如何採取某些屬性的最後一個值從JSON在Angularjs

Question

我有我的app.js這個代碼發送聊天和看我的聊天應用

$scope.messageshistory = {}; 
$scope.tmp = {}; 

    // send message 
    $scope.sendMessage = function(){ 
     $scope.messages = { 
      from  : $scope.datauser['data']['_id'], 
      fromname : $scope.datauser['data']['nama'], 
      to  : $scope.tmpuserid, 
      message : $scope.tmp['sendmessage'], 
      time  : moment() 
     }; 
      //event emit message 
     socket.emit('message',$scope.messages,function(callback){  
      if(!callback['error']){ 
       $scope.messages['time'] = moment($scope.messages['time']).format('DD-MMMM-YYYY HH:MM'); 


       if ($scope.messageshistory.hasOwnProperty($scope.tmpuserid)){ //yg di json yg paling awal 
        $scope.messageshistory[$scope.tmpuserid].push($scope.messages); 
       }else{ 
        $scope.messageshistory[$scope.tmpuserid] = []; 
        $scope.messageshistory[$scope.tmpuserid].push($scope.messages); 
       } 
       $scope.tmp['sendmessage'] = ''; 
      }else{ 
       var msg = callback['error']; 
       navigator.notification.alert(msg,'','Error Report', 'Ok'); 
      } 

      $scope.$apply(); 
     }); 
    }; 
     //event read message 
    socket.on('message', function (data) { 

     window.plugin.notification.local.add({ 
      id  : moment(), 
      title  : data['fromname'], 
      message : data['message'].substr(0,20) + ' ...', 
      led  : 'A0FF05', 
      json  : JSON.stringify({ routes:'chat', nama :data['fromname'],from:data['from'] }) 
     }); 
     data['time'] = moment(data['time']).format('DD-MMMM-YYYY HH:MM'); 
     if ($scope.messageshistory.hasOwnProperty(data['from'])){ 
      $scope.messageshistory[data['from']].push(data); 
     }else{ 
      $scope.messageshistory[data['from']] = []; 
      $scope.messageshistory[data['from']].push(data); 
     } 

     for(var i = 0; i<= $scope.datauser['data']['friends'].length; i++){ 
      if($scope.datauser['data']['friends'][i]['userid'] == data['from']){ 
       $scope.datauser['data']['friends'][i]['ischat'] = true; 
       break; 
      } 
     }; 
     $scope.$apply(); 
    }); 

我的問題是如何利用最後的消息屬性中的值來自$scope.messageshistory，因爲$scope.messages用於發送消息，$scope.messageshistory用於保留聊天記錄。這是聊天活動圖像：

剛剛從這個活動，$scope.messageshistory將數據保存在它的JSON爲：

{ 
    "5512": [{ 
     "from": "561c", 
     "fromname": "ryan", 
     "to": "5512", 
     "message": "hey", 
     "time": "18-Maret-2016 21:03" 
    }, { 
     "from": "5512", 
     "fromname": "sasa", 
     "to": "561c", 
     "message": "hello", 
     "time": "18-Maret-2016 21:03", 
     "_id": "593s" 
    }] 
} 

我使用angular.toJson($scope.messageshistory)得到這個值，而這個數組如果聊天活動仍在繼續，總是加起來。我的意圖是從$scope.messageshistory獲得消息屬性中的最後一個值，即在我的應用程序中使用Text-to-Speech功能。這是代碼：

$scope.speakText = function() { 

    TTS.speak({ 
      text: **this the place for the code**, 
      locale: 'en-GB', 
      rate: 0.75 
     }, function() { 
      // handle the success case 
     }, function (reason) { 
      // Handle the error case 
     }); 
    }; 

它將讀取$scope.messageshistory中的最後一條消息。那麼，我必須編寫哪些代碼才能獲取最後一個值？

Answer 1

你必須做到以下幾點：

var msgs = $scope.messageshistory[$scope.tmpuserid] 
var yourLastMessage = msgs[msgs.length-1].message 
// you could also add a quick check so you don't get 
// an error if the messages array is emtpy : 
// var yourLastMessage = (msgs && msgs[msgs.length-1] ? msgs[msgs.length-1].message : null) 

編輯

按您的評論的一些解釋：

var msgs = $scope.messageshistory[$scope.tmpuserid] 
// msgs is now an Array containing Objects 
// [{message : 'xxx'},{message : 'yyy'}] 
// we take the last entry of the msgs Array (msgs.length-1) 
// so msgs[msgs.length-1] is the last object ({message : 'yyy'}) 
// and finally we take the 'message' property' of that object: 
var yourLastMessage = msgs[msgs.length-1].message 

Answer 2

假設歷史記錄對象的鍵升數，並考慮到並非由W3C指定的鍵的對象的順序，你必須做到以下幾點：

1. 得到所有鑰匙
2. 找到 「最新」（因此最大數量）
3. 把它拿來

，所以你可以例如做

var keys = Object.keys($scope.messagehistory); 
keys.sort (function (a, b) { 
    return a - b; 
}); 
var result = keys[keys.length - 1];