const functions = require('firebase-functions');
var admin = require('firebase-admin');
var request = require('request');
var firebase = require('firebase');

admin.initializeApp();
firebase.initializeApp();

exports.takeValues = functions.database.ref('/users/{uid}/')
.onWrite((change, context) => {

       var user_id = context.auth.uid;
       var FB_access_token = change.after.val().Facebook_Access_Token;
       var twitter_username = change.after.val().twitter_handle;
       var insta_username = change.after.val().insta_handle;
       var test_at_src = change.after.val().Test_Score.Arts;
       var test_sp_src = change.after.val().Test_Score.Sports;
       var test_ac_src = change.after.val().Test_Score.Academics;

       if( FB_access_token!==undefined||'undefined' && twitter_username!==undefined||'undefined' && insta_username!==undefined||'undefined' && user_id!==undefined||'undefined' && test_at_src!==undefined||'undefined' && test_ac_src!==undefined||'undefined' && test_sp_src!==undefined||'undefined'){ 
        apicall(user_id,FB_access_token,twitter_username,insta_username,test_ac_src,test_at_src,test_sp_src)
      }
    });

    function apicall(user_id,FB_access_token,twitter_username,insta_username,test_ac_src,test_at_src,test_sp_src){

        return new Promise((resolve, reject)=> {

           var dataString = `user_id=${user_id}&FB_access_token=${FB_access_token}&twitter_username=${twitter_username}&insta_username=${insta_username}&test_ac_src=${test_ac_src}&test_at_src=${test_at_src}&test_sp_src=${test_sp_src}`;             

            var options = {
                url: 'http://ip/api',
                method: 'POST',
                body : dataString

            };

            function callback(error, response, body) 
{
    if (!error && response.statusCode === 200)
     {
         console.log(body);
         resolve(body);
    }
}
request(options, callback);
});
      }

字符串
上面的代码每次在firebase数据库上写入数据时都会调用apicall（）函数，即使所有变量都没有值。但是需要调用api的代码，即apicall（）函数只有在所有变量都有值之后才被触发一次。对此可以做些什么？
数据库看起来像这样：
TtUBAOgi0aWBdPou2oCu
Facebook_Access_Token：“.kwBAHXDtyxSauOZAnzENAnA3d88Igbxxjzz2...”
测试_评分
学术：1
艺术：4人
体育：3
insta_handle：“ab”
twitter_handle：“hj”