OS.File for the main thread

This page details how to use File I/O from the main thread. For other uses of OS.File, please see the corresponding page.

Using OS.File from a JSM

To import OS.File into your chrome code, add the following line at the start of your script:

Components.utils.import("resource://gre/modules/osfile.jsm")

Promises

Before using OS.File from the main thread, you need some understanding of the Promise library.

See the Promise object documentation for details.

Also, OS.File plays very nicely with Task.jsm.

Example: Read the contents of a file as text

The following snippet opens a file "file.txt" and read its contents as a string, using the default encoding (utf-8).

The content is read asynchronously. The result is a Promise.

let decoder = new TextDecoder();        // This decoder can be reused for several reads
let promise = OS.File.read("file.txt"); // Read the complete file as an array
promise = promise.then(
  function onSuccess(array) {
    return decoder.decode(array);        // Convert this array to a text
  }
);

Example: Write a string to a file

The following snippet writes the text "This is some text" to a string "file.txt", using the default encoding (utf-8). It uses an atomic write to ensure that the file is not modified if, for some reason, the write cannot complete (typically because the computer is turned off, the battery runs out, or the application is stopped.)

let encoder = new TextEncoder();                                   // This encoder can be reused for several writes
let array = encoder.encode("This is some text");                   // Convert the text to an array
let promise = OS.File.writeAtomic("file.txt", array,               // Write the array atomically to "file.txt", using as temporary
    {tmpPath: "file.txt.tmp"});                                    // buffer "file.txt.tmp".

The following variant does the same thing but will fail if "file.txt" already exists:

let encoder = new TextEncoder();                                   // This encoder can be reused for several writes
let array = encoder.encode("This is some text");                   // Convert the text to an array
let promise = OS.File.writeAtomic("file.txt", array,               // Write the array atomically to "file.txt", using as temporary
    {tmpPath: "file.txt.tmp", noOverwrite: true});                 // buffer "file.txt.tmp".

Example: Rename a file

You have to use OS.File.move to rename a file:

let promise = OS.File.move("oldname.txt", "newname.txt", {noOverwrite:true});

Here's a working example which renames test.txt to testRenamed.txt if the file is located in directory C:\Jean\

var promise = OS.File.move(OS.Path.join('C:', 'Jean', 'test.txt'), OS.Path.join('C:', 'Jean', 'testRenamed.txt'));
promise.then(
    function() {
       console.log('rename successful')
    },
    function(aRejectReason) {
       console.log('rename failed, aRejectReason = ', aRejectReason)
    }
)

The noOverwrite true is important, as default is false which means if a file in the directory exists already with the same name it will no longer be there after this "rename" operation, which is a "move".

Example: Copy a file

The following snippet copies file "oldname.txt" to "newname.txt". On most operating systems, this operation is handled directly by the operating system itself, which makes it as fast as possible.

let promise = OS.File.copy("oldname.txt", "newname.txt");

Example: Path manipulation

The following snippet obtains the path to file "sessionstore.js", contained in the user's profile directory.

let sessionstore = OS.Path.join(OS.Constants.Path.profileDir, "sessionstore.js");
   // Under Linux, this is generally "$HOME/.firefox/Profiles/$PROFILENAME/sessionstore.js"
   // Under MacOS, this is generally "$HOME/Library/Application Support/Firefox/$PROFILENAME/sessionstore.js"
   // Under Windows, this is generally "%APPDATA%\Local\temp\%PROFILENAME%"\sessionstore.js
   // etc.

Example: Determine if a file is a directory

The following snippet determines if some path represents a file or a directory:

let promise = OS.File.stat(somePath);
promise = promise.then(
  function onSuccess(stat) {
    if (stat.isDir) {
      // The path represents a directory
    } else {
      // The path represents a file, not a directory
    }
  },
  function onFailure(reason) {
    if (reason instanceof OS.File.Error && reason.becauseNoSuchFile) {
      // The file does not exist
    } else {
      // Some other error
      throw reason;
    }
  }
);

Example: copy a file by chunks

The following snippet writes a (presumably large) buffer by chunks. Note that this snippet is useful as a demonstration of complex asynchronous programming with OS.File – in most cases, function OS.File.writeAtomic is a better choice.

let writeStream = function writeStream(data, outFile, chunkSize) {
  let view = new Uint8Array(data);
  let loop = function loop(pos) {                                         // Define a recursive asynchronous loop.
    if (pos <= view.byteLength) {  // Note: Should this be pos >= view.byteLength ?
      return Promise.resolve(true);                                       // Loop end.
    }
    let promise = file.write(view.subarray(pos, chunkSize));              // Write a subset of |data|
    return promise.then(function onSuccess(bytes) {
      return loop(pos + bytes);                                           // ... and loop.
    });
  };
  let promise = loop(0);                                                  // Enter the loop.
  promise = promise.then(function onSuccess() {                           // Once loop is complete, finalize.
    file.close();
  }, function onError(reason) {
    file.close();
    throw reason;
  });
  return promise;
}

Or a variant using Task.js (or at least the subset already present on mozilla-central):

let writeStream = function writeStream(data, outFile, chunkSize) {
  return Task.spawn(function() {
    let view = new Uint8Array(data);
    let pos = 0;
    while (pos < view.byteLength) {
      pos += yield outFile.write(view.subarray(pos, chunkSize));
    }
    outFile.close();
  }).then(
    null,
    function onFailure(reason) {
      outFile.close();
      throw reason;
    }
  );
}

Example: Save Canvas to Disk

This exmaple uses Image to load an image from a path (note: if your path is a file on disk you must use local file; this is accomplished with OS.Path.toFileURI, which accepts a string). After image loads it then draws it to canvas, makes it a blob, and uses FileReader to turn it into ArrayBuffer(View), then uses OS.File.writeAtomic to save to disk.

var img = new Image();
img.onload = function() {
    var canvas = document.createElementNS('http://www.w3.org/1999/xhtml', 'canvas');
    canvas.width = img.naturalWidth;
    canvas.height = img.naturalHeight;
    var ctx = canvas.getContext('2d');
    ctx.drawImage(img, 0, 0);
    (canvas.toBlobHD || canvas.toBlob).call(canvas, function(b) {
        var r = Cc['@mozilla.org/files/filereader;1'].createInstance(Ci.nsIDOMFileReader); //new FileReader();
        r.onloadend = function() {
            // r.result contains the ArrayBuffer.
            var writePath = OS.Path.join(OS.Constants.Path.desktopDir, 'savedImage.png');
            var promise = OS.File.writeAtomic(writePath, new Uint8Array(r.result), { tmpPath: writePath + '.tmp' });
            promise.then(
                function(aVal) {
                    console.log('successfully saved image to disk');
                },
                function(aReason) {
                    console.log('writeAtomic failed for reason:', aReason);
                }
            );
        };
        r.readAsArrayBuffer(b);
    }, 'image/png');
};
//var path = OS.Path.toFileURI(OS.Path.join(OS.Contants.Path.desktopDir, 'my.png')); //do it like this for images on disk
var path = 'https://mozorg.cdn.mozilla.net/media/img/firefox/channel/toggler-beta.png?2013-06'; //do like this for images online
img.src = path;

Example: Append to File

This example shows how to use open, write, and close to append to a file. If the file does not exist, it is created. At the time of this writing, write does not support encoding option so the text to be written has to be encoded with TextEncoder. This example also shows the resolve value of open (an instance of OS.File, this is a file, so you can do any of the methods on it found here), write (a number indicating bytes written), and close (undefined, meaning there is no resolve value).

var pth = OS.Path.join(OS.Constants.Path.desktopDir, 'app.txt');
OS.File.open(pth, {write: true, append: true}).then(valOpen => {
    console.log('valOpen:', valOpen);
    var txtToAppend = 'append some text \n';
    var txtEncoded = new TextEncoder().encode(txtToAppend);
    valOpen.write(txtEncoded).then(valWrite => {
        console.log('valWrite:', valWrite);
        valOpen.close().then(valClose => {
            console.log('valClose:', valClose);
            console.log('successfully appended');
        });
    });
});

Global object OS.File

Method overview

Methods

OS.File.open()

Use method OS.File.open() to open a file.

Promise<File> open(
  in string path,
  [optional] in object mode,
  [optional] in object options
)

Arguments

path

The full native name of the file to open.

mode Optional

The opening mode for the file, as an object that may contain a subset of the following fields:

read

If true, the file will be opened for reading. Depending on other fields, it may also be opened for writing.

write

If true, the file will be opened for writing. Depending on other fields, it may also be opened for reading.

Prior to Gecko 27, unless create or truncate are set or explicit unixFlags are given, the file will be opened for appending on Unix/Linux. However, the file is not opened for appending on Windows. See bug 924858. Starting with Gecko 27, you may use the append flag instead. For an example using append see here.

truncate | trunc

If true, the file will be opened for writing. If the file does not exist, it will be created. If the file exists, its contents will be removed. Cannot be used with create.

create

If true, file will be opened for writing. The file must not exist. If the file already exists, throw an error. Cannot be used with truncate or existing.

existing

If true, the file must already exist. If the file does not exist, throw an error. Cannot be used with create.

append

If true, the file will be opened for appending, meaning the equivalent of .setPosition(0, POS_END) is executed before each write. The default is true, i.e. opening a file for appending. Specify append: false to open the file in regular mode.

options Optional

Platform-specific options for opening the file. For advanced users only. Most users will never have need of these options. To specify options, pass an object that may contain some of the following flags:

unixFlags

(ignored under non-Unix platforms) If specified, file opening flags, as per libc function open. If unspecified, build these flags from the contents of mode. You can build these flags from values OS.Constants.libc.O_*.

unixMode

(ignored under non-Unix platforms) If specified, file creation mode, as per libc function open. If unspecified, files are created with a default mode of 0600 (file is private to the user, the user can read and write). You can build this mode from values OS.Constants.libc.S_I*.

winShare

(ignored under non-Windows platforms) If specified, a sharing policy, as per Windows function CreateFile. If unspecified, files are opened with a default sharing policy (file is not protected against being read/written/removed by another process or another use in the same process). You can build this policy from constants OS.Constants.Win.FILE_SHARE_*.

winSecurity

(ignored under non-Windows platforms) If specified, a security policy, as per Windows function CreateFile. If unspecified, no security attributes.

winAccess

(ignored under non-Windows platforms) If specified, access mode, as per Windows function CreateFile. This also requires option winDisposition and this replaces argument mode. If unspecified, value is built from mode.

winDisposition

(ignored under non-Windows platforms) If specified, disposition mode, as per Windows function CreateFile. This also requires option winAccess and this replaces argument mode. If unspecified, value is built from mode.

Promise resolves to

An instance of OS.File representing the expected file.

When opening files for writing, they will be opened for appending unless you specify append: false in Gecko 27 and later. In Gecko 26 and earlier, on platforms other than Windows, the files will be opened for appending unless you specify explicit unixFlags or open the file with either create or truncate flags. In Gecko 26 and earlier on Windows, files will never be opened for appending.

To open an existing file for writing without appending in a compatible way on all platforms in both Gecko 27 and later and Gecko 26 and earlier, you should specify both the append flag and unixFlags.

// Open a file for writing without appending to it.
Task.spawn(function() {
  // Under Unix, you'll have to specify your own unixFlags for Gecko < 27 to avoid append mode.
  var options = {};
  if (OS.Constants.libc) {
    // Own flags omitting O_APPEND, e.g.
    options.unixFlags = OS.Constants.libc.O_CREAT | OS.Constants.libc.O_WRONLY;
  }
  // For Gecko >= 27, it is enough, but crucial, to set the correct append flag.
  var outfile = yield OS.File.open("file.tmp", {write: true, append: false}, options);
  try {
    // ... do something with that file
  } finally {
    yield outfile.close();
  }
});

Example of opening file and keeping it locked

This uses Tasks.jsm to open a file and keep it open. When you are done with it, like in shutdown of restartless add-on, you should close the file so it becomes editable again.

let options = {
  winShare: 0 // Exclusive lock on Windows
};
if (OS.Constants.libc.O_EXLOCK) {
  // Exclusive lock on *nix
  options.unixFlags = OS.Constants.libc.O_EXLOCK;
}
let file = yield OS.File.open(..., options);

Then when you want to unlock the file so it can be edited from other places, close the file.

file.close();

This example is from Stackoverflow: OS.File check last modified date before OS.read

OS.File.openUnique()

Creates and opens a file with a unique name. By default, generate a random hex number and use it to create a unique new file name.

Promise<object> openUnique(
  in string path,
  [optional] in object options
) throws OS.File.Error

Arguments

path

The full native name of the file to open.

options Optional

Additional options for file opening. This implementation interprets the following fields:

humanReadable

If true, create a new filename appending a decimal number, e.g., filename-1.ext, filename-2.ext. If false use hex numbers, e.g., filename-A65BC0.ext.

maxAttempts

Used to limit the amount of tries after a failed file creation. Default is 99.

Promise resolves to

An object contains a file object{file} and the path{path}.

Promise can be rejected with

OS.File.Error

If the file could not be opened.

OS.File.copy()

Copy a file.

void copy(
  in string sourcePath,
  in string destPath
  [optional] in object options)
throws OS.File.Error

Arguments

sourcePath

The full path of the file to copy. At the time of this writing, this function does not copy directories.

destPath

The full path of the destination. Note that this is not a directory but a file.

options Optional

An optional object used to control the behavior of this function. You may pass an object with a subset of the following fields:

noOverwrite

If destPath already exists, do not overwrite it, but rather launch an exception.

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the file does not exist.

OS.File.exists()

Determine whether a file exists

Promise<bool> exists(
  in string path
)

Arguments

path

The name of the file

Promise resolves to

true if the file exists, false otherwise

OS.File.getCurrentDirectory()

Return the current directory

Promise<string> getCurrentDirectory()

Promise resolves to

The path to the current directory.

OS.File.makeDir()

Create a new directory

Promise<void> makeDir(
  in string path,
  [optional] in object options
) throws OS.File.Error

Arguments

path

The full name of the directory to create.

options Optional

Options for creating the directory. To specify options, pass an object that may contain some of the following flags:

ignoreExisting

If true, succeed even if the directory already exists (default behavior). Otherwise, fail if the directory already exists. NOTE: If from is specified then even if ignoreExisting is specified as false, it will not fail due to pre-existence of directories, because the from option tells makeDir to make the folders if not found.

unixMode

(ignored under non-Unix platforms) If specified, file creation mode, as per libc function mkdir. If unspecified, directories are created with a default mode of 0600 (file is private to the user, the user can read and write). You can build this mode from values OS.Constants.libc.S_I*.

winSecurity

(ignored under non-Windows platforms) If specified, a security policy, as per Windows function CreateDirectory. If unspecified, no security attributes.

from

If specified, the call to makeDir creates all the ancestors of path that are descendents of from. Note that from and its existing descendents must be user-writeable and that path must be a descendent of from.

OS.File.move()

Move a file.

Promise<void> move(
  in string sourcePath,
  in string destPath
  [optional] in object options
)

Arguments

sourcePath

The full path of the file to move. At the time of this writing, the behavior of this function is unspecified if sourcePath is a directory.

destPath

The full path of the destination. At the time of this writing, the behavior of this function is unspecified if destPath is a directory.

options Optional

An optional object used to control the behavior of this function. You may pass an object with a subset of the following fields:

noOverwrite

If destPath already exists, do not overwrite it, but rather launch an exception.

noCopy

If moving the file would require a copy (i.e. if the destination path resides on another drive/device/file system as the source path), fail.

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the file does not exist.

OS.File.read()

Read the contents of a file

Promise<Uint8Array> read(
  in string path,
  [optional] in number bytes
)

Arguments

path

The full path to the file to read.

bytes Optional

The number of bytes to read. If unspecified, read the complete file.

Promise resolves to

An array holding bytes bytes (or less if the file did not contain as many bytes).

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the file does not exist or if the process does not have the authorization to read it.

Promise<Uint8Array> read(
  in string path,
  [optional] in object options
)

OS.File.remove()

Remove an existing file.

Promise<void> remove(
  in string path,
  [optional] in object options
)

Arguments

path

A string representing the path of the file to remove. At the time of this writing, this function does not remove directories.

options Optional

An optional object used to control the behavior of this function. You may pass an object with a subset of the following fields:

ignoreAbsent

Succeed if the file doesn't exist.

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the file does not exist.

OS.File.removeEmptyDir()

Remove an empty directory

Promise<void> removeEmptyDir(
  in string path,
  [optional] in object options
)

Arguments

path

The complete path to the directory.

options Optional

An optional object used to control the behavior of this function. You may pass an object with a subset of the following fields:

ignoreAbsent

Succeed if the directory doesn't exist.

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the file does not exist.

OS.File.removeDir()

Remove an existing directory and its contents.

Promise<void> removeDir(
  in string path,
  [optional] in object options
)

Arguments

path

A string representing the name of the file to remove.

options

An object that may contain the following fields

ignoreAbsent

If false, this function will throw an error if the directory doesn't exist.

ignorePermissions

If true, this function will remove the directory even when lacking write permissions.

Promise can be rejected with

OS.File.Error

In case of any error, in particular if path isn't a directory.

OS.File.setCurrentDirectory()

Change the current directory of the process.

Promise<void> setCurrentDirectory(
  in string path
)

Arguments

path

The complete path to use as current directory.

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the path does not represent an existing directory.

OS.File.setDates()

Set the last access and modification date of the file.

The time stamp resolution is one second at best, but might be worse depending on the platform, file system, etc.

Promise<void> setDates(
  in string path,
  in Date|number accessDate,
  in Date|number modificationDate
)

Arguments

path

The complete path to the file.

accessDate

The last access date. If numeric, milliseconds since epoch. If omitted or null, the current date will be used.

modificationDate

The last modification date. If numeric, milliseconds since epoch. If omitted or null, the current date will be used.

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the path does not represent an existing file.

OS.File.setPermissions()

Sets the file's access permission bits.

Promise<void> setPermissions(
  in string path,
  in object options
)

Arguments

path

The complete path to the file.

options

The new attributes to set

winAttributes

This is an object with following optional keys. Ignored under non-Windows platforms.

hidden

Boolean. Set to true to make the target hidden, or false to make it visible.

readOnly

Boolean. Set to true to make the target "read only".

system

Boolean. Toggles the "system" attribute, this is equivalent .

unixMode

Number. This is an number can be created with the constants available in OS.Constants.libc.S_I* or OS.Constants.libc.S_O*. Ignored under non-Unix platforms. To make a file hidden on Unix based platforms, including Mac, simply rename the file with OS.File.move to have "." at the start of the file name.

unixHonorUmask

Toggles the OS.Constants.Sys.umask flag. Ignored under non-Unix platforms..

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the path does not represent an existing file.

OS.File.stat()

Obtain information about a file, such as size, creation date, etc.

Promise<File.Info> stat(
  in string path
)

Arguments

path

The complete path to the file.

Promise resolves to

An instance of File.Info holding information about a file.

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the path does not represent an existing file.

OS.File.unixSymLink()

Create a symoblic link file, also known as "Alias" files on Mac OS. This is similar to "Shortcut" files on Windows systems. This function is specific to UNIX baed systems such as Linux and Mac OS X.

Promise<undefined> unixSymLink(
  in string pathTarget,
  in string pathCreate
)

Arguments

pathTarget

The complete path to the file that should be launced by the symbolic link.

pathCreate

The complete path to the file that should launch target. The file extension should be .link.

Promise resolves to

undefined

Promise can be rejected with

OS.File.Error

In case of any error. If the file exists already, unixErrorco of 17 will be returned.

OS.File.writeAtomic()

Write data to a file, atomically.

Unlike a regular write, this operation ensures that, until the contents are fully written, the destination file is not modified.

Promise<void> writeAtomic(
  in string path,
  in ArrayBufferView data,
  in object options
)

Arguments

path

The full path to the destination file.

data

An ArrayBufferView holding the data to write.

Firefox 37 note

As of Firefox 37, this method will neuter the array buffer.

 

options

An object that may contain the following fields

tmpPath

If null or unspecified, write the data directly to path. If specified, write the data to a temporary file called tmpPath and, once the write is complete, rename the file to replace path. Performing this operation is a little slower but also a little safer.

Firefox 25 note

tmpPath is required in Firefox 24 or lower version, but optional in Firefox 25 or higher version

noOverwrite

If specified and true, and if path already exists, this function will throw an error without overwriting path.

flush

If false or unspecified, return immediately once the write is complete. If true, before writing, force the operating system to write its internal disk buffers to the disk. This is considerably slower (not just for the application but for the whole system) and more battery expensive but also safer: if the system shuts down improperly (typically due to a kernel freeze or a power failure) or if the device is disconnected before the buffer is flushed, the file has more chances of not being corrupted.

backupTo

Available since Firefox 30. If specified, backup the destination file as backupTo. Note that this function renames the destination file before overwriting it. If the process or the operating system freezes or crashes during the short window between these operations, the destination file will have been moved to its backup.

encoding

Available since Firefox 22. Instead of using TextEncoder, you can supply a string to this option. For example, instead of:

let encoder = new TextEncoder();
let array = encoder.encode("This is some text");
let promise = OS.File.writeAtomic("file.txt", array, {tmpPath: "file.txt.tmp"});

You can simply do:

let promise = OS.File.writeAtomic("file.txt", "This is some text", { encoding: "utf-8", tmpPath: "file.txt.tmp" })

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the destination file cannot be overwritten, or if tmpPath is not on the same device as path.

Instances of OS.File

To obtain an instance of OS.File, use function OS.File.open.

Methods overview

Methods

close()

Close a file and release any associated resource.

Once the file is closed, any attempt to call methods of the file object will raise an error.

An example is seen here. In this example the contents is not written to file until .close is called.

Promise<void> close()

flush()

Flushes the file's internal buffers, ensuring that all data still in these buffers is now written to disk.

Disk flushes are very expensive and therefore should be used carefully, sparingly, and only in scenarios where it is vital that data survives system crashes. Even though the function will be executed off the main-thread, it might still affect the overall performance of any running application.

Promise<void> flush()

getPosition()

Return the current position in the file.

Promise<number> getPosition()

Promise resolves to

The current position in the file, as a number of bytes from the start.

Promise can be rejected with

OS.File.Error

If the file is closed.

read()

Read bytes from this file to a new buffer. Bytes are read from the current position in the file and the position is advanced accordingly.

Promise<Uint8Array> read(
  [optional] in number bytes
)

Arguments

bytes

If specified, read bytes bytes, or less if the file does not contain that many bytes. If unspecified, read all the remaining bytes from this file.

Promise resolves to

An array containing the bytes read.

Promise can be rejected with

OS.File.Error

In case of I/O error.

setDates()

Set the last access and modification date of the file.

The time stamp resolution is one second at best, but might be worse depending on the platform, file system, etc.

Promise<void> setDates(
  in Date|number accessDate,
  in Date|number modificationDate
)

Arguments

accessDate

The last access date. If numeric, milliseconds since epoch. If omitted or null, the current date will be used.

modificationDate

The last modification date. If numeric, milliseconds since epoch. If omitted or null, the current date will be used.

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the path does not represent an existing file.

setPosition()

Change the current position in the file.

Promise<void> setPosition(
  in number offset,
  in object origin
)

Arguments

offset

The new position, as a number of bytes from the origin.

origin

One of the following:

• OS.File.POS_START (bytes are counted from the start of the file)
• OS.File.POS_CUR (bytes are counted from the current position in the file)
• OS.File.POS_END (bytes are counted from the end of the file)

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the new position is before the start of the file, or if the file is closed.

stat()

Obtain information about the file, such as size, creation date, etc.

Promise<File.Info> stat()

Promise resolves to

An instance of File.Info holding information about the file.

Promise can be rejected with

OS.File.Error

In case of any error, in particular if the file is closed.

write()

Write bytes from a buffer to this file.

Note that, by default, this function may perform several I/O operations to ensure that the buffer is fully written.

An example is seen here.

Promise<number> write(
  in ArrayBufferView source
  [optional] in object options
)

Arguments

source

The array in which the the bytes are stored.

Firefox 37 note

As of Firefox 37, this method will neuter the array buffer.

options Optional

An object that may contain some of the following fields:

bytes

An upper bound to the number of bytes to write to the file. If unspecified, write up to source.byteLength bytes. If specified, this must be less than source.byteLength.

Promise resolves to

The number of bytes effectively written to the file.

Promise can be rejected with

OS.File.Error

In case of any I/O error.

TypeError

If options.bytes is specified and is larger than source.byteLength.