Make GetStaticProps and GetServerSideProps types use generic props

Feature request

The GetStaticProps and GetServerSideProps typescript types allow us to use the new data fetching methods with typescript, but there is no type check for props content.

Is your feature request related to a problem? Please describe.

The current implementation is as follows:

export type GetStaticProps = (ctx: {
  params?: ParsedUrlQuery
  preview?: boolean
  previewData?: any
}) => Promise<{
  props: { [key: string]: any }
  revalidate?: number | boolean
}>

Describe the solution you’d like

export type GetStaticProps<T = {}> = (ctx: {
  params?: ParsedUrlQuery
  preview?: boolean
  previewData?: any
}) => Promise<{
  props: T
  revalidate?: number | boolean
}>

This way, when defining a Props interface in our page, we can have type safety:

interface Props {
    name: string
}

const Page: NextPage<Props> = (props) => {
    return <div>{props.name}</div>
}

export const getStaticProps: GetStaticProps<Props> = async (context) => {
    const name = await getName(); // string
    return {
        props: {
            name, // yay! Type checked props
        },
    };
};

Something very similar would be implemented for GetServerSideProps.

Describe alternatives you’ve considered

As the new methods are not static properties of pages anymore, we cannot infer them via the page, so the user may need to add the generic Props type parameter in many places.

Perhaps the types could have more parameters to allow typing previewData?

[abhijithvijayan]

What about types in context params?

in [id].ts

export const getServerSideProps: GetServerSideProps = async (context) => {
    const { id } = context.params;
}

I get this error

Property 'id' does not exist on type 'ParsedUrlQuery | undefined'.
    27 | 
    28 | export const getServerSideProps: GetServerSideProps = async (context) => {
  > 29 |     const { id } = context.params;

In VSCODE

[jareks]

You got to check first if context.params is not undefined before destructuring this valu

Under what circumstances context.params can be undefined in getServerSideProps?