How to extract keys from JSON where its value is true

I have below JSON, from this I want to extract Skills where it is true.

[  
   {  
      "_id":"5de9f351baca28556c6a4b71",
      "Name":"Harsha",
      "Age":20,
      "Gender":"M",
      "Skills":{  
         "Java":"",
         "Mule":true,
         "Angular":""
      }
   },
   {  
      "_id":"5de9f358baca28556c6a4b72",
      "Name":"Anji",
      "Age":21,
      "Gender":"M",
      "Skills":{  
         "Java":"",
         "Mule":true,
         "Angular":true
      }
   },
   {  
      "_id":"5dea110297c2b65298b136e4",
      "Name":"Abhi",
      "Age":25,
      "Gender":"M",
      "Skills":{  
         "Java":"",
         "Mule":true,
         "Angular":""
      }
   }
]

I am able to print rest of the data using below code

<table *ngIf="formTemplate">
        <tr>
            <th *ngFor="let header of questionTitleArray">{{header}}</th>
        </tr>

        <tr *ngFor="let data of surveyDataFromDB">
                <ng-container *ngFor="let head of questionTitleArray">
                <td>{{data[head]}}</td>         
        </ng-container>
        </tr>
    </table>

(Here JSON is "surveyDataFromDB")

Below is the output I am getting

Name    Age Gender  Skills
Harsha  20  M   [object Object]
Anji    21  M   [object Object]
Abhi    25  M   [object Object]

I want Skills which are true in place of [object Object]. Please help.