class Buffer {
  getLines() {
    return new Promise((resolve) => resolve(['exciting', 'lines']));
  }
}

class Neovim {
  getBuffer() {
    return new Promise((resolve) => resolve(new Buffer()));
  }
}

// Get lines of current buffer
nvim.getBuffer().then(buffer => buffer.getLines()).then(lines => {
  // Do something with `lines`
});