Chrome 抛出了 Uncaught TypeError: Cannot set property 'key' of undefined 错误，但我真的不知道代码出了什么问题。我试过 console.log(item) 并且它是未定义的。但使用 lodash 和 clone，我不确定如何设置值。抱歉，我问了这么一个菜鸟问题，但是如果有人能向我解释发生了什么事情，那就太好了，我一定会花很多时间从中学习！

下面是我的代码...

  class Actions {

  initSession() {
    return (dispatch) => {

      Firebase.auth().onAuthStateChanged(function(result) {
        var profile = null;

        if (result) {
          profile = {
            id: result.uid,
            name: result.providerData[0].displayName,
            avatar: result.providerData[0].photoURL
          }
        }

        dispatch(profile);
      });
    }
  }

  login() {
    return (dispatch) => {
      var provider = new Firebase.auth.FacebookAuthProvider();
      Firebase.auth().signInWithPopup(provider).then(function(result) {
        var user = result.user;

        var profile = {
          id: user.uid,
          name: user.providerData[0].displayName,
          avatar: user.providerData[0].photoURL
        }

        Firebase.database().ref('/users/'+user.uid).set(profile);
        dispatch(profile);

      }).catch(function(error) {
        console.log('Failed!', error);
      });
    }
  }

  logout() {
    return(dispatch) => {
      Firebase.auth().signOut().then(function() {
        // Sign-out successful.
        dispatch(null);
      }, function(error) {
        // An error happened.
        console.log(error);
      });
    }
  }

  getComments(productId) {
    return (dispatch) => {
      var commentRef = Firebase.database().ref('comments/'+productId);

      commentRef.on('value', function(snapshot) {
        var commentsValue = snapshot.val();
        var comments = _(commentsValue).keys().map((commentKey) => {
          var item = _.clone(commentsValue[commentKey]);
          item.key = commentKey;
          return item;
        })
        .value();
        dispatch(comments);
      });
    }
  }

  getProducts() {
    return(dispatch) => {
      Firebase.database().ref('products').on('value', function(snapshot) {
        var productsValue = snapshot.val();
        var products = _(productsValue).keys().map((productKey) => {
          var item = _.clone(productsValue[productKey]);
          item.key = productKey;
          return item;
        })
        .value();
        console.log(item);
        dispatch(products);
      });
    }
  }

  getProducts() {
    return(dispatch) => {
      Firebase.database().ref('products').on('value', function(snapshot) {
        var productsValue = snapshot.val();
        var products = _(productsValue).keys().map((productKey) => {
          var item = _.clone(productsValue[productKey]);
          item.key = productKey;
          return item;
        })
        .value();
        dispatch(products);
      });
    }
  }

  addProduct(product) {
    return (dispatch) => {
      Firebase.database().ref('products').push(product);
    }
  }

  addVote(productId, userId) {
    return (dispatch) => {
      var voteRef = Firebase.database().ref('votes/'+productId+'/'+userId);
      var upvoteRef = Firebase.database().ref('products/'+productId+'/upvote');

      voteRef.on('value', function(snapshot) {
        if(snapshot.val() == null) {
          voteRef.set(true);
          var vote = 0;
          upvoteRef.on('value', function(snapshot) {
            vote = snapshot.val();
          });
          upvoteRef.set(vote+1);
        }
      });
    }
  }

  addComment(productId, comment) {
    return (dispatch) => {
      Firebase.database().ref('comments/'+productId).push(comment);
    }
  }

  getComments(productId) {
    return (dispatch) => {
      var commentRef = Firebase.database().ref('comments/'+productId);

      commentRef.on('value', function(snapshot) {
        var commentsValue = snapshot.val();
        var comments = _(commentsValue).keys().map((commentKey) => {
          var item = _.clone(commentsValue[commentKey]);
          item.key = commentKey;
          return item;
        })
        .value();
        dispatch(comments);
      });
    }
  }

}

export default alt.createActions(Actions);

这是我收到的错误...

Chrome 控制台错误 ：

未捕获的类型错误：无法设置未定义的属性“key” ：