Async/Await Class Constructor

Async/Await Class Constructor

This can never work.

The async keyword allows await to be used in a function marked as async but it also converts that function into a promise generator. So a function marked with async will return a promise. A constructor on the other hand returns the object it is constructing. Thus we have a situation where you want to both return an object and a promise: an impossible situation.

You can only use async/await where you can use promises because they are essentially syntax sugar for promises. You can't use promises in a constructor because a constructor must return the object to be constructed, not a promise.

There are two design patterns to overcome this, both invented before promises were around.

1. Use of an init() function. This works a bit like jQuery's .ready(). The object you create can only be used inside it's own init or ready function:

   Usage:

   var myObj = new myClass();
   myObj.init(function() {
       // inside here you can use myObj
   });
   

   Implementation:

   class myClass {
       constructor () {
   
       }
   
       init (callback) {
           // do something async and call the callback:
           callback.bind(this)();
       }
   }
   

2. Use a builder. I've not seen this used much in javascript but this is one of the more common work-arounds in Java when an object needs to be constructed asynchronously. Of course, the builder pattern is used when constructing an object that requires a lot of complicated parameters. Which is exactly the use-case for asynchronous builders. The difference is that an async builder does not return an object but a promise of that object:

   Usage:

   myClass.build().then(function(myObj) {
       // myObj is returned by the promise, 
       // not by the constructor
       // or builder
   });
   
   // with async/await:
   
   async function foo () {
       var myObj = await myClass.build();
   }
   

   Implementation:

   class myClass {
       constructor (async_param) {
           if (typeof async_param === 'undefined') {
               throw new Error('Cannot be called directly');
           }
       }
   
       static build () {
           return doSomeAsyncStuff()
              .then(function(async_result){
                  return new myClass(async_result);
              });
       }
   }
   

   Implementation with async/await:

   class myClass {
       constructor (async_param) {
           if (typeof async_param === 'undefined') {
               throw new Error('Cannot be called directly');
           }
       }
   
       static async build () {
           var async_result = await doSomeAsyncStuff();
           return new myClass(async_result);
       }
   }
   

Note: although in the examples above we use promises for the async builder they are not strictly speaking necessary. You can just as easily write a builder that accept a callback.

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Note on calling functions inside static functions.

This has nothing whatsoever to do with async constructors but with what the keyword this actually mean (which may be a bit surprising to people coming from languages that do auto-resolution of method names, that is, languages that don't need the this keyword).

The this keyword refers to the instantiated object. Not the class. Therefore you cannot normally use this inside static functions since the static function is not bound to any object but is bound directly to the class.

That is to say, in the following code:

class A {
    static foo () {}
}

You cannot do:

var a = new A();
a.foo() // NOPE!!

instead you need to call it as:

A.foo();

Therefore, the following code would result in an error:

class A {
    static foo () {
        this.bar(); // you are calling this as static
                    // so bar is undefinned
    }
    bar () {}
}

To fix it you can make bar either a regular function or a static method:

function bar1 () {}

class A {
    static foo () {
        bar1();   // this is OK
        A.bar2(); // this is OK
    }

    static bar2 () {}
}

Can async/await be used in constructors?

Without trying to fortune-tell about future decisions, let's concentrate on practicality and what is already known.

ES7, like ES6 before it will try to be a backwards compatible expansion to the language. With that in mind, a backwards compatible constructor function is essentially a regular function (with some runtime restrictions) that's meant to be invoked with the new keyword. When that happens, the function's return value gets special treatment, specifically, non-object return values are ignored and the newly allocated object is returned while object return values are returned as is (and the newly allocated object is thrown away). With that, your code would result in a promise being returned and no "object construction" would take place. I don't see the practicality of this and I suppose if anybody takes the time to find what to do with such code it will be rejected.

Can constructors be async?

Constructor acts very similarly to a method returning the constructed type. And async method can't return just any type, it has to be either “fire and forget” void, or Task.

If the constructor of type T actually returned Task<T>, that would be very confusing, I think.

If the async constructor behaved the same way as an async void method, that kind of breaks what constructor is meant to be. After constructor returns, you should get a fully initialized object. Not an object that will be actually properly initialized at some undefined point in the future. That is, if you're lucky and the async initialization doesn't fail.

All this is just a guess. But it seems to me that having the possibility of an async constructor brings more trouble than it's worth.

If you actually want the “fire and forget” semantics of async void methods (which should be avoided, if possible), you can easily encapsulate all the code in an async void method and call that from your constructor, as you mentioned in the question.

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:

1. Make your public setup async.

2. Do not call it from the constructor.

3. 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);
       }
   }
   

How to await an async function in the constructor in JavaScript

• Constructors must return an instance of the class being constructed
• Async functions must return a promise

Since a function can only do one of those things, a constructor function can't be async.

Helper Function

You can use a helper function:

// index.js
async loadWasm() {
   // load wasm file
}

export class MyClass {
  constructor(wasm) {
    this.val = wasm;
  }
}

async function myClassFactory() {
    const wasm = await loadWasm();
    const instance = new MyClass(wasm);
    return instance;
}

You'll need to await the result of calling myClassFactory().

Store a promise in the object

You could also store a promise in the object you create:

// index.js
async loadWasm() {
   // load wasm file
}

export class MyClass {
  constructor() {
    const wasmPromise = loadWasm();
    this.val = wasmPromise;
  }

  async doSomethingWithValue() {
    const wasm = await this.val;
    console.log(wasm);
  }
}

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