Using Mongoose query builders

Every Mongoose model has static helper methods to do several kinds of operations, such as retrieving a document. When a callback is passed to these helper methods, the operation is executed immediately:

      const user = await User.findOne({ 
          firstName: 'Jonh', 
          age: { $lte: 30 }, 
      }, (error, document) => { 
          if (error) return console.log(error) 
          console.log(document) 
      }) 

Otherwise, if there is no defined callback, a query builder interface is returned, which can be later executed:

      const user = User.findOne({ 
          firstName: 'Jonh', 
          age: { $lte: 30 }, 
      }) 
      user.exec((error, document) => { 
          if (error) return console.log(error) 
          console.log(document) 
      }) 

Queries also have a .then function which can be used as a...