FileSystemSyncAccessHandle: flush() method

Baseline Widely available

This feature is well established and works across many devices and browser versions. Its been available across browsers since March 2023.

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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 flush() method of the FileSystemSyncAccessHandle interface persists any changes made to the file associated with the handle via the write() method to disk.

Bear in mind that you only need to call this method if you need the changes committed to disk at a specific time, otherwise you can leave the underlying operating system to handle this when it sees fit, which should be OK in most cases.

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.

Syntax

flush()

InvalidStateError DOMException: Thrown if the associated access handle is already closed.

Examples

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();
};