Troubleshooting problem with MongoDB queries within a for loop

Question

Troubleshooting problem with MongoDB queries within a for loop

I have an array of user emails obtained from the post data. My goal is to find the _id associated with each email. Here's the for loop I attempted:

var studentIds = [];

for (var i = studentEmails.length - 1; i >= 0; i--) {
  var email = studentEmails[i];

  User.findOne({"email": email}, (err, student) => {
    if (err) {
      console.log("ERROR" + err);
    }
    if (student) {
      var id = student._id;
      studentIds.push(id);
      console.log("STUDENT: " + student);
    }
  });

}

// Outside for loop
console.log('END');

However, the output shows:

END
STUDENT: { _id: 5a11e667d7333203337cd9a4,
  name: 'Patrick Jacobs',
  email: '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="5d2a3433392b3c3c331d31342b38733331">[email protected]</a>',
  password: '$2a$10$CiSw/VH1HCaPtW6Sjz0X4.4avVoLsAH6iyF3FhidorahwLt1WDXoC',
  __v: 0 }
STUDENT: { _id: 5a0f7dfb64b5a6000417c662,
  name: 'Carlo Jacobs',
  email: '<a href="/cdn-cgi/l/email-protection" class="__cf_email__" data-cfemail="f69597849a999c9795999485cfc7b6919b979f9ad895999b">[email protected]</a>',
  password: '$2a$10$fiIosS4Jo5ehuCp3TfltSOnpypPMWSMvzlb7phRWmNGBtDz5W1rCG',
  __v: 0 }

The issue is that END gets printed before the student details. It seems like the for loop is asynchronous. How can I adjust it to be synchronous?

Thank you in advance!

javascript mongodb express

Answer 1

Answer №1

This proposed code snippet aims to accomplish the following task:

const idsOfStudents = [];
const iterateOverRecords = function (records, callback) {
  if (!records.length) return callback();
  
  const email = records.shift();
  
  User.findOne({"email": email}, (error, student) => {
    if (error) {
      console.log("ERROR: " + error);
    }
    
    if (student) {
      const studentId = student._id;
      idsOfStudents.push(studentId);
      console.log("STUDENT: " + student);
    }
    
    return iterateOverRecords(records, callback);
  });
}

iterateOverRecords(emailsOfStudents, function () {
  console.log('COMPLETED');
});

Answer 2

This proposed code snippet aims to accomplish the following task:

const idsOfStudents = [];
const iterateOverRecords = function (records, callback) {
  if (!records.length) return callback();
  
  const email = records.shift();
  
  User.findOne({"email": email}, (error, student) => {
    if (error) {
      console.log("ERROR: " + error);
    }
    
    if (student) {
      const studentId = student._id;
      idsOfStudents.push(studentId);
      console.log("STUDENT: " + student);
    }
    
    return iterateOverRecords(records, callback);
  });
}

iterateOverRecords(emailsOfStudents, function () {
  console.log('COMPLETED');
});

Answer 3

Answer №2

When dealing with asynchronous tasks in JavaScript, it's essential to understand that the `for` loop itself may not be the culprit. In fact, it is often the `User.findOne` function call that introduces asynchrony into your code.

To address this issue, one approach recommended in this insightful response involves leveraging a tool like streamline.js to synchronize such calls. Below is a snippet demonstrating how you can adapt your code for synchronous execution:

var result = User.findOne({"email": email}, _);
if (result === null) {
      console.log("An error occurred: " + err);
}

var id = result._id;
studentIds.push(id);
console.log("STUDENT INFO: " + result);

Answer 4

When dealing with asynchronous tasks in JavaScript, it's essential to understand that the `for` loop itself may not be the culprit. In fact, it is often the `User.findOne` function call that introduces asynchrony into your code.

To address this issue, one approach recommended in this insightful response involves leveraging a tool like streamline.js to synchronize such calls. Below is a snippet demonstrating how you can adapt your code for synchronous execution:

var result = User.findOne({"email": email}, _);
if (result === null) {
      console.log("An error occurred: " + err);
}

var id = result._id;
studentIds.push(id);
console.log("STUDENT INFO: " + result);

Troubleshooting problem with MongoDB queries within a for loop

Answer №1

Answer №2

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