Better way to parse data from nested JSON in Javascript/ NextJS

The nested JSON file contains names of US states and corresponding cities. City.json

[
    {
      "header": "Michigan ",
      "id": "MI",
      "PlaceList": [
        {
          "title": "Detroit",
          "slug": "detroit"
        },
        {
          "title": "Lansing ",
          "slug": "lansing"
        }
      ]
    },
    {
      "header": "Iowa ",
      "id": "IA",
      "PlaceList": [
        {
          "title": "Ames",
          "slug": "ames"
        },
        {
          "title": "Davenport ",
          "slug": "davenport"
        }
      ]
    }
  ]

The following code returns details of the cities stored in markdown file.

import React, { useMemo } from 'react'
import fs from "fs"
import Link from "next/link"
import Markdown from "react-markdown"
import rehypeRaw from "rehype-raw"
import matter from "gray-matter"
import City from '@/app/(posts)/Blog/City.json'
import path from "path"
import { Metadata, ResolvingMetadata } from "next"

const allPosts = City.flatMap(({ header, id, PlaceList }) =>
contentList.map((topics) => ({

    stateName: header,
    stateId: id,
    ...topics,

  })),
);

// generates static pages 
export async function generateStaticParams() {
  return allPosts.map((post) => ({
    chapterID: post.stateId,
    postID: post.slug

  }))
}

interface Post {
  stateId: string;
  title: string;
  slug: string;
}

interface Pagination {
  prev: string | null;
  next: string | null;
  post: Post | null;
  switchState: boolean;
}

const BlogPage = ({params}:{params:any) => {
  //gets post ids from matching slug
  const postID = params.postID;
  // calculate prev/next page slug and switch state
  const { prev, next, post, switchState } = useMemo<Pagination>(() => {
    const postIndex = allPosts.findIndex(({ slug }) => slug === postID);

    if (postIndex === -1) {
    return {
      prev: null,
      next: null,
      post: null,
      switchState: false
    };
  }

    const currentPost = allPosts[postIndex];
    const isLastPostInHeader =
      postIndex === allPosts.length - 1 ||
      allPosts[postIndex + 1]?.stateId !== currentPost.stateId;

    return {
       prev: allPosts[postIndex - 1]?.slug || null,
       post: currentPost,
       next: isLastPostInState ? null : allPosts[postIndex + 1]?.slug || null,
       switchState: isLastPostInState
  };
  }, [postID]);


  if(!post) {
    return null;
  }
  const curr_post = GetPostContent(`${post.stateId}`,`${post.slug}`)
  return (

    <>

      <h1>{curr_post.title}</h1>
      <hr className="my-6 border border-dashed bg-sky-600 h-0.5" />
      <article lang="bn" className="prose lg:prose-xl">
        <Markdown rehypePlugins={[rehypeRaw]}>
          {curr_post.content}</Markdown>
      
      </article>
     <Link
        className="float-left text-teal-600"
        aria-disabled={!prev}
        href={prev ? `${prev}` : ''}
      >
        prev
      </Link>
      <Link
        className="float-right text-teal-600"
        aria-disabled={!next}
        href={next ? `${next}` : ''}
      >
        next
      </Link>
      
      )}
    </>

  )
}

export default BlogPage

Is there a better way to parse data from JSON in NextJS?