Question

预期输出：“x为2” 实际输出：“x未定义”

app.js

var x = db_Save(req.session.user);
console.log('x is ' + x);

dbFile.js

var db_Save= function (user) {

    // return 2; /* 'x is 2' would print;

    console.log('function returns "undefined" before following');
    userProfile.find({Email: profileInstance.Email}, function(err, doc){
        console.log('prints after "x is undefined"');
        return 2; // does not get returned  
    });
}

Answer 1

使用回调函数：

db_Save(req.session.user,function(x){
    console.log('x is ' + x);
});

var db_Save= function (user,callback) {
    userProfile.find({Email: profileInstance.Email}, function(err, doc){                                
        callback(2);
    });
};

Answer 2

userProfile.find是异步的，这意味着它会被启动但直到你的console.log被触发后才返回2。你的回调就是这个功能：

 function(err, doc){                              
    console.log('prints after "x is undefined"');
    return 2; // does not get returned  
  }

您将此作为调用userProfile.find的第二个参数。当find完成时，该函数被称为返回2，但此时已经太晚了，你已经控制了当时未定义的console.logged x。

函数返回值的错误

2 个答案: