How to use explain() in Node.js mongoose

I am trying to see The executionStats on my console but I always get undefined or [object, object]. I just have a simple query where I want to see some stats. Can someone explain why this isn’t working? If I just use my query for querying (while not using explain()) the query is successful.

This is my query when trying to use explain with all the other code. I want to see the performance of the query so I found out that the best way to get the performance time is by using explain().

  const timeFunction2 = new Promise((resolve, reject) => {
    var startTime = performance.now();

    setTimeout(() => {

      conn.collection('galery').find({ "user_id": req.session.userId}).explain("executionStats", (err, explain) => {
        console.log('MongoDebug: ' + explain[0]);
      });

      var endTime = performance.now();
      resolve(endTime - startTime);
    });
  });

  timeFunction2.then(time => {
    console.log(`${time} ms.`);
  });

And this is my query code where I can sucssessfuly get data and show them on my page

  const timeFunction2 = new Promise((resolve, reject) => {
    var startTime = performance.now();

    setTimeout(() => {


conn.collection('galery').find({ "user_id": req.session.userId}).toArray( (err, resultImg) =>{
.
.
.
});

      var endTime = performance.now();
      resolve(endTime - startTime);
    });
  });

  timeFunction2.then(time => {
    console.log(`${time} ms.`);
  });

>Solution :

You need to change how you are logging the explain object to console.

      conn.collection('galery').find({ "user_id": req.session.userId}).explain("executionStats", (err, explain) => {
        console.log('MongoDebug: ' + explain[0]);
      });

By using 'MongoDebug: ' + explain[0] you are implicitly calling .toString() on explain[0] as you are appending it to a string, so it must be a string. For objects, this converts them to [object, Object].

Instead you can pass it as a separate object to your console.log:

console.log('MongoDebug: ', explain[0]);

Or convert it to JSON with extra spacing for better readability:

console.log(JSON.stringify(explain[0], null, 2));