next-context.tsx

import { NextContext } from 'next'

// we define our type
type Context = NextContext<{ subreddit: string }>

// ...
  
const getInitialProps = async (context: Context) => {
  // now we can get subreddit value from the query!
  const subreddit = context.query.subreddit
  const response = await fetch(`https://www.reddit.com/r/${subreddit}.json`)
  const result = await response.json() as RedditResult

  return {
    subreddit,
    posts: result.data.children
  }
}

/*
 * But we get an error here :(
 *
 * Type '(context: NextContext<{ subreddit: string; }, {}>) => 
 * Promise<{ subreddit: string; posts: readonly RedditPost[]; }>' is not assignable to type 
 * 'GetInitialProps<{ subreddit: string; posts: readonly RedditPost[]; }, 
 * NextContext<Record<string, string | string[] | undefined>, {}>>'.
 */
Posts.getInitialProps = getInitialProps