giving access to a directory, where it might not be immediately clear to a user just how many files actually exist in that directory.

A related risk is having a user give access to particularly sensitive data. This could include some of a user agent’s configuration data, network cache or cookie store, or operating system configuration data such as password files. To protect against this, user agents are encouraged to restrict which directories a user is allowed to select in a directory picker, and potentially even restrict which files the user is allowed to select. This will make it much harder to accidentally give access to a directory that contains particularly sensitive data. Care must be taken to strike the right balance between restricting what the API can access while still having the API be useful. After all, this API intentionally lets the user use websites to interact with some of their most private personal data.

Examples of directories that user agents might want to restrict as being too sensitive or dangerous include:

6.2. Websites trying to use this API for tracking.

This API could be used by websites to track the user across clearing browsing data. This is because, in contrast with existing file access APIs, user agents are able to grant persistent access to files or directories and can re-prompt. In combination with the ability to write to files, websites will be able to persist an identifier on the users' disk. Clearing browsing data will not affect those files in any way, making these identifiers persist through those actions.

This risk is somewhat mitigated by the fact that clearing browsing data will clear all handles that a website had persisted (for example in IndexedDB), so websites won’t have any handles to re-prompt for permission after browsing data was cleared. Furthermore user agents are encouraged to make it clear what files and directories a website has access to, and to automatically expire permission grants except for particularly well trusted origins (for example persistent permissions could be limited to "installed" web applications).

User agents also are encouraged to provide a way for users to revoke permissions granted. Clearing browsing data is expected to revoke all permissions as well.

6.3. First-party vs third-party contexts.

In third-party contexts (e.g. an iframe whose origin does not match that of the top-level frame) websites can’t gain access to data they don’t already have access to. This includes both getting access to new files or directories via the local file system handle factories, as well as requesting more permissions to existing handles via the requestPermission API.

Handles can also only be post-messaged to same-origin destinations. Attempts to send a handle to a cross-origin destination will result in a messageerror event.

7. Security Considerations

This section is non-normative.

This API gives websites the ability to modify existing files on disk, as well as write to new files. This has a couple of important security considerations:

7.1. Malware

This API could be used by websites to try to store and/or execute malware on the users system. To mitigate this risk, this API does not provide any way to mark files as executable (on the other hand files that are already executable likely remain that way, even after the files are modified through this API). Furthermore user agents are encouraged to apply things like Mark-of-the-Web to files created or modified by this API.

Finally, user agents are encouraged to verify the contents of files modified by this API via malware scans and safe browsing checks, unless some kind of external strong trust relation already exists. This of course has effects on the performance characteristics of this API.

7.2. Ransomware attacks

Another risk factor is that of ransomware attacks. The limitations described above regarding blocking access to certain sensitive directories helps limit the damage such an attack can do. Additionally user agents can grant write access to files at whatever granularity they deem appropriate.

Conformance

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[ECMASCRIPT]
ECMAScript Language Specification. URL: https://tc39.es/ecma262/
[ENCODING]
Anne van Kesteren. Encoding Standard. Living Standard. URL: https://encoding.spec.whatwg.org/
[FILE-API]
Marijn Kruisselbrink; Arun Ranganathan. File API. 11 September 2019. WD. URL: https://www.w3.org/TR/FileAPI/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[INFRA]
Anne van Kesteren; Domenic Denicola. Infra Standard. Living Standard. URL: https://infra.spec.whatwg.org/
[MIMESNIFF]
Gordon P. Hemsley. MIME Sniffing Standard. Living Standard. URL: https://mimesniff.spec.whatwg.org/
[PERMISSIONS]
Mounir Lamouri; Marcos Caceres; Jeffrey Yasskin. Permissions. 20 July 2020. WD. URL: https://www.w3.org/TR/permissions/
[PERMISSIONS-REQUEST]
Requesting Permissions. cg-draft. URL: https://wicg.github.io/permissions-request/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[STORAGE]
Anne van Kesteren. Storage Standard. Living Standard. URL: https://storage.spec.whatwg.org/
[STREAMS]
Adam Rice; Domenic Denicola; 吉野剛史 (Takeshi Yoshino). Streams Standard. Living Standard. URL: https://streams.spec.whatwg.org/
[WebIDL]
Boris Zbarsky. Web IDL. 15 December 2016. ED. URL: https://heycam.github.io/webidl/

Informative References

[ENTRIES-API]
File and Directory Entries API. cg-draft. URL: https://wicg.github.io/entries-api/
[FILE-SYSTEM-API]
Eric Uhrhane. File API: Directories and System. 24 April 2014. NOTE. URL: https://www.w3.org/TR/file-system-api/

IDL Index

enum FileSystemPermissionMode {
  "read",
  "readwrite"
};

dictionary FileSystemPermissionDescriptor : PermissionDescriptor {
  required FileSystemHandle handle;
  FileSystemPermissionMode mode = "read";
};

dictionary FileSystemHandlePermissionDescriptor {
  FileSystemPermissionMode mode = "read";
};

enum FileSystemHandleKind {
  "file",
  "directory",
};

[Exposed=(Window,Worker), SecureContext, Serializable]
interface FileSystemHandle {
  readonly attribute FileSystemHandleKind kind;
  readonly attribute USVString name;

  Promise<boolean> isSameEntry(FileSystemHandle other);

  Promise<PermissionState> queryPermission(optional FileSystemHandlePermissionDescriptor descriptor = {});
  Promise<PermissionState> requestPermission(optional FileSystemHandlePermissionDescriptor descriptor = {});
};

dictionary FileSystemCreateWritableOptions {
  boolean keepExistingData = false;
};

[Exposed=(Window,Worker), SecureContext, Serializable]
interface FileSystemFileHandle : FileSystemHandle {
  Promise<File> getFile();
  Promise<FileSystemWritableFileStream> createWritable(optional FileSystemCreateWritableOptions options = {});
};

dictionary FileSystemGetFileOptions {
  boolean create = false;
};

dictionary FileSystemGetDirectoryOptions {
  boolean create = false;
};

dictionary FileSystemRemoveOptions {
  boolean recursive = false;
};

[Exposed=(Window,Worker), SecureContext, Serializable]
interface FileSystemDirectoryHandle : FileSystemHandle {
  async iterable<USVString, FileSystemHandle>;

  Promise<FileSystemFileHandle> getFileHandle(USVString name, optional FileSystemGetFileOptions options = {});
  Promise<FileSystemDirectoryHandle> getDirectoryHandle(USVString name, optional FileSystemGetDirectoryOptions options = {});

  Promise<undefined> removeEntry(USVString name, optional FileSystemRemoveOptions options = {});

  Promise<sequence<USVString>?> resolve(FileSystemHandle possibleDescendant);
};

enum WriteCommandType {
  "write",
  "seek",
  "truncate",
};

dictionary WriteParams {
  required WriteCommandType type;
  unsigned long long? size;
  unsigned long long? position;
  (BufferSource or Blob or USVString)? data;
};

typedef (BufferSource or Blob or USVString or WriteParams) FileSystemWriteChunkType;

[Exposed=(Window,Worker), SecureContext]
interface FileSystemWritableFileStream : WritableStream {
  Promise<undefined> write(FileSystemWriteChunkType data);
  Promise<undefined> seek(unsigned long long position);
  Promise<undefined> truncate(unsigned long long size);
};

dictionary FilePickerAcceptType {
    USVString description;
    record<USVString, (USVString or sequence<USVString>)> accept;
};

dictionary FilePickerOptions {
    sequence<FilePickerAcceptType> types;
    boolean excludeAcceptAllOption = false;
};

dictionary OpenFilePickerOptions : FilePickerOptions {
    boolean multiple = false;
};

dictionary SaveFilePickerOptions : FilePickerOptions {
};

dictionary DirectoryPickerOptions {
};

[SecureContext]
partial interface Window {
    Promise<sequence<FileSystemFileHandle>> showOpenFilePicker(optional OpenFilePickerOptions options = {});
    Promise<FileSystemFileHandle> showSaveFilePicker(optional SaveFilePickerOptions options = {});
    Promise<FileSystemDirectoryHandle> showDirectoryPicker(optional DirectoryPickerOptions options = {});
};

partial interface DataTransferItem {
    Promise<FileSystemHandle?> getAsFileSystemHandle();
};

[SecureContext]
partial interface StorageManager {
  Promise<FileSystemDirectoryHandle> getDirectory();
};

Issues Index

TODO: Explain better how entries map to files on disk (multiple entries can map to the same file or directory on disk but an entry doesn’t have to map to any file on disk).
Ideally this user activation requirement would be defined upstream. <https://github.com/WICG/permissions-request/issues/2>
Currently FileSystemPermissionMode can only be "read" or "readwrite". In the future we might want to add a "write" mode as well to support write-only handles. <https://github.com/wicg/file-system-access/issues/119>
The reading and snapshotting behavior needs to be better specified in the [FILE-API] spec, for now this is kind of hand-wavy.
There has been some discussion around and desire for a "inPlace" mode for createWritable (where changes will be written to the actual underlying file as they are written to the writer, for example to support in-place modification of large files or things like databases). This is not currently implemented in Chrome. Implementing this is currently blocked on figuring out how to combine the desire to run malware checks with the desire to let websites make fast in-place modifications to existing large files. <https://github.com/wicg/file-system-access/issues/67>
In the future we might want to add arguments to the async iterable declaration to support for example recursive iteration. <https://github.com/wicg/file-system-access/issues/173>
Better specify what possible exceptions this could throw. <https://github.com/wicg/file-system-access/issues/68>
Better specify what possible exceptions this could throw. <https://github.com/wicg/file-system-access/issues/68>
Better specify what possible exceptions this could throw. <https://github.com/wicg/file-system-access/issues/68>
Not clear if this should reject, and if it does, is this really the right error type? Chrome’s implementation is actually inconsistent about this rejecting or not.
Rather than requiring the website to prompt separately for a writable directory, we should provide some kind of API to request a writable directory in one step. <https://github.com/wicg/file-system-access/issues/89>
This currently does not block access to too sensitive or dangerous directories, to be consistent with other APIs that give access to dropped files and directories. This is inconsistent with the local file system handle factories though, so we might want to reconsider this.
Storage endpoints should be defined in [storage] itself, rather than being defined here. So merge this into the table there.