docs: function writeFileUtf8 (filePath, content, callback) {
    // Argument callback defaults to throwError.
    if (typeof callback !== 'function') callback = throwError
  
    if (typeof content === 'string') {
      fs.writeFile(filePath, content, 'utf8', callback)
    } else {
      throw TypeError(error.contentIsNotString)
    }
  }