Typescript: Invalid 'new' expression

Question

In an attempt to help this person out I was stubbing out the typescript definitions for the Ampersand.js library. Given the following typescript code, why does the compiler output an Invalid 'new' experession error?

[Here's a link to run the code using the typescript playground.]

declare module ampersand {
    interface AmpersandState {
        // todo...
    }

    interface AmpersandCollection {
        // todo...
    }

    interface ModelExtendOptions {
        parse?: boolean;
        parent?: AmpersandState;
        collection?: AmpersandCollection;
    }

    interface ModelSaveOptions {
        patch?: boolean;
    }

    class AmpersandModel<T> {
        constructor(attrs: T, options?: ModelExtendOptions);
        save: (attrs?: T, options?: ModelSaveOptions) => void;
        // todo:  fetch, destroy, sync, etc...
    }

    interface ExtendOptions {
        props?: {};
        session?: {};
        derived?: {};
    }   

    interface AmpersandModelStatic {
        extend: <T>(options: ExtendOptions) => AmpersandModel<T>;
    }
} 

declare var AmpersandModel: ampersand.AmpersandModelStatic;


interface PersonProps {
    firstName: string;
    lastName: string;
}

var Person = AmpersandModel.extend<PersonProps>({ props: { firstName: 'string', lastName: 'string' } });

var me = new Person({ firstName: 'Phil', lastName: 'Roberts' });

Invalid 'new' expression.

Answer 1

My extend method was returning an instance of AmpersandModel, not the ApersandModel's constructor function. Corrected code:

declare module ampersand {
    interface AmpersandState {
        // todo...
    }

    interface AmpersandCollection {
        // todo...
    }

    interface ModelExtendOptions {
        parse?: boolean;
        parent?: AmpersandState;
        collection?: AmpersandCollection;
    }

    interface ModelSaveOptions {
        patch?: boolean;
    }

    interface AmpersandModel<T> {
        save: (attrs?: T, options?: ModelSaveOptions) => void;
        // todo:  fetch, destroy, sync, etc...
    }

    interface AmpersandModelConstructor<T> {
        new (attrs: T, options?: ModelExtendOptions): AmpersandModel<T>;
    }   

    interface ExtendOptions {
        props?: {};
        session?: {};
        derived?: {};
    }   

    interface AmpersandModelStatic {
        extend: <T>(options: ExtendOptions) => AmpersandModelConstructor<T>;
    }
} 

declare var AmpersandModel: ampersand.AmpersandModelStatic;

interface PersonProps {
    firstName: string;
    lastName: string;
}

interface IPerson extends PersonProps, ampersand.AmpersandModel<PersonProps>{}

var Person = AmpersandModel.extend<PersonProps>({ props: { firstName: 'string', lastName: 'string' } });

var me = new Person({ firstName: 'Jeremy', lastName: 'Danyow' });