For nearly all mongoose queries, the provided callback function will be called with two arguments in the node callback pattern - callback(err, results) as stated in the documentation.

I want to know when the err part of the callback will be executed.

Suppose below query is executing in a loop and in case of callback err, I am coming out of loop. Now, if it will not find any document with the mentioned conditions in any one iteration of loop, then err part will be executed or it will be executed only in case of some run time error only. I mean the next iteration will continue or not??

Model.findOneAndUpdate([conditions], [update], [options], [callback])

Thanks in advance.