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

FileSystemWritableFileStream: truncate() method - Web APIs

Limited availability

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 truncate() method of the FileSystemWritableFileStream interface resizes the file associated with the stream to the specified size in bytes.

If the size specified is larger than the current file size the file is padded with 0x00 bytes.

The file cursor is also updated when truncate() is called. If the offset is smaller than the size, it remains unchanged. If the offset is larger than size, the offset is set to that size. This ensures that subsequent writes do not error.

No changes are written to the actual file on disk until the stream has been closed. Changes are typically written to a temporary file instead.

size

A number specifying the number of bytes to resize the stream to.

A Promise that returns undefined.

NotAllowedError DOMException

Thrown if PermissionStatus.state is not granted.

QuotaExceededError DOMException

Thrown if the new size of the file is larger than the original size of the file, and exceeds the browser's storage quota.

TypeError

Thrown if size 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 truncate() method is used to resize the file to 8 bytes.
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("This is my first file content");
    await writableStream.truncate(8);
    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 "This is my second file content".

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