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Showing content from http://developer.mozilla.org/en-US/docs/Web/API/FileSystemSyncAccessHandle/write below:

FileSystemSyncAccessHandle: write() method - Web APIs

Baseline 2023

Newly available

Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.

Note: This feature is only available in Dedicated Web Workers.

The write() method of the FileSystemSyncAccessHandle interface writes the content of a specified buffer to the file associated with the handle, optionally at a given offset.

Files within the origin private file system are not visible to end-users, therefore are not subject to the same security checks as methods running on files within the user-visible file system. As a result, writes performed using FileSystemSyncAccessHandle.write() are much more performant. This makes them suitable for significant, large-scale file updates such as SQLite database modifications.

buffer

An ArrayBuffer or ArrayBufferView (such as a DataView) representing the buffer to be written to the file.

options Optional

An options object containing the following properties:

at

A number representing the offset in bytes from the start of the file that the buffer should be written at.

Note: You cannot directly manipulate the contents of an ArrayBuffer. Instead, you create a typed array object like an Int8Array or a DataView object, which represents the buffer in a specific format, and use that to read and write the contents of the buffer.

A number representing the number of bytes written to the file.

InvalidStateError DOMException

Thrown if the associated access handle is already closed, or if the modification of the file's binary data completely fails.

QuotaExceededError DOMException

Thrown if the increased data capacity exceeds the browser's storage quota.

TypeError

Thrown if the underlying file system does not support writing the file from the specified file offset.

The following asynchronous event handler function is contained inside a Web Worker. On receiving a message from the main thread it:

• Creates a synchronous file access handle.
• Gets the size of the file and creates an ArrayBuffer to contain it.
• Reads the file contents into the buffer.
• Encodes the message and writes it to the end of the file.
• Persists the changes to disk and closes the access handle.

onmessage = async (e) => {
  // Retrieve message sent to work from main script
  const message = e.data;

  // Get handle to draft file
  const root = await navigator.storage.getDirectory();
  const draftHandle = await root.getFileHandle("draft.txt", { create: true });
  // Get sync access handle
  const accessHandle = await draftHandle.createSyncAccessHandle();

  // Get size of the file.
  const fileSize = accessHandle.getSize();
  // Read file content to a buffer.
  const buffer = new DataView(new ArrayBuffer(fileSize));
  const readBuffer = accessHandle.read(buffer, { at: 0 });

  // Write the message to the end of the file.
  const encoder = new TextEncoder();
  const encodedMessage = encoder.encode(message);
  const writeBuffer = accessHandle.write(encodedMessage, { at: readBuffer });

  // Persist changes to disk.
  accessHandle.flush();

  // Always close FileSystemSyncAccessHandle if done.
  accessHandle.close();
};

Note: In earlier versions of the spec, close(), flush(), getSize(), and truncate() were wrongly specified as asynchronous methods, and older versions of some browsers implement them in this way. However, all current browsers that support these methods implement them as synchronous methods.

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