Are JavaScript ES6 Classes of any use with asynchronous code bases?

Solution 1:

Can I do async constructor()

No, that's a syntax error - just like constructor* (). A constructor is a method that doesn't return anything (no promise, no generator), it only initialises the instance.

And, if not how should a constructor work that does this

Such a constructor should not exist at all, see Is it bad practice to have a constructor function return a Promise?

Can ES6 classes support any form of asynchrony that operates on the object's state? Or, are they only for purely synchronous code bases?

Yes, you can use asynchronous methods (even with the proposed async syntax) on classes, and getters can return promises as well.

However, you will need to decide what should happen when a method is called while some asynchronous process is still active. If you want it to sequence all your operations, you should store your instance's state inside a promise for the end of that sequence that you can chain onto. Or, if you want to allow parallel operations, the best approach is to make your instances immutable and return a promise for another instance.

Solution 2:

Another way that Classes can be useful for arranging asynchronous tasks is with the exclusive use of static methods.

class Organizer {
    static async foo() {
        const data = await this.bar();
        data.key = value;
        return data;
    }
    static async bar() {
        return {foo:1, bar:2}
    }
};

Organizer.foo();

Of course, this is no different than creating a simple object literal, or a new file and including it, except you can more cleanly extend it.

Solution 3:

ECMAScript 2017 is intended to be classes of async methods.

Invoking another async or promise-returning function is a one-liner!

The highly expressive code reads without interruption top to bottom regardless of deferred execution

If you have callbacks, alternative error handlers, parallel execution or other unmet needs, instantiate promises in function body. It is better to have code in the function body rather than in a promise executor, and note that there is no try-catch wrapping callback code: do next-to-nothing there.

The async method can return a promise, a regular value, or throw

The callback apis that Node.js people used to love, we will now hate with a passion: they must all be wrapped in promises

The beauty of async/await is that errors bubble up implicitly

class MyClass {
  async doEverything() {
    const sumOfItAll = await http.scrapeTheInternet() +
      await new Promise((resolve, reject) =>
        http.asyncCallback((e, result) => !e ? resolve(result) : reject(e)))
    return this.resp = sumOfItAll
  }
}

If limited to ECMAScript 2015 and no async, return promise values:

class ES2015 {
  fetch(url) {
    return new Promise((resolve, reject) =>
      http.get(url, resolve).on('error', reject))
      .then(resp => this.resp = resp) // plain ECMAScript stores result
      .catch(e => { // optional internal error handler
        console.error(e.message)
        throw e // if errors should propagate
      })
  }
}

This ECMAScript 2015 version is what you are really asking about, any desired behavior can be coded up using the returned promise construct.

If you really, really want to execute promises in the constructor, it is a good idea to pass in then-catch functions or provide some callback construct so that consumers can take action on promise fulfillment or rejection. In the constructor, it is also good practice to wait for nextTick/.then before doing real work.

Every promise needs a final catch or there will be trouble