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

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;    }}

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();