NodeJS / Express Why am I getting "Cannot Read Property 'then' of Undefined"? - node.js

Question

I apologize in advance because I have seen other questions on this very topic, but I do not understand what I am doing wrong and how to fix it. Please have a look at my code:
function getSum(productID) {
Rating.aggregate(
[
{
$group: {
_id: "$productID",
total: {
$sum: "$rating"
}
}
}
],
function (err, result) {
if (err) {
res.send(err);
} else {
//console.log("product-sum: " + req.body.productID)
const count = result.find(item => item._id === productID.productID);
console.log("getSum count: ", count.total);
return count.total;
}
}
);
}
router.route('/compute-rating').post((req, res) => {
console.log("compute Rating: ", req.body.data);
var productID = req.body.data;
var sum = getSum(productID).then( //getting the error here
res.json({ sum })
);
});
getSum() returns a valid number from count.total.
Once I get the sum, I plan to chain another .then onto the existing then and call another function with the productID, but I need to use the sum later in the computer-rating route.
In the code, I have a comment that shows where the error, "Cannot Read Property 'then' of Undefined", is occurring. Why am I getting this error and how can I fix it?
Thanks.
Edit:
I wanted to show my final solution so others could benefit from my experience. Hopefully I did not create any major Javascript violations in my code. I ended up using Promise.all because I had to perform a calculation based on 2 returned values.
function getSum(productID) {
return new Promise(async (resolve, reject) => {
const result = await Rating.aggregate( //sum of a column in MongoDB
[
{
$group: {
_id: "$productID",
total: {
$sum: "$rating"
}
}
}
]
);
try {
var sum = result.find(item => item._id === productID.productID);
if (sum !== undefined) {
resolve(sum);
console.log("getSum sum: ", sum);
}
else {
reject("invalid product id");
}
}
catch (e) {
reject(e);
}
});
}
function getCount(productID) {
return new Promise(async (resolve, reject) => {
var result = await Rating.countDocuments(productID)
.then(count => {
console.log("getCount count:", result);
var documentCount = { count: count }
resolve(documentCount);
})
.catch(err => reject(err));
});
}
router.route('/compute-rating').post((req, res) => {
console.log("compute Rating: ", req.body.data);
var productID = req.body.data;
Promise.all([getSum(productID), getCount(productID)])
.then(results => {
console.log("compute rating results: ", results);
if (results[1].count > 0) {
res.status(200).json({ rating: results[0].total / results[1].count });
}
else {
res.status(200).json({ rating: 0 });
}
})
.catch(err => {
res.status(400).json({ error: err });
})
});

Answer 1

Your getSum doesn't return anything. You are returning only from the callback function, not the getSum function.
You should make it async.
And also you are doing res.send in that function, while not having access to res object.
For example you can do it like this:
async function getSum(productID) {
const result = await Rating.aggregate(
[
{
$group: {
_id: "$productID",
total: {
$sum: "$rating"
}
}
}
]
);
const count = result.find(item => item._id === productID.productID);
console.log("getSum count: ", count.total);
return count.total;
}
router.route('/compute-rating').post((req, res) => {
console.log("compute Rating: ", req.body.data);
var productID = req.body.data;
// EDIT: This should work
getSum(productID).then(sum => {
res.json({ sum })
});
});

Answer 2

then only works on functions that return a promise. your function getSum is not returning anything, if Rating.aggregate function returns a promise, aka accepts a then, then you should return this aggregate, simply add return before calling it.
Now if aggregate doesn't return a promise, and I'm guessing so because you're passing a callback function to it, you might want to return a promise that resolves using promise.resolve in this callback body.