Mongoose, Node.js get sum of a field from a bunch of documents

I have this mongoose method/query which finds all 'incomes' from a certain user but only if the dates of the 'incomes' are within the current month.

Code:

module.exports.getMonthlyIncome = function(userId, callback){
const now = new Date();

const year = now.getFullYear();
const month = now.getMonth();
const date = now.getDate();

const start = new Date(year, month, 1);
const end = new Date(year, month, 30);

Income.find({owner: userId, date: { $gte: start, $lt: end }}, callback);
}

Result:

[
{

"_id": "58cc9ee50fe27e0d2ced5193",
"amount": 600,
"description": "Ripco Salary",
"owner": "58cc9e950fe27e0d2ced5192",
"__v": 0,
"date": "2017-03-17T00:00:00.000Z"
},

{

"_id": "58ccc3cfca6ea10980480d42",
"amount": 450,
"description": "Another Ripped co salary",
"owner": "58cc9e950fe27e0d2ced5192",
"__v": 0,
"date": "2017-03-26T00:00:00.000Z"
}

]

Result is as expected, gives me the 2 income documents belonging to a certain user during the month.

Now, I want to get the total sum of every 'amount' field from these documents.

So in this case, the sum would be 1050.

How would I achieve this in Mongoose?

Any help is greatly appreciated, cheers.

via Berimbolo