async constructor functions in TypeScript?

Question

I have some setup I want during a constructor, but it seems that is not allowed

Which means I can\'t use:

How else should I do this?

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Answer 1

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);
   };
}