MongoDB Function clarification sync insert with auto increment - mongodb

Question

In the following link from mongoDB documentation:
https://docs.mongodb.org/manual/tutorial/create-an-auto-incrementing-field/
there is an explanation on how to build a "safe" insert with auto-incremental _id.
My questions are:
Where should i define the function at?
How can i call it later on?
I couldn't quit understand if the function is stored as a JS function is a JS file? or is it stored in the DB? or something else.
Thanks.
Here is the actual function as it is brought in the link above:
function insertDocument(doc, targetCollection) {
while (1) {
var cursor = targetCollection.find( {}, { _id: 1 } ).sort( { _id: -1 } ).limit(1);
var seq = cursor.hasNext() ? cursor.next()._id + 1 : 1;
doc._id = seq;
var results = targetCollection.insert(doc);
if( results.hasWriteError() ) {
if( results.writeError.code == 11000 /* dup key */ )
continue;
else
print( "unexpected error inserting data: " + tojson( results ) );
}
break;
}
}

Answer 1

Implementation approach
Here's an example of how to implement an auto-increment field with mongoose:
var CounterSchema = Schema({
_id: {type: String, required: true},
seq: { type: Number, default: 0 }
});
var counter = mongoose.model('counter', CounterSchema);
var entitySchema = mongoose.Schema({
testvalue: {type: String}
});
entitySchema.pre('save', function(next) {
var doc = this;
counter.findByIdAndUpdate({_id: 'entityId'}, {$inc: { seq: 1} }, function(error, counter) {
if(error)
return next(error);
doc.testvalue = counter.seq;
next();
});
});
Source: https://stackoverflow.com/a/30164636/236660
Please note that {_id: 'entityId'} should be set differently per collection you're using this code with. So if you're generating an ID for entitySchema, you'll have {_id: 'entityId'}. For userSchema you would use {_id: 'userId'}, etc. You need to do this so that every collection has its own auto-incremented sequence.
The code above is thread safe. The findByIdAndUpdate operation is atomic; its consistency and concurrency are handled by MongoDB.
Existing module
Also, there is a mongoose module created specifically for handling auto-incremented IDs. You may actually be better off using it:
https://www.npmjs.com/package/mongoose-auto-increment