Use the Shopify GraphQL API without having to install any libraries or additional frameworks. This example includes metafields

shopify-graphql-no-library.js

// You need to make your fields public first:
// https://shopify.dev/tutorials/retrieve-metafields-with-storefront-api#expose-metafields-to-the-storefront-api
await (await fetch("/api/2020-04/graphql.json", {
  "method": "POST",
  "credentials": "include",
  "headers": {
    // /admin/apps/private and create new app with storefront access
    "x-shopify-storefront-access-token": "xxxxx",
    "accept": "application/json",
    "content-type": "application/json"
  },
  "body": JSON.stringify({
    "query": `query($filter: String!, $namespace: String!) {
      products(first: 10, query: $filter) {
        edges {
          node {
            title
            handle
            metafields(first: 10, namespace: $namespace) {
              edges {
                node {
                  key
                  value
                }
              }
            }
          }
        }
      }
    }`,
    "variables": {
      "filter": "id:000000000001",
      "namespace": "metafield-namespace-to-include"
    }
  })
})).json();