FileSystemWritableFileStream

Baseline 2025

Newly available

Since ⁨September 2025⁩, this feature works across the latest devices and browser versions. This feature might not work in older devices or browsers.

Secure context: This feature is available only insecure contexts (HTTPS), in some or allsupporting browsers.

Note: This feature is available inWeb Workers.

TheFileSystemWritableFileStream interface of theFile System API is aWritableStream object with additional convenience methods, which operates on a single file on disk. The interface is accessed through theFileSystemFileHandle.createWritable() method.

Instance properties

Inherits properties from its parent,WritableStream.

Instance methods

Inherits methods from its parent,WritableStream.

FileSystemWritableFileStream.write(): Writes content into the file the method is called on, at the current file cursor offset.
FileSystemWritableFileStream.seek(): Updates the current file cursor offset to the position (in bytes) specified.
FileSystemWritableFileStream.truncate(): Resizes the file associated with the stream to be the specified size in bytes.

Examples

The following asynchronous function opens the 'Save File' picker, which returns aFileSystemFileHandle once a file is selected. From this, a writable stream is created using theFileSystemFileHandle.createWritable() method.

A text string is then written to the stream, which is subsequently closed.

async function saveFile() {  // create a new handle  const newHandle = await window.showSaveFilePicker();  // create a FileSystemWritableFileStream to write to  const writableStream = await newHandle.createWritable();  // write our file  await writableStream.write("This is my file content");  // close the file and write the contents to disk.  await writableStream.close();}

The following examples show different options that can be passed into thewrite() method.

// just pass in the data (no options)writableStream.write(data);// writes the data to the stream from the determined positionwritableStream.write({ type: "write", position, data });// updates the current file cursor offset to the position specifiedwritableStream.write({ type: "seek", position });// resizes the file to be size bytes longwritableStream.write({ type: "truncate", size });