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)