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FileSystemWritableFileStream

files/en-us/web/api/filesystemwritablefilestream/index.md

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{{securecontext_header}}{{APIRef("File System API")}}{{AvailableInWorkers}}

The FileSystemWritableFileStream interface of the {{domxref("File System API", "File System API", "", "nocode")}} is a {{domxref('WritableStream')}} object with additional convenience methods, which operates on a single file on disk. The interface is accessed through the {{domxref('FileSystemFileHandle.createWritable()')}} method.

{{InheritanceDiagram}}

Instance properties

Inherits properties from its parent, {{DOMxRef("WritableStream")}}.

Instance methods

Inherits methods from its parent, {{DOMxRef("WritableStream")}}.

  • {{domxref('FileSystemWritableFileStream.write()')}}
    • : Writes content into the file the method is called on, at the current file cursor offset.
  • {{domxref('FileSystemWritableFileStream.seek()')}}
    • : Updates the current file cursor offset to the position (in bytes) specified.
  • {{domxref('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 {{domxref('FileSystemFileHandle')}} once a file is selected. From this, a writable stream is created using the {{domxref('FileSystemFileHandle.createWritable()')}} method.

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

js
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.

js
// 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 });

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also