Improve code with a Promise for a mongodb request - node.js

Question

I'm new to javascript (and coding) and I'm looking into Promises.
I have the following working code:
router.get("/test", function(req, res){
var mainCategory = new Promise(function(resolve, reject){
Maincategory.find().populate("subcategory").exec(function(err, allMaincategories){
if (err) {
console.log("error 1");
reject("error 2");
}
else {
resolve(allMaincategories);
}
});
});
var itemQuery = new Promise(function(resolve, reject){
Items.find({}, function(err, allItems){
if (err) {
console.log("error 3");
reject("error 4");}
else {
resolve(allItems);
reject("error 5");
}
});
});
Promise.all([
mainCategory,
itemQuery
]).then(function(allQueries){
console.log(allQueries);
var allCategories = allQueries[0];
var allItems = allQueries[1]
var userId = "true";
var show = "list";
res.render("test.ejs", {
allCategories: allCategories,
allItems: allItems,
userId: userId,
show: show
});
}).catch(function(error){
console.log("error 6");
res.render("error.ejs", {error: error});
});
});
Question 1: Is it correct to use a callback inside a promise like that?
Question 2: Is there a (shorter) way to put all mongoose requests in one promise?

Answer 1

Mongoose queries return Promises already - since you're using Promises, better to just use the Promise it returns rather than to construct a new one to use with the callback:
Promise.all([
Maincategory.find().populate("subcategory").exec(),
Items.find({})
]).then(function(allQueries){
// ...

Answer 2

mongoose already supports promises natively, so there's no reason at all to create a promise like you are doing. This:
var mainCategory = new Promise(function(resolve, reject){
Maincategory.find().populate("subcategory").exec(function(err, allMaincategories){
if (err) {
console.log("error 1");
reject("error 2");
}
else {
resolve(allMaincategories);
}
});
});
should be replaced with
const subcatPromise = Maincategory.find().populate("subcategory").exec();
and
var itemQuery = new Promise(function(resolve, reject){
Items.find({}, function(err, allItems){
if (err) {
console.log("error 3");
reject("error 4");}
else {
resolve(allItems);
reject("error 5");
}
});
});
is equivalent to
const itemsPromise = Items.find({});

Answer 3

router.get("/test", async function (req, res) {
try {
const allQueries = await Promise.all([Maincategory.find().populate("subcategory").exec(), Items.find({}).exec()]),
allCategories = allQueries[0],
allItems = allQueries[1],
userId = "true",
show = "list";
res.render("test.ejs", {
allCategories: allCategories,
allItems: allItems,
userId: userId,
show: show
});
} catch (error) {
console.log("error 6");
res.render("error.ejs", {
error: error
});
};
});