The return type of an async function must be the global Promise<T> type

Question

Why does TSlint still says "The return type of an async function or method must be the global Promise type"?

I can't understand what's wrong.

UPDATED:

Outpoint

I suppose success is not boolean type. Try to put double negation !! in the return before res... and let me know if the error is still there – Australasia

@MU, I tried this and explicit cast too – Yiyid

@MU, I think that tslint does not work correctly, but I don't know how to fix it – Yiyid

Did you try removing the : Promise<boolean> at all? It will implicitly type your method. It might also help you see what it is actually returning. – Imagination

@НикитаЛебедев Please make sure this returned type result.data.success is indeed of boolean type and not enquoted boolean which makes it a string, like "true" instead of true. Can you please check in the Network tab of the browser console, or put a breakpoint on the method? – Cns

For me it worked after removing async and also await from the function and changed the return type to false | Promise<boolean>. The function continued to work as before. – Kila

Answer 1

Try returning a Promise-wrapped value corresponding to the expected generic type of the Promise, something like so:

@Action
public async register(registerInfo: Account): Promise<boolean> {
  const res = await http.post('users/', registerInfo);

  return new Promise<boolean>((resolve, reject) => {
    resolve(res.data.success);

    // Or reject() if something wrong happened
  });

  // Or simply return a resolved Promise
  return Promise.resolve(res.data.success);
}

Actually, you should also be able to then() the result:

@Action
public async register(registerInfo: Account): Promise<boolean> {
  return await http
            .post('users/', registerInfo)
            .then(res => res.data.success);
}

Cns

Property 'resolve' does not exist on type '<T>(resolver: (resolve: (val?: T | PromiseLike<T> | undefined) => void, reject: (reason?: any) => void, notify: (progress: any) => void) => void) => Promise<T>' – Yiyid

Can you please update your question with this attempted code? – Cns

Answer 2

I think the problem is that you are trying to return the result of await instead of capturing the result and processing it:

@Action
public async register(registerInfo: Account): Promise<boolean> {
  const result = await http.post('users/', registerInfo);

  return result.data.success;
}

Imagination

Answer 3

The method http.post returns Observable type, you can convert it to Promise using toPromise() method.

Like - http.post.toPromise()

Alexaalexander

Answer 4

there can be 3 ways.

1. You can use generic type to avoid this.

2. you can check the success key before returning whether its boolean or not.

3. As you know the result from api response will be containing result.data.success as true/false boolean value but the typescript doesn't know that(thats why there is error while showing in typescript code) so you need to explicitly define the interface maybe for your api response, there you can make the success property as boolean.

Hardening