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FileSystemWritableFileStream: seek() method

files/en-us/web/api/filesystemwritablefilestream/seek/index.md

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

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

Syntax

js-nolint
seek(position)

Parameters

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

Return value

A {{jsxref('Promise')}} that returns undefined.

Exceptions

  • NotAllowedError {{domxref("DOMException")}}
    • : Thrown if {{domxref('PermissionStatus.state')}} is not granted.
  • {{jsxref("TypeError")}}
    • : Thrown if position is not a number or not defined.

Examples

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

js
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".

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also