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The write() method of the FileSystemWritableFileStream interface writes content into the file the method is called on, 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. This method can also be used to seek to a byte point within the stream and truncate to modify the total bytes the file contains.




Can be either the file data to write, in the form of a BufferSource, Blob or USVString. Or an object containing the following properties:
  • type: One of 'write', 'seek' or 'truncate'. This is required if the object is passed into the write() method.
  • data: The file data to write. Can be a BufferSource, Blob or USVString. This is required if the type is set to 'write'.
  • position: The byte position the current file cursor should move to if type 'seek' is used. Can also be set with 'write' in which case the write will start at the position.
  • size: An unsigned long value representing the amount of bytes the stream should contain. This is required if the type is set to 'truncate'

Return value

Promise which returns undefined


If PermissionStatus is not granted.
If data is undefined, or if position or size aren't valid.
If the position is set and larger than the bytes available.


This asynchronous function opens the 'Save File' picker, which returns a FileSystemFileHandle once a file is selected. From which a writable stream is then created using the FileSystemFileHandle.createWritable() method.

A user defined Blob 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(imgBlob);

  // close the file and write the contents to disk.
  await writableStream.close();

The following show different examples of options that can be passed into the write() method.

// just pass in the data (no options)

// writes the data to the stream from the determined position
writableStream.write({ type: "write", position: position, data: data })

// updates the current file cursor offset to the position specified
writableStream.write({ type: "seek", position: position })

// resizes the file to be size bytes long
writableStream.write({ type: "truncate", size: size })


Specification Status Comment
The definition of 'write' in that specification.
Working Draft Initial definition.

Browser compatibility

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See also