Is it bad practice to have a constructor function return a Promise?

Question

Yes, it is a bad practise. A constructor should return an instance of its class, nothing else. It would mess up the new operator and inheritance otherwise.

Moreover, a constructor should only create and initialize a new instance. It should set up data structures and all instance-specific properties, but not execute any tasks. It should be a pure function without side effects if possible, with all the benefits that has.

What if I want to execute things from my constructor?

That should go in a method of your class. You want to mutate global state? Then call that procedure explicitly, not as a side effect of generating an object. This call can go right after the instantiation:

var engine = new Engine()
engine.displayPosts();

If that task is asynchronous, you can now easily return a promise for its results from the method, to easily wait until it is finished.
I would however not recommend this pattern when the method (asynchronously) mutates the instance and other methods depend on that, as that would lead to them being required to wait (become async even if they're actually synchronous) and you'd quickly have some internal queue management going on. Do not code instances to exist but be actually unusable.

What if I want to load data into my instance asynchronously?

Ask yourself: Do you actually need the instance without the data? Could you use it somehow?

If the answer to that is No, then you should not create it before you have the data. Make the data ifself a parameter to your constructor, instead of telling the constructor how to fetch the data (or passing a promise for the data).

Then, use a static method to load the data, from which you return a promise. Then chain a call that wraps the data in a new instance on that:

Engine.load({path: '/path/to/posts'}).then(function(posts) {
    new Engine(posts).displayPosts();
});

This allows much greater flexibility in the ways to acquire the data, and simplifies the constructor a lot. Similarly, you might write static factory functions that return promises for Engine instances:

Engine.fromPosts = function(options) {
    return ajax(options.path).then(Engine.parsePosts).then(function(posts) {
        return new Engine(posts, options);
    });
};

…

Engine.fromPosts({path: '/path/to/posts'}).then(function(engine) {
    engine.registerWith(framework).then(function(framePage) {
        engine.showPostsOn(framePage);
    });
});

I encountered the same problem and came up with this simple solution.

Instead of returning a Promise from the constructor, put it in this.initialization property, like this:

function Engine(path) {
  var engine = this
  engine.initialization = Promise.resolve()
    .then(function () {
      return doSomethingAsync(path)
    })
    .then(function (result) {
      engine.resultOfAsyncOp = result
    })
}

Then, wrap every method in a callback that runs after the initialization, like that:

Engine.prototype.showPostsOnPage = function () {
  return this.initialization.then(function () {
    // actual body of the method
  })
}

How it looks from the API consumer perspective:

engine = new Engine({path: '/path/to/posts'})
engine.showPostsOnPage()

This works because you can register multiple callbacks to a promise and they run either after it resolves or, if it's already resolved, at the time of attaching the callback.

This is how mongoskin works, except it doesn't actually use promises.

---

Edit: Since I wrote that reply I've fallen in love with ES6/7 syntax so there's another example using that. You can use it today with babel.

class Engine {

  constructor(path) {
    this._initialized = this._initialize()
  }

  async _initialize() {
    // actual async constructor logic
  }

  async showPostsOnPage() {
    await this._initialized
    // actual body of the method
  }

}

Edit: You can use this pattern natively with node 7 and --harmony flag!

To avoid the separation of concerns, use a factory to create the object.

class Engine {
    constructor(data) {
        this.data = data;
    }

    static makeEngine(pathToData) {
        return new Promise((resolve, reject) => {
            getData(pathToData).then(data => {
              resolve(new Engine(data))
            }).catch(reject);
        });
    }
}

The return value from the constructor replaces the object that the new operator just produced, so returning a promise is not a good idea. Previously, an explicit return value from the constructor was used for the singleton pattern.

The better way in ECMAScript 2017 is to use a static methods: you have one process, which is the numerality of static.

Which method to be run on the new object after the constructor may be known only to the class itself. To encapsulate this inside the class, you can use process.nextTick or Promise.resolve, postponing further execution allowing for listeners to be added and other things in Process.launch, the invoker of the constructor.

Since almost all code executes inside of a Promise, errors will end up in Process.fatal

This basic idea can be modified to fit specific encapsulation needs.

class MyClass {
  constructor(o) {
    if (o == null) o = false
    if (o.run) Promise.resolve()
      .then(() => this.method())
      .then(o.exit).catch(o.reject)
  }

  async method() {}
}

class Process {
  static launch(construct) {
    return new Promise(r => r(
      new construct({run: true, exit: Process.exit, reject: Process.fatal})
    )).catch(Process.fatal)
  }

  static exit() {
    process.exit()
  }

  static fatal(e) {
    console.error(e.message)
    process.exit(1)
  }
}

Process.launch(MyClass)