How to Connect Algolia with Sanity and a Next.js Website

import indexer from 'sanity-algolia';

import { algoliaInstance } from '../../graphql/utils/algolia';
import sanityServerClient, { algoliaSanityQuery } from '../../utils/sanity';

const handler = async (req) => {
  const passphrase = 'ALGOLIA_SECRET';
  if (passphrase !== process.env.ALGOLIA_SECRET) {
    return {
      status: 401,
    };
  }
  const index = algoliaInstance.initIndex(process.env.ALGOLIA_INDEX);
  const sanityAlgolia = indexer(
    {
      product: {
        index,
        // The projection is the piece of the GROQ query
        // where we determine what data to fetch
        projection: algoliaSanityQuery,
      },
      // 💡 Could have many other document types here!
    },

    // Serializer function for manipulating documents with Javascript
    // I'm not using it as GROQ is doing all the work
    (document) => document,
    // (document) => console.log(document, 'document'),
    // Visibility function to determine which document should be included
    (document) => !['unapproved'].includes(document.status),
  );
  // console.log('sanityAlgolia', sanityAlgolia);
  return sanityAlgolia
    .webhookSync(sanityServerClient, req.body)
    .then(() => ({
      status: 200,
      body: 'Success!',
    }))
    .catch(() => ({
      status: 500,
      body: 'Something went wrong',
    }));
};

export default handler;

 'categories.lvl0',
    'categories.lvl1',
    'categories.lvl2',
    'categories.lvl3',

export const algoliaSanityQuery = `
*[
  _type == 'product'
  && !(_id in path('drafts.**'))
  && defined(slug.current)
] {
  "categories": categories[]->{
    title,
    "slug": slug.current,
    _id,
  },
 
}`;