Empty properties in event user

[timursevimli]

const user = {
  ....,
  premium: false,
  attachMenuEnabled: false,
  flags2: 16,
  botCanEdit: false,
  closeFriend: false,
  storiesHidden: false,
  storiesUnavailable: true,
  contactRequirePremium: false,
  botBusiness: false,
  id: Integer { value: 356024551n },
  accessHash: Integer { value: -54182806721850885n },
  firstName: 'Mustafa',
  lastName: null,
  username: null,
  phone: null,
  ......,
  };

Some properties in user object is empty. How can I identify empty properties, such as 'username', in the user (message._sender) object?