async constructor functions in TypeScript?

A constructor must return an instance of the class it 'constructs'. Therefore, it's not possible to return Promise<...> and await for it.

You can:

Make your public setup async. Do not call it from the constructor. Call it whenever you want to 'finalize' object construction. async function run() { let topic; debug("new TopicsModel"); try { topic = new TopicsModel(); await topic.setup(); } catch (err) { debug("err", err); } }

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Readiness design pattern

Don't put the object in a promise, put a promise in the object.

Readiness is a property of the object. So make it a property of the object.

The awaitable initialise method described in the accepted answer has a serious limitation. Using await like this means only one block of code can be implicitly contingent on the object being ready. This is fine for code with guaranteed linear execution but in multi-threaded or event driven code it's untenable.

You could capture the task/promise and await that, but how do you manage making this available to every context that depends on it?

The problem is more tractable when correctly framed. The objective is not to wait on construction but to wait on readiness of the constructed object. These are two completely different things. It is even possible for something like a database connection object to be in a ready state, go back to a non-ready state, then become ready again.

How can we determine readiness if it depends on activities that may not be complete when the constructor returns? Quite obviously readiness is a property of the object. Many frameworks directly express the notion of readiness. In JavaScript we have the Promise, and in C# we have the Task. Both have direct language support for object properties.

Expose the construction completion promise as a property of the constructed object. When the asynchronous part of your construction finishes it should resolve the promise.

It doesn't matter whether .then(...) executes before or after the promise resolves. The promise specification states that invoking then on an already resolved promised simply executes the handler immediately.

class Foo {
  public Ready: Promise.IThenable<any>;
  constructor() {
    ...
    this.Ready = new Promise((resolve, reject) => {
      $.ajax(...).then(result => {
        // use result
        resolve(undefined);
      }).fail(reject);
    });
  }
}

var foo = new Foo();
foo.Ready.then(() => {
  // do stuff that needs foo to be ready, eg apply bindings
});
// keep going with other stuff that doesn't need to wait for foo

// using await
// code that doesn't need foo to be ready
await foo.Ready;
// code that needs foo to be ready

Why resolve(undefined); instead of resolve();? Because ES6. Adjust as required to suit your target.

From the peanut gallery

Using await

In a comment it has been suggested that I should have framed this solution with await to more directly address the question as asked.

You can use await with the Ready property as shown in the example above. I'm not a big fan of await because it requires you to partition your code by dependency. You have to put all the dependent code after await and all the independent code before it. This can obscure the intent of the code.

I encourage people to think in terms of call-backs. Mentally framing the problem like this is more compatible with languages like C. Promises are arguably descended from the pattern used for IO completion.

Lack of enforcement as compared to factory pattern

One punter thinks this pattern "is a bad idea because without a factory function, there's nothing to enforce the invariant of checking the readiness. It's left to the clients, which you can practically guarantee will mess up from time to time."

How will he stop people from building factory methods that don't enforce the check? Where do you draw the line? The answer is you learn the difference between domain specific code and framework code and apply different standards, seasoned with some common sense: would you forbid the division operator because there's nothing stopping people from passing a zero divisor?

This is original work by me. I devised this design pattern because I was unsatisfied with external factories and other such workarounds. Despite searching for some time, I found no prior art for my solution, so I'm claiming credit as the originator of this pattern until disputed.

In 2020 I discovered that in 2013 Stephen Cleary posted a very similar solution to the problem. Looking back through my own work the first vestiges of this approach appear in code I worked on around the same time. I suspect Cleary put it all together first but he didn't formalise it as a design pattern or publish it where it would be easily found by others with the problem. Moreover, Cleary deals only with construction which is only one application of the Readiness pattern (see below).

Summary

The pattern is

put a promise in the object it describes

expose it as a property named Ready

always reference the promise via the Ready property (don't capture it in a client code variable)

This establishes clear simple semantics and guarantees that

the promise will be created and managed

the promise has identical scope to the object it describes

the semantics of readiness dependence are conspicuous and clear in client code

if the promise is replaced (eg a connection goes unready then ready again due to network conditions) client code referring to it via thing.Ready will always use the current promise

This last one is a nightmare until you use the pattern and let the object manage its own promise. It's also a very good reason to refrain from capturing the promise into a variable.

Some objects have methods that temporarily put them in an invalid condition, and the pattern can serve in that scenario without modification. Code of the form obj.Ready.then(...) will always use whatever promise property is returned by the Ready property, so whenever some action is about to invalidate object state, a fresh promise can be created.

Closing notes

The Readiness pattern isn't specific to construction. It is easily applied to construction but it's really about ensuring that state dependencies are met. In these days of asynchronous code you need a system, and the simple declarative semantics of a promise make it straightforward to express the idea that an action should be taken ASAP, with emphasis on possible. Once you start framing things in these terms, arguments about long running methods or constructors become moot.

Deferred initialisation still has its place; as I mentioned you can combine Readiness with lazy load. But if chances are that you won't use the object, then why create it early? It might be better to create on demand.

There are other solutions. When I write embedded software I create everything up front including resource pools. This makes leaks impossible and memory demands are known at compile time. But that's only a solution for a small closed problem space.

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Use an asynchronous factory method instead.

class MyClass {
   private mMember: Something;

   constructor() {
      this.mMember = await SomeFunctionAsync(); // error
   }
}

Becomes:

class MyClass {
   private mMember: Something;

   // make private if possible; I can't in TS 1.8
   constructor() {
   }

   public static CreateAsync = async () => {
      const me = new MyClass();
      
      me.mMember = await SomeFunctionAsync();

      return me;
   };
}

This will mean that you will have to await the construction of these kinds of objects, but that should already be implied by the fact that you are in the situation where you have to await something to construct them anyway.

There's another thing you can do but I suspect it's not a good idea:

// probably BAD
class MyClass {
   private mMember: Something;

   constructor() {
      this.LoadAsync();
   }

   private LoadAsync = async () => {
      this.mMember = await SomeFunctionAsync();
   };
}

This can work and I've never had an actual problem from it before, but it seems to be dangerous to me, since your object will not actually be fully initialized when you start using it.

Another way to do it, which might be better than the first option in some ways, is to await the parts, and then construct your object after:

export class MyClass {
   private constructor(
      private readonly mSomething: Something,
      private readonly mSomethingElse: SomethingElse
   ) {
   }

   public static CreateAsync = async () => {
      const something = await SomeFunctionAsync();
      const somethingElse = await SomeOtherFunctionAsync();

      return new MyClass(something, somethingElse);
   };
}

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I've found a solution that looks like

export class SomeClass {
  private initialization;

  // Implement async constructor
  constructor() {
    this.initialization = this.init();
  }

  async init() {
    await someAsyncCall();
  }

  async fooMethod() {
    await this.initialization();
    // ...some other stuff
  }

  async barMethod() {
    await this.initialization();
    // ...some other stuff
  }

It works because Promises that powers async/await, can be resolved multiple times with the same value.

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I know it's quite old but another option is to have a factory that will create the object and wait for its initialization:

// Declare the class
class A {

  // Declare class constructor
  constructor() {

    // We didn't finish the async job yet
    this.initialized = false;

    // Simulates async job, it takes 5 seconds to have it done
    setTimeout(() => {
      this.initialized = true;
    }, 5000);
  }

  // do something usefull here - thats a normal method
  useful() {
    // but only if initialization was OK
    if (this.initialized) {
      console.log("I am doing something useful here")

    // otherwise throw an error which will be caught by the promise catch
    } else {
      throw new Error("I am not initialized!");
    }
  }

}

// factory for common, extensible class - that's the reason for the constructor parameter
// it can be more sophisticated and accept also params for constructor and pass them there
// also, the timeout is just an example, it will wait for about 10s (1000 x 10ms iterations
function factory(construct) {

  // create a promise
  var aPromise = new Promise(
    function(resolve, reject) {

      // construct the object here
      var a = new construct();

      // setup simple timeout
      var timeout = 1000;

      // called in 10ms intervals to check if the object is initialized
      function waiter() {
    
        if (a.initialized) {
          // if initialized, resolve the promise
          resolve(a);
        } else {

          // check for timeout - do another iteration after 10ms or throw exception
          if (timeout > 0) {     
            timeout--;
            setTimeout(waiter, 10);            
          } else {            
            throw new Error("Timeout!");            
          }

        }
      }
  
      // call the waiter, it will return almost immediately
      waiter();
    }
  );

  // return promise of the object being created and initialized
  return a Promise;
}


// this is some async function to create object of A class and do something with it
async function createObjectAndDoSomethingUseful() {

  // try/catch to capture exceptions during async execution
  try {
    // create object and wait until its initialized (promise resolved)
    var a = await factory(A);
    // then do something usefull
    a.useful();
  } catch(e) {
    // if class instantiation failed from whatever reason, timeout occured or useful was called before the object finished its initialization
    console.error(e);
  }

}

// now, perform the action we want
createObjectAndDoSomethingUsefull();

// spaghetti code is done here, but async probably still runs

---

Use a private constructor and a static factory method FTW. It is the best way to enforce any validation logic or data enrichment, encapsulated away from a client.

class Topic {
  public static async create(id: string): Promise<Topic> {
    const topic = new Topic(id);
    await topic.populate();
    return topic;
  }

  private constructor(private id: string) {
    // ...
  }

  private async populate(): Promise<void> {
    // Do something async. Access `this.id` and any other instance fields
  }
}

// To instantiate a Topic
const topic = await Topic.create();

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Use a setup async method that returns the instance

I had a similar problem in the following case: how to instanciate a 'Foo' class either with an instance of a 'FooSession' class or with a 'fooSessionParams' object, knowing that creating a fooSession from a fooSessionParams object is an async function? I wanted to instanciate either by doing:

let foo = new Foo(fooSession);

or

let foo = await new Foo(fooSessionParams);

and did'nt want a factory because the two usages would have been too different. But as we know, we can not return a promise from a constructor (and the return signature is different). I solved it this way:

class Foo {
    private fooSession: FooSession;

    constructor(fooSession?: FooSession) {
        if (fooSession) {
            this.fooSession = fooSession;
        }
    }

    async setup(fooSessionParams: FooSessionParams): Promise<Foo> {
        this.fooSession = await getAFooSession(fooSessionParams);
        return this;
    }
}

The interesting part is where the setup async method returns the instance itself. Then if I have a 'FooSession' instance I can use it this way:

let foo = new Foo(fooSession);

And if I have no 'FooSession' instance I can setup 'foo' in one of these ways:

let foo = await new Foo().setup(fooSessionParams);

(witch is my prefered way because it is close to what I wanted first) or

let foo = new Foo();
await foo.setup(fooSessionParams);

As an alternative I could also add the static method:

    static async getASession(fooSessionParams: FooSessionParams): FooSession {
        let fooSession: FooSession = await getAFooSession(fooSessionParams);
        return fooSession;
    }

and instanciate this way:

let foo = new Foo(await Foo.getASession(fooSessionParams));

It is mainly a question of style…

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You may elect to leave the await out of the equation altogether. You can call it from the constructor if you need to. The caveat being that you need to deal with any return values in the setup/initialise function, not in the constructor.

this works for me, using angular 1.6.3.

import { module } from "angular";
import * as R from "ramda";
import cs = require("./checkListService");

export class CheckListController {

    static $inject = ["$log", "$location", "ICheckListService"];
    checkListId: string;

    constructor(
        public $log: ng.ILogService,
        public $loc: ng.ILocationService,
        public checkListService: cs.ICheckListService) {
        this.initialise();
    }

    /**
     * initialise the controller component.
     */
    async initialise() {
        try {
            var list = await this.checkListService.loadCheckLists();
            this.checkListId = R.head(list).id.toString();
            this.$log.info(`set check list id to ${this.checkListId}`);
         } catch (error) {
            // deal with problems here.
         }
    }
}

module("app").controller("checkListController", CheckListController)

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Create holder for promise status:

class MyClass {
    constructor(){
        this.#fetchResolved = this.fetch()
    }
    #fetchResolved: Promise<void>;
    fetch = async (): Promise<void> => {
        return new Promise(resolve => resolve()) // save data to class property or simply add it by resolve() to #fetchResolved reference
    }
    isConstructorDone = async (): boolean => {
        await this.#fetchResolved;
        return true; // or any other data depending on constructor finish the job
    }
}

To use:

const data = new MyClass();
const field = await data.isConstructorDone();

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Or you can just stick to the true ASYNC model and not overcomplicate the setup. 9 out of 10 times this comes down to asynchronous versus synchronous design. For example I have a React component that needed this very same thing were I was initializing the state variables in a promise callback in the constructor. Turns out that all I needed to do to get around the null data exception was just setup an empty state object then set it in the async callback. For example here's a Firebase read with a returned promise and callback:

        this._firebaseService = new FirebaseService();
        this.state = {data: [], latestAuthor: '', latestComment: ''};

        this._firebaseService.read("/comments")
        .then((data) => {
            const dataObj = data.val();
            const fetchedComments = dataObj.map((e: any) => {
                return {author: e.author, text: e.text}
            });

            this.state = {data: fetchedComments, latestAuthor: '', latestComment: ''};

        });

By taking this approach my code maintains it's AJAX behavior without compromising the component with a null exception because the state is setup with defaults (empty object and empty strings) prior to the callback. The user may see an empty list for a second but then it's quickly populated. Better yet would be to apply a spinner while the data loads up. Oftentimes I hear of individuals suggesting overly complicated work arounds as is the case in this post but the original flow should be re-examined.

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