JavaScript for循環不工作？

Question

我還沒有真正使用循環之前，我似乎無法弄清楚爲什麼我的代碼不工作。希望有人幫助我指出正確的方向。

return firebase.database().ref('Users/' + uid + "/PDR").once('value').then(function display(dataSnapshot) { 

    //Number of times the loop should run returning a value 
    var number = dataSnapshot.val().Total; 

    var i; 
    for (i = 1; i < number; i++) { 

    //Each time loop runs changes Firebase reference by "1" 
    return firebase.database().ref('/Users/' + uid + "/PDR/" + i).once('value').then(function display(dataSnapshot) { 

     var num = dataSnapshot.val().number; 
     var dateFrom = dataSnapshot.val().dateFrom; 
     var dateTo = dataSnapshot.val().dateTo; 
     var dbActivity = dataSnapshot.val().activity; 

     //Each loop adds different data to table 
     document.getElementById("PDRTable").innerHTML += '<tr><td>' + num + '</td><td>' + dateFrom + '</td><td>' + dateTo + '</td><td>' + dbActivity + '</td></tr>' 

    }) 
    } 
}) 

我試圖從我的Firebase後端提取數據並將其顯示在表中，目前它只運行一次。

感謝您的任何幫助和建議！

Answer 1

不確定dataSnapshot對象中發生了什麼，但在javascript調試中的第一步是添加alert（「Here」）;在你的代碼，看看有什麼（如果有的話）正在返回

//Number of times the loop should run returning a value 
    var number = dataSnapshot.val().Total;  

    var i; 
    for (i = 1; i < number; i++){ 
    alert("Here " + i); //Show me the loop variable 
    //Each time loop runs changes Firebase reference by "1" 
    return firebase.database().ref('/Users/' + uid + "/PDR/" + i).once('value').then(function display(dataSnapshot) { 
    alert(num); 
    alert(dateFrom); 
    alert(dateTo); 
    var num = dataSnapshot.val().number; 
    var dateFrom = dataSnapshot.val().dateFrom; 
    var dateTo = dataSnapshot.val().dateTo; 
    var dbActivity = dataSnapshot.val().activity;        

    //Each loop adds different data to table 
    document.getElementById("PDRTable").innerHTML += '<tr><td>'+ num +'</td><td>'+ dateFrom + '</td><td>'+ dateTo +'</td><td>'+ dbActivity +'</td></tr>'