mongodb for loop insert update

This for loop is working and updating all records using mongodb loop and the example as given below.

//Find Items from collection.

var items = db.collection.find(),  i = 0;

//For-each loop execution.

items.forEach(function(item){

    db.collection.update(

            {_id: item._id},

             {$set:{new_field:i}

    });

   $i++;

});

For more detail, go to link

https://docs.mongodb.com/manual/tutorial/query-documents/#read-operations-cursors