I am trying to create a function that accepts extensions as arguments, and returns an object that is typed as the combination of all the extensions.

To better illustrate, here is a basic example:

const withNotification = () => ({ notify: (message: string): void => void 0 });
const withDialog = () => ({ dialog: (data: any): void => void 0 });

const compose = (...extensions: any[]): any => null!;

const composed = compose(withNotification(), withDialog());

// When writing the following, the signature should be recognized
composed.notify('OK'); // Works
composed.notify(0);    // Does not work (not a string)
composed.dialog({});  // Works
composed.dialog();    // Does not work (no parameter)

My approach so far is the following:

type Extension<T extends object> = { [key in keyof T]: T[key] };

const withNotification = () => ({ notify: (message: string): void => void 0 });
const withDialog = () => ({ dialog: (data: any): void => void 0 });

const compose = <T extends object>(...extensions: Extension<T>[]): T => null!;

const composed = compose(withNotification(), withDialog());

composed.notify('OK');
composed.notify(0);
composed.dialog({});
composed.dialog();

What happens is that calling notify is recognized, whereas dialog is not. I understand that this is because my T generic that is not “spreading” to all extensions, but I don’t know how to achieve that.

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