Appending an array into a object with forEach()

Im trying to add the array of objects(transactions) into the the array(bills) using forEach to loop and array[key].transactions to add the objects. I have also tried to use push but that didn't work neither.

Some help would be lovely.

var bills = [{
    "_id": "5499aece1d7be6c6a3000001",
    "billName": "Insurance",
    "startDate": "2014-12-15T00:00:00.000Z",
    "amount": 2400,
    "type": 4,
    "timestamp": "2014-12-23T18:05:02.987Z",
    "__v": 0
}, {
    "_id": "549bf0597886c3763e000001",
    "billName": "Leasing",
    "startDate": "2014-12-25T00:00:00.000Z",
    "endDate": "2017-10-14T22:00:00.000Z",
    "amount": 16500,
    "type": 4,
    "timestamp": "2014-12-25T11:09:13.957Z",
    "__v": 0
}];

var transactions = [{
    "_id": "549ea8c957b654ef64000003",
    "billId": "5499aece1d7be6c6a3000001",
    "paymentDate": "2014-12-27T12:40:41.311Z",
    "amount": 2400,
    "timestamp": "2014-12-27T12:40:41.311Z",
    "__v": 0
}, {
    "_id": "549ea8e632ed3f6265000001",
    "billId": "549bf0597886c3763e000001",
    "paymentDate": "2014-12-27T12:41:10.582Z",
    "amount": 16500,
    "timestamp": "2014-12-27T12:41:10.582Z",
    "__v": 0
}, {
    "_id": "549ea93452ebbd8366000001",
    "billId": "549bf0597886c3763e000001",
    "paymentDate": "2014-12-27T12:42:28.744Z",
    "amount": 16500,
    "timestamp": "2014-12-27T12:42:28.745Z",
    "__v": 0
}];

and

 var test = [];
 bills.forEach(function (bill, key) {
     test.push(bill);
     transactions.forEach(function (transaction) {
         if (transaction.billId == bill._id) test[key].transactions = transaction;
     });
 });

 console.log(test);

The end result should be the test object would carry the bills plus the transaction inside each bill objects as an object array.

Muchas gracias!