TypeScript obtain type for class instance

Question

I'm using a library that exposes an SDK, but not the type (UserModule) of one of the objects (user) you can create with it.

import { Magic } from 'magic-sdk';

// ...

function useMagicLinkAuthentication() {
  const [magicReady, setMagicReady] = usePromise<Magic>();
  const [isMagicInitialized, setMagicInitialized] = useState(false);
  const [user, setUser] = useState<UserModule | null>(null);

  // ...

  const signIn = async (email: string) => {
    const magic = await magicReady;

    if (await magic.user.isLoggedIn()) {
      return setUser(magic.user);
    }

    await magic.auth.loginWithMagicLink({ email });
    return setUser(magic.user);
  };

  // ...
}

As you can see, I need access to the UserModule in useState which I have no access to (because I can't do import { UserModule } from 'magic-sdk';.

For functions there is ReturnType. So if I have a regular function, I can do:

export const createFoo = () => { /* ... returns a Foo ... */ } 
export type Foo = ReturnType<typeof createFoo>;

Is there an equivalent for classes in TypeScript so that I can access the instance of new MyClass and get

// broken pseudo-code
export type Foo  = Something<typeof MyClass.foo>

?

I need to somehow get typeof magic.user aka the UserModule type.

FYI there is a built-in utility type InstanceType: typescriptlang.org/docs/handbook/… But it sounds like that's not actually what you need here. — Linda Paiste
– Linda Paiste, Commented Jan 31, 2021 at 3:12

J. Hesters · Accepted Answer · 2020-12-08 20:39:39Z

1

You can use a type alias from the constructor:

type UserModule = Magic['user'];

answered Dec 8, 2020 at 20:39

J. Hesters

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