如何处理firebase数据库的异步特性

firebase.database().ref('users').once('value', function(snapshot) {
  var users = snapshot.val();
  console.log(users);
});

firebase.database().ref('users').once('value')
  .then(function(snapshot) {
    var users = snapshot.val();
    console.log(users);
  })
  .catch(function(error) {
    console.error("Error reading data: ", error);
  });

firebase.database().ref('users').once('value')
  .then(function(snapshot) {
    var users = snapshot.val();
    console.log(users);
  })
  .catch(function(error) {
    console.error("Error reading data: ", error);
  });

async function readUserData() {
  try {
    const snapshot = await firebase.database().ref('users').once('value');
    const users = snapshot.val();
    console.log(users);
  } catch (error) {
    console.error("Error reading data: ", error);
  }
}

readUserData();