Error: Type is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor

Question

I've written this function in TypeScript:

export class LoginService {

  async isLoggedIn(): boolean {
    const r = await this.http.get('http://localhost:3000/api/user/isLoggedIn').toPromise();
    return r.body;
  }
}

When I try to run the Angular 6 application, I'm getting this error message:

ERROR in src/app/login.service.ts(28,23): error TS1055: Type 'boolean' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value.

I've used async/await in other applications before without hitting this before.

UPDATE: The question I would like answered is: how do I get the "isLoggedIn" function to return a boolean?

Answer 1

An async function can ONLY return a promise by definition - all async functions return promises. It can't return a boolean.

That's what TypeScript is telling you. The async function can return a promise that resolves to a boolean.

The value that you return inside your async function becomes the resolved value of the promise that the async function returns. So, the return type for your async function is a promise (that resolves to a boolean).

The caller of isLoggedIn() will have to either use .then() with it or await with it.

export class LoginService {

    async isLoggedIn(): Promise<any> {
      const r = await this.http.get('http://localhost:3000/api/user/isLoggedIn').toPromise();
      return r.body;
    }

}

Answer 2

If your endpoint /api/user/isLoggedIn returns only a boolean value, you should just be able to use below by casting the http get method. But indeed you can only return a promise from an async function.

export class LoginService {
  async isLoggedIn(): Promise<boolean> {
    return this.http.get<boolean>('http://localhost:3000/api/user/isLoggedIn').toPromise();
  }
}

You could have an async function that consumes isLoggedIn() like this:

async doSomething() {
  const loggedIn: boolean = await service.isLoggedIn();
  if (loggedIn) {
    doThis();
  }    
}

Which would be called like this

await doSomething();