Loop with nested promise - node.js

Question

What is the best way to get result from a promise inside a For loop.
In this example code resultArray is not complete when the loops ends.
var resultArr = [];
var itemArray1 = [1, 2, 3, 4, 5];
var itemArray2 = ['a','b','c','d','e'];
for (var a = 0; a < itemArray1.length; a++) {
for (var b = 0; b < itemArray2.length; b++) {
myPromise(a,b)
.then(function(result) {
if (result != null) {
resultArr.push(result);
}
});
}
}
// resultArray is still not complete
function myPromise(a,b) {
return new Promise(function(resolve, reject) {
// request to mongodb
myTable.findOne({ _id:a, name:b }, function(err,result) {
if (err) throw err;
resolve(result);
});
});
}

Answer 1

In my opinion, the cleanest way to do things like this is to use Promise.all() with Array#map. Also, make sure to give keep your functions clean and concise, and give them meaningful names!
var itemArray1 = [1, 2, 3, 4, 5];
var itemArray2 = ['a','b','c','d','e'];
function flatten(arrays) {
return [].concat(arrays);
}
function queryAs() {
return Promise.all(itemArray1.map(queryBs))
// the result is an array of arrays, so we'll flatten them here
.then(flatten);
}
function queryBs(a) {
return Promise.all(itemArray2.map(function (b) {
return performQuery(a, b);
}));
}
// resultArray is still not complete
function performQuery(a, b) {
return new Promise(function(resolve, reject) {
// request to mongodb
myTable.findOne({ _id:a, name:b }, function(err,result) {
if (err) throw err;
resolve(result);
});
});
}
queryAs().then(function (results) {
console.log(results);
});

Answer 2

You could use a combination of forEach and Promise.all
var resultArr = [];
var itemArray1 = [1, 2, 3, 4, 5];
var itemArray2 = ['a', 'b', 'c', 'd', 'e'];
var myPromise = (item1, item2) => new Promise((resolve, reject) => {
myTable.findOne({
_id: item1,
name: item2
}, function(err, result) {
if (err) reject(err);
else resolve(result);
});
});
var promises = [];
itemArray1.forEach(item1 => {
itemArray2.forEach(item2 => {
promises.push(myPromise(item1, item2));
});
});
Promise.all(promises).then(result => {
console.log(result);
}).catch(err => {
console.error(err.message);
});