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Showing content from https://developer.cdn.mozilla.net/en-US/docs/Web/API/FileSystemSyncAccessHandle/read below:

FileSystemSyncAccessHandle: read() 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 read() method of the FileSystemSyncAccessHandle interface reads the content of the file associated with the handle into a specified buffer, optionally at a given offset.

buffer

An ArrayBuffer or ArrayBufferView (such as a DataView) representing the buffer that the file content should be read into. Note that you cannot directly manipulate the contents of an ArrayBuffer. Instead, you create one of the typed array objects 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.

options Optional

An options object containing the following properties:

at

A number representing the offset in bytes that the file should be read from.

A number representing the number of bytes read from the file.

InvalidStateError DOMException

Thrown if the associated access handle is already closed.

TypeError

Thrown if the underlying file system does not support reading 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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