Unable to perform $merge aggregation in cron job in nodejs+mongodb

I tried to create a cron job that gets data from one collection and do some operations using aggregate method and finally dump the output to another collection in same database (using $merge ). If I try the code for aggregate in mongo shell, then the $merge happens successfully. But when it is done in a cron job, I could see that the aggregation is being performed, but the result is not getting dumped to the destined collection (which is supposed to be done by $merge ).

Here is the code I tried to implement

const CronJob = require('cron').CronJob;
const job = new CronJob( '* * * * *', () => {
     console.log('Cron Job started');
       some calculations
       { $merge: { into: { db: 'db-name', coll: 'collection-name' } } }
     ], (err, docs) => {
          if(!err && docs){
             console.log('Cron Job ended');
             console.log(docs);  //I'm getting the correct output for the docs


Cron Job started
Cron Job ended

Note: There is no issue with aggregate function as I have tested it in mongo shell. The issue happens when the aggregate function is performed in a cron job

Please help me with this one