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Showing content from https://developer.mozilla.org/en-US/docs/Web/API/FileSystemWritableFileStream/seek below:

FileSystemWritableFileStream: seek() method - Web APIs

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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 available in Web Workers.

The seek() method of the FileSystemWritableFileStream interface updates the current file cursor offset to the position (in bytes) specified when calling the method.

position

A number specifying the byte position from the beginning of the file.

A Promise that returns undefined.

NotAllowedError DOMException

Thrown if PermissionStatus.state is not granted.

TypeError

Thrown if position is not a number or not defined.

The following asynchronous function opens the 'Save File' picker, which returns a FileSystemFileHandle once a file is selected. From this, a writable stream is created using the FileSystemFileHandle.createWritable() method.

Next, we write to the stream:

1. A text string is written to the stream.
2. The seek() method is used to put the cursor at the start of the stream.
3. A second text string is written to the start of the stream, overwriting the first write.

The stream is then closed.

async function saveFile() {
  try {
    // create a new handle
    const newHandle = await window.showSaveFilePicker();

    // create a FileSystemWritableFileStream to write to
    const writableStream = await newHandle.createWritable();

    // write our file
    await writableStream.write("My first file content");
    await writableStream.seek(0);
    await writableStream.write("My second file content");

    // close the file and write the contents to disk.
    await writableStream.close();
  } catch (err) {
    console.error(err.name, err.message);
  }
}

If you run the above function and then open the resulting file created on disk, you should see the text "My second file content".

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