ContextQMD
Back to Content

FileSystemSyncAccessHandle: read() method

files/en-us/web/api/filesystemsyncaccesshandle/read/index.md

latest3.5 KB
Original Source

{{securecontext_header}}{{APIRef("File System API")}}{{AvailableInWorkers("dedicated")}}

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

Syntax

js-nolint
read(buffer, options)

Parameters

  • buffer
    • : An {{jsxref("ArrayBuffer")}} or ArrayBufferView (such as a {{jsxref("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 {{jsxref("Int8Array")}} or a {{jsxref("DataView")}} object which represents the buffer in a specific format, and use that to read and write the contents of the buffer.
  • options {{optional_inline}}
    • : An options object containing the following properties:
      • at
        • : A number representing the offset in bytes that the file should be read from.

Return value

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

Exceptions

  • InvalidStateError {{domxref("DOMException")}}
    • : Thrown if the associated access handle is already closed.
  • {{jsxref("TypeError")}}
    • : Thrown if the underlying file system does not support reading the file from the specified file offset.

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 {{jsxref("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.
js
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, {{domxref("FileSystemSyncAccessHandle.close()", "close()")}}, {{domxref("FileSystemSyncAccessHandle.flush()", "flush()")}}, {{domxref("FileSystemSyncAccessHandle.getSize()", "getSize()")}}, and {{domxref("FileSystemSyncAccessHandle.truncate()", "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.

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also