Not getting response if find inside loop node.js mongodb

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?

yes, If I use it out of loop to get only one result then it's working fine. — Rajat
– Rajat, Commented Apr 26, 2019 at 6:39

Jonas Wilms · Accepted Answer · 2019-04-26 10:47:55Z

2

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;
     }
  });

edited Apr 26, 2019 at 10:47

answered Apr 26, 2019 at 8:17

Jonas Wilms

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5 Comments

Jonas Wilms Over a year ago

@rajat no, Shops.find is asynchronous.

Jonas Wilms Over a year ago

No, not really. Wether a loop is syncrhonous or not depends on how you use it.

Jonas Wilms Over a year ago

@rajat oh and by the way, mongodb has a .count() method (not sure how that is named in mongoose (?)) which will greatly speed up your code.

Jonas Wilms Over a year ago

Do you have Node 12.0.0 ?

Jonas Wilms Over a year ago

I added a polyfill.

Simon Thiel · Accepted Answer · 2019-04-26 07:02:41Z

0

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 
})

edited Apr 26, 2019 at 7:02

answered Apr 26, 2019 at 6:54

Simon Thiel

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4 Comments

Rajat Over a year ago

Should I need to apply above both functions?

Simon Thiel Over a year ago

the first code blocks declares the helper function asyncForEach and second code block shows how to use the helper function

Simon Thiel Over a year ago

I use this helper function several time in my project and refactored it to a util functions class

Jonas Wilms Over a year ago

for(const mile of miles) { await Shops.find(...) } why do you need a helper for that?

Vikash_Singh · Accepted Answer · 2019-04-26 08:08:38Z

0

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);
    }
  }
}

answered Apr 26, 2019 at 8:08

Vikash_Singh

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1 Comment

Rajat Over a year ago

Thanks for your answer. But neither it gives me error not it gives me response

Rajat · Accepted Answer · 2019-04-26 11:22:13Z

0

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.

answered Apr 26, 2019 at 11:22

Rajat

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