Extract Schema from Gatsby

.eslintrc.js

module.exports = {
  extends: ["react-app"],
  env: {
    browser: true,
    node: true,
    jest: true
  },
  rules: {
    "graphql/template-strings": [
      "error",
      {
        env: "relay",
        tagName: "graphql",
        schemaJson: require("./schema.json")
      }
    ],
  }
};

gatsby-node.js

import path from "path"
import { writeFile } from "fs"
import { introspectionQuery, graphql } from "gatsby/graphql"

exports.onPostBootstrap = async ({ store }) => {
  try {
    const { schema } = store.getState() 

    const data = await graphql(schema, introspectionQuery)

    await fs.writeFile(
      path.resolve(process.cwd(), `schema.json`),
      JSON.stringify(data)
    )
  } catch (error) {
    return error
  }
}