Adding TypeScript type checking to getInitialProps

[tomevans18]

Hi all,

Within our codebase I am facing an issue with type checking for getInitialProps.
When a page requires initial props and no getInitialProps is added for that page then no type checking is completed.
This leads to undefined content being rendered to the page.

On looking through the source code, NextComponentType seems to allow for getInitialProps to be optional.

export declare type NextComponentType<C extends BaseContext = NextPageContext, IP = {}, P = {}> = ComponentType<P> & {
    /**
     * Used for initial page load data population. Data returned from `getInitialProps` is serialized when server rendered.
     * Make sure to return plain `Object` without using `Date`, `Map`, `Set`.
     * @param ctx Context of `page`
     */
    getInitialProps?(context: C): IP | Promise<IP>;
};

With this optional value being in place the following code will not throw an error despite initial props being needed to render the page correctly.

interface IHomePageProps {
  userAgent: string
}

const Home: NextPage<IHomePageProps> = ({ userAgent }) => (
  <h1>Hello world! - user agent: {userAgent}</h1>
);

export default Home;

But the following will error as getInitialProps is present but returning an incorrect object:

interface IHomePageProps {
  userAgent: string
}

const Home: NextPage<IHomePageProps> = ({ userAgent }) => (
  <h1>Hello world! - user agent: {userAgent}</h1>
);

Home.getInitialProps = async () => {
  return { hello: "world" };
};

export default Home;

My thinking is that getInitialProps should only be optional when there are no initial props provided to the NextComponentType type but tbh this is beyond my knowledge of TS as you would need the key to be optional base on the present of a generic having a key which is also required.

Has anyone found a solution to this issue?