Fill string with data from mongodb

Question

I'm new to nodejs and mongodb. Can someone explain the behaviour of the following code to me?

var express = require('express')
var app = express();
var MongoClient = require('mongodb').MongoClient
const assert = require('assert')
const mongourl = 'mongodb://localhost:27017/test'
var str=""

MongoClient.connect(mongourl, function(err, client){
    assert.equal(null, err);
    var db = client.db('test');
    var cursor = db.collection('projects.testproject.exampleChannel.germany.parameter').find();
    cursor.forEach(function(item){
      if(item!=null){
        str=str+JSON.stringify(item)
        console.log("1 "+str)
      }
    })
    client.close();
    console.log("2 "+str)
  })

I get the following output and don't understand, why output 1 is what I expected and output 2 is empty.

2
1 {"_id":"5e67960fd92ba91300f5d718","basicInfo":{"projectName":"exampleProject","salesChannel":"exampleChannel","country":"germany"},"categories":{"pieceItems":{"amount":0},"weightItems":{"amount":0},"lengthItems":{"amount":0},"volumeItems":{"amount":0},"weightCodedItems":{"amount":0},"lengthCodedItems":{"amount":0},"volumeCodedItems":{"amount":0},"priceCodedItems":{"amount":0},"deposit&containerItems":{"amount":0},"ageRestrictedItems":{"amount":0},"prepaidItems":{"amount":0},"e-loadingItems":{"amount":0},"guranteeItems":{"amount":0},"paperReadingItems":{"amount":0},"technicalItems":{"amount":0},"giftItems":{"amount":0},"commonCustomerCardItems":{"amount":0},"payBackCardItems":{"amount":0},"deutschlandCardItems":{"amount":0},"serviceItems":{"amount":0}}}

This is because of the event loop. You will need to have a look at event loop, how callStack works. hackernoon.com/understanding-js-the-event-loop-959beae3ac40. have a look at this and try to understand callstack. — Suresh Shetiar
– Suresh Shetiar, Commented Mar 11, 2020 at 9:19
youtube.com/watch?v=v0QTqHXiVNw this video from Dev Ed explains it in very easy way. Please have a look at this. — Suresh Shetiar
– Suresh Shetiar, Commented Mar 11, 2020 at 9:21
Inshort: .find() is an async operation and subsequently .forEach takes a callback function to get you results. The execution is not going to wait for that loop to finish off before reaching console.log("2 "+str) — ambianBeing
– ambianBeing, Commented Mar 11, 2020 at 9:38

Ashish Modi · Accepted Answer · 2020-03-11 10:51:08Z

1

forEach takes a callback function and you are not waiting for the loop to finish. try something like this

const express = require("express");
const app = express();
const MongoClient = require("mongodb").MongoClient;
const assert = require("assert");
const mongourl = "mongodb://localhost:27017/test";
let str = "";

MongoClient.connect(mongourl, async function (err, client) {
  assert.equal(null, err);
  const db = client.db("test");
  const cursor = db.collection("projects.testproject.exampleChannel.germany.parameter").find();
  while (await cursor.hasNext()) {
    const item = await cursor.next();
    if (item != null) {
      str += JSON.stringify(item);
      console.log(`1 ${str}`);
    }
  }
  client.close();
  console.log(`2 ${str}`);
});

answered Mar 11, 2020 at 10:51

Ashish Modi

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