Built-in Promises

Mongoose async operations, like .save() and queries, return thenables. This means that you can do things like MyModel.findOne({}).then() and await MyModel.findOne({}).exec() if you're using async/await.

You can find the return type of specific operations in the api docs

var gnr = new Band({
  name: "Guns N' Roses",
  members: ['Axl', 'Slash']
});

var promise = gnr.save();
assert.ok(promise instanceof Promise);

promise.then(function (doc) {
  assert.equal(doc.name, "Guns N' Roses");
});

Queries are not promises

Mongoose queries are not promises. They have a .then() function for co and async/await as a convenience. If you need a fully-fledged promise, use the .exec() function.

var query = Band.findOne({name: "Guns N' Roses"});
assert.ok(!(query instanceof Promise));

// A query is not a fully-fledged promise, but it does have a `.then()`.
query.then(function (doc) {
  // use doc
});

// `.exec()` gives you a fully-fledged promise
var promise = query.exec();
assert.ok(promise instanceof Promise);

promise.then(function (doc) {
  // use doc
});

Plugging in your own Promises Library

If you're an advanced user, you may want to plug in your own promise library like bluebird. Just set mongoose.Promise to your favorite ES6-style promise constructor and mongoose will use it.

var query = Band.findOne({name: "Guns N' Roses"});

// Use bluebird
mongoose.Promise = require('bluebird');
assert.equal(query.exec().constructor, require('bluebird'));

// Use q. Note that you **must** use `require('q').Promise`.
mongoose.Promise = require('q').Promise;
assert.ok(query.exec() instanceof require('q').makePromise);

Want to learn how to check whether your favorite npm modules work with async/await without cobbling together contradictory answers from Google and Stack Overflow? Chapter 4 of Mastering Async/Await explains the basic principles for determining whether frameworks like React and Mongoose support async/await. Get your copy!