Script to fetch metadata w/ Sanity before initiating Gatsby/Next/etc.

fetch-meta.js

const path = require('path')
const fs = require('fs/promises')
require('dotenv').config({
                                   // vvvvvvvvvv replace w/ your env
  path: path.resolve(process.cwd(), '.env.local')
})

const createClient = require('@sanity/client')
const client = createClient({
  projectId: process.env.NEXT_PUBLIC_SANITY_PROJECT_ID,
  dataset: process.env.NEXT_PUBLIC_SANITY_DATASET,
  token: process.env.SANITY_EDITOR_TOKEN,
  useCdn: process.env.NEXT_PUBLIC_ENV === 'production',
})

const fetchMeta = async () => {
  console.log(`Pre-export: Fetching metadata`)

  const data = await client.fetch(`*[_id == 'config_site'][0]`)
  // { "url": "https://www.foo.bar", etc. }
    
  if (!data) {
    throw new Error(`No data found.`)
  }

  await fs.writeFile(path.resolve(process.cwd(), 'temp/data.json'), JSON.stringify(data))
  console.log(`Pre-export: Successfully fetched metadata.`)
}

package.json

{
  "name": "foo",
  "scripts": {
    "preexport": "node ./scripts/fetch-meta.js",
    "export": "next build && next export",
    
    "prebuild": "node ./scripts/fetch-meta.js",
    "build": "next build"
  }
}