var bills = [
          {
            "refNo": 17,
            "billDate": "1-apr-2016",
            "dueDate": "30-apr-2016",
            "pendingAmount": 4500,
            "overdueDays": 28
          },
          {
            "refNo": 20,
            "billDate": "15-apr-2016",
            "dueDate": "3-may-2016",
            "pendingAmount": 56550,
            "overdueDays": 15
          }
        ];
var res = bills.map(bill => bill.pendingAmount).reduce((acc, bill) => bill + acc);
console.log(res)