gatsby-node-onPostBuild.js

const fs = require('fs');

exports.onPostBuild = async ({ graphql }) => {
    await graphql(`
        {
            activeAds: allStoreProducts(filter: { cms: { isAdvertised: { eq: true } } }) {
                nodes {
                    productId
                    cms {
                        title
                        collectionTitle
                        shortDescription
                        discount
                        slug
                    }
                    cloudinary {
                        secure_url
                    }
                }
            }
            storeDetails: markdownRemark(fileAbsolutePath: { regex: "/siteLaunch/" }) {
                frontmatter {
                    siteUrl
                    siteName
                    title
                }
            }
        }
    `).then((results) => {
        const adsPath = './public/activeAds';
        const adsList = results.data.activeAds.nodes.map((item) => item);
        const storeDetails = results.data.storeDetails.frontmatter;

        if (!fs.existsSync(adsPath)) {
            fs.mkdirSync(adsPath);
        }

        const adsItems = [];

        adsList.forEach((ad) => {
            const { cms, cloudinary } = ad;

            adsItems.push({
                title: cms.title,
                collectionTitle: cms.collectionTitle,
                shortDescription: cms.shortDescription,
                image: cloudinary[0].secure_url,
                discount: cms.discount,
                url: `${storeDetails.siteUrl}${cms.slug}`,
            });
        });

        const adsData = {
            ...storeDetails,
            adsItems,
        };

        fs.writeFileSync(`${adsPath}/ads.json`, JSON.stringify(adsData));
    });
};