javascript - 返回 promise 方式的差异

Question

我实现了以下两个 promise :

function getUserSportListPromise(_uid){
        return firebase.database().ref('users/' + user.uid + '/sports/').once('value').then(function(snap){
          var userSportList = [];
          if(snap.exists()){
            snap.forEach(function(sport){
              userSportList.push(sport.val().name);
            });
          }

          return userSportList;
        });
      }

      function getRegistrationsFromSportList(sportList){
        var promiseHash = {};
        sportList.forEach(function (sport){
          promiseHash[sport] = firebase.database().ref('/sport_registrations/' + sport).once('value');
        });
        return $q.all(promiseHash);
      }

为什么此代码显示结果:

var userSportListPromise = getUserSportListPromise(user.uid);
      var registrationHashPromise = userSportListPromise.then(function(sportList){
        return getRegistrationsFromSportList(sportList);
      }).then(function(registrationsHash){
        angular.forEach(registrationsHash, function(regsSnap, sport){
          console.info(sport + ' has the following registrations: ');
          console.dir(regsSnap);
        });
      });

而此代码显示未定义:

getUserSportListPromise(user.uid)
      .then(function(sportList){
          getRegistrationsFromSportList(sportList);
      })
      .then(function(registrationsHash){
        angular.forEach(registrationsHash, function(regsSnap, sport){
          console.dir(registrationsHash);
          console.info(sport + ' has the following registrations: ' + regsSnap);
        });
      });

函数 getUserSportListPromise 返回一个 promise 的哈希值，那么为什么上面的代码在最后一个 then() 调用中传递了一个未定义的 RegistrationsHash 呢？

Answer 1

您在这里缺少返回:

  .then(function(sportList){
      getRegistrationsFromSportList(sportList);
  })

该匿名函数没有 return 语句，因此它返回 undefined。应该是:

  .then(function(sportList){
      return getRegistrationsFromSportList(sportList);
  })

或者只是:

  .then(getRegistrationsFromSportList)