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 in secure contexts (HTTPS), in some or all supporting browsers.

Note: This feature is available in Web Workers.

The FileSystemWritableFileStream interface of the File System API is a WritableStream object with additional convenience methods, which operates on a single file on disk. The interface is accessed through the FileSystemFileHandle.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 a FileSystemFileHandle once a file is selected. From this, a writable stream is created using the FileSystemFileHandle.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 the write() method.

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