mongoose query without immediate callback

mongooosefindquery.js

(function() {
  "use strict";
  const util = require('util');
  const mongoose = require('mongoose');

  mongoose.connect('mongodb://localhost/chuchu');
  const CardSchema = mongoose.Schema({ 
    "rank" : String,
    "suit" : String,
  });

  const Card = mongoose.model("Card", CardSchema);

  let c1, c2, c3, c4;
  Card.remove().then( () => {
    c1 = new Card({"rank":"ace", "suit":"spades ♠"   });
    c2 = new Card({"rank":"2",   "suit":"hearts ♥"   });
    c3 = new Card({"rank":"3",   "suit":"clubs ♣"    });
    c4 = new Card({"rank":"4",   "suit":"diamonds ♦" });


    let p1 = c1.save(function (err) {
      if (err) { console.log(`Hubieron errores:\n${err}`); return err; }
    });

    let p2 = c2.save(function (err) {
      if (err) { console.log(`Hubieron errores:\n${err}`); return err; }
    });

    let p3 = Card.create(c3, function (err, x) {
      if (err) { console.log(`Hubieron errores:\n${err}`); return err; }
    });

    let p4 = Promise.all([p1, p2, p3]).then( (value) => { 
      let query = Card.find({suit: /spa|hea/});
      query.select('rank suit');
      query.exec((err, cards) => { 
        if (err) { console.log("ERROR: " + err); }
        let c = 1;
        cards.forEach(function (card) {
          console.log (`${c++}: ${card.rank} of ${card.suit}`);
        }); 
      }).then( function() { mongoose.connection.close(); } );  
    });
  });
})();

In mongoose 4, a Query has a .then() function, and thus can be used as a promise.

Any model method which involves specifying query conditions can be executed two ways:

When a callback function:

• is passed, the operation will be executed immediately with the results passed to the callback.
• is not passed, an instance of Query is returned, which provides a special query builder interface.