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The FileSystemSyncAccessHandle interface of the File System Access API represents a synchronous handle to a file system entry. The synchronous nature of the file reads and writes allows for higher performance for critical methods in contexts where asynchronous operations come with high overhead, e.g., WebAssembly.

This class is only accessible inside dedicated Web Workers for files within the origin private file system.

The interface is accessed through the FileSystemFileHandle.createSyncAccessHandle() method.

Instance properties


Instance methods


Closes an open synchronous file handle, disabling any further operations on it and releasing the exclusive lock previously put on the file associated with the file handle.


Persists any changes made to the file associated with the handle via the write() method to disk.


Returns the size of the file associated with the handle in bytes.


Reads the content of the file associated with the handle into a specified buffer, optionally at a given offset.


Resizes the file associated with the handle to a specified number of bytes.


Writes the content of a specified buffer to the file associated with the handle, optionally at a given 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 =;

  // Get handle to draft file
  const root = await;
  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 =, { 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.

  // Always close FileSystemSyncAccessHandle if done.

Note: In earlier versions of the spec, close(), flush(), getSize(), and truncate() were wrongly specified as asynchronous methods. This has now been amended, but some browsers still support the asynchronous versions.


File System Standard
# api-filesystemsyncaccesshandle

Browser compatibility

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See also