Question

我试图返回一个返回值并将其显示在控制台中。（目前）用户ID传递给“功能”。该函数将在数据库中检查此ID，并应返回userName。我知道找到了正确的数据，因为console.log在函数本身中返回了正确的确认（请参阅：此工作）但是，使用return childData.userName;

时返回的值是“ undefined”

调用函数

console.log( f_returnUserDetails(uid)

函数本身

  function f_returnUserDetails(a){
    console.log(a)
    var key;
    var childData;
    firebase.database().ref('/dataman-blabla/').orderByChild("uid").equalTo(a).on('value', function (snapshot) {
      snapshot.forEach(function(childSnapshot) {
          key = childSnapshot.key;
          childData = childSnapshot.val();
          console.log(childData.userName); //THIS WORKS
          return childData.userName; //THIS DOES NOT
      });
    });
  };

Answer 1

您必须返回promise，因为此函数是异步的。

    function f_returnUserDetails(a){
      console.log(a)
      var key;
      var childData;
      return new Promise(function(resolve, reject) { //return promise
        firebase.database().ref('/dataman-blabla/').orderByChild("uid").equalTo(a).on('value', function (snapshot) {
          snapshot.forEach(function(childSnapshot) {
            key = childSnapshot.key;
            childData = childSnapshot.val();
            console.log(childData.userName);
            resolve(childData.userName); 
          });
        });
      });
    };
   // function call should be like this   
   f_returnUserDetails(uid).then((username) => {
     console.log(username);
   });

Answer 2

我不确定100％，但是我认为.forEach不能很好地利用休息和回报。尝试将其更改为传统的for循环，看看是否可行。

来源：https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/forEach#Description

除了通过以下方式之外，没有其他方法可以停止或中断forEach（）循环：引发异常。如果您需要这种行为，请使用forEach（）方法是错误的工具。

return firebase.database().ref('/dataman-blabla/').orderByChild("uid").equalTo(a).on('value', function (snapshot) {
      for(childSnapshot of snapshot) {
          key = childSnapshot.key;
          childData = childSnapshot.val();
          console.log(childData.userName); //THIS WORKS
          return childData.userName; //THIS DOES NOT
      });
    });

Answer 3

调用新函数并传递响应

function f_returnUserDetails(a){
    console.log(a)
    var key;
    var childData;
    firebase.database().ref('/dataman-blabla/').orderByChild("uid").equalTo(a).on('value', function (snapshot) {
      snapshot.forEach(function(childSnapshot) {
          key = childSnapshot.key;
          childData = childSnapshot.val();
          console.log(childData.userName); //THIS WORKS
          processData(childData.userName); 
      });
    });
  };


function processDate(name){
   // You have your name here.
}

我无法使return语句正常工作

3 个答案: