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FileSystemFileHandle - Web APIs | MDN

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 available in Web Workers.

The FileSystemFileHandle interface of the File System API represents a handle to a file system entry. The interface is accessed through the window.showOpenFilePicker() method.

Note that read and write operations depend on file-access permissions that do not persist after a page refresh if no other tabs for that origin remain open. The queryPermission method of the FileSystemHandle interface can be used to verify permission state before accessing a file.

Inherits properties from its parent, FileSystemHandle.

Inherits methods from its parent, FileSystemHandle.

getFile()

Returns a Promise which resolves to a File object representing the state on disk of the entry represented by the handle.

createSyncAccessHandle()

Returns a Promise which resolves to a FileSystemSyncAccessHandle object that can be used to synchronously read from and write to a file. The synchronous nature of this method brings performance advantages, but it is only usable inside dedicated Web Workers.

createWritable()

Returns a Promise which resolves to a newly created FileSystemWritableFileStream object that can be used to write to a file.

The following asynchronous function presents a file picker and once a file is chosen, uses the getFile() method to retrieve the contents.

async function getTheFile() {
  const pickerOpts = {
    types: [
      {
        description: "Images",
        accept: {
          "image/*": [".png", ".gif", ".jpeg", ".jpg"],
        },
      },
    ],
    excludeAcceptAllOption: true,
    multiple: false,
  };

  // open file picker
  const [fileHandle] = await window.showOpenFilePicker(pickerOpts);
  // get file contents
  const fileData = await fileHandle.getFile();
  return fileData;
}

The following asynchronous function writes the given contents to the file handle, and thus to disk.

async function writeFile(fileHandle, contents) {
  // Create a FileSystemWritableFileStream to write to.
  const writable = await fileHandle.createWritable();

  // Write the contents of the file to the stream.
  await writable.write(contents);

  // Close the file and write the contents to disk.
  await writable.close();
}

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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