Not getting response if find inside loop node.js mongodb - node.js

Question

I am using mongodb find query inside loop as I need to run the find query 5 times. And I used below code for that:
let result = {};
let miles = ['5','10','15','25'];
let i = 0;
while (i < miles.length) {
Shops.find({ 'shopInfo.address':{ $geoWithin:{ $centerSphere: [ [ 75.83183541365247, 30.902146005639267 ], miles[i] / 3959 ] } } }).then(response=>{
if(i==4){
result[miles[i]] = response.length;
res.json(result);
}else{
result[miles[i]] = response.length;
i++;
}
})
.catch(err=>{
console.log(err)
});
}
And when I hit the api on browser. It's not returning with anything and getting below error in console:
Please help me, How can I solve the issue?

Answer 1

The following happens:
Your while loop runs and starts an async action. However as that async action is not finished yet, the i++ will not be executed, therefore the loop runs forever, creates more and more async actions that fill up your memory, and finally NodeJS crashes because it runs out of memory. To prevent that, you should not synchronously iterate with an asynchronous task. Either await the asynchronous task inside a loop, or .map the miles to the asynchronous tasks then await them all:
const entries = Promise.all(miles.map(mile =>
Shops.find({ 'shopInfo.address':{ $geoWithin:{ $centerSphere: [ [ 75.83183541365247, 30.902146005639267 ], mile / 3959 ] } } })
.then(entry => ([mile, entry.length]))
));
entries.then(entries => {
const result = Object.fromEntries(entries);
//...
});
// Polyfill: Object.fromEntries
if(!Object.fromEntries)
Object.defineProperty(Object, "fromEntries", {
value(entries) {
const result = {};
for(const [k, v] of entries)
result[k] = v;
return result;
}
});

Answer 2

Normal loops have issues with looping over async code. You can implement an asyncForEach helper function:
async function asyncForEach(array, callback) {
if (array)
for (let index = 0; index < array.length; index++) {
await callback(array[index], index, array);
}
}
And call it executing your mongoQueries:
await asyncForEach(miles, async mile => {
const response = await Shops.find({ 'shopInfo.address':{ $geoWithin:{ $centerSphere: [ [ 75.83183541365247, 30.902146005639267 ], miles[i] / 3959 ] } } })
// do soming with the response
})

Answer 3

Use async/await to wait mongo promise get resolved and then increment variable. Try this :
async function test() {
let result = {};
let miles = ["5", "10", "15", "25"];
let i = 0;
while (i < miles.length) {
try {
let response = await Shops.find({
"shopInfo.address": {
$geoWithin: {
$centerSphere: [
[75.83183541365247, 30.902146005639267],
miles[i] / 3959
]
}
}
});
if (i == 4) {
result[miles[i]] = response.length;
res.json(result);
} else {
result[miles[i]] = response.length;
i++;
}
} catch (err) {
console.log(err);
}
}
}

Answer 4

Below is the code which worked for me:
let miles = ['5','10','15','25','50'];
async.map(miles,getDistance , function(err, results) {
if(err){
console.log(err);
}
res.json(results);
});
function getDistance(mile, callback) {
Shops.count({ 'shopInfo.address':{ $geoWithin:{ $centerSphere: [ [ 75.83183541365247, 30.902146005639267 ], mile / 3959 ] } } }).then(response=>{
if(response){
callback(null,response);
}
})
}
I Hope It will work for you as well.