Sample code: https://shopify.dev/apps/online-store/verify-support contains bugs. The code should be checking `templateJSONFiles.length > 0` but doesn't. The gist also doesn't not use Shopify's node library and HTTP requests are made to Shopify's REST API using node-fetch, but any HTTP library can be used instead

Shopify verify theme support

const fetch = require('node-fetch');

try {
    // Specify the name of the template the app will integrate with
    const APP_BLOCK_TEMPLATES = ['product', 'collection'];
    const getOptions = { method: 'GET', headers: { 'X-Shopify-Access-Token': accessToken } };

    // Get published theme
    const themeResponse = await fetch(`https://${shop}/admin/api/${API_VERSION}/themes.json`, getOptions);
    this.checkError(themeResponse, 'ThemeFetchFailure');
    const { themes } = await themeResponse.json();
    const themeId = themes.filter(t => t.role === 'main').map(t => t.id);

    // Retrieve a list of assets in the published theme
    let themeAssetsResponse = await fetch(`https://${shop}/admin/api/${API_VERSION}/themes/${themeId}/assets.json`, getOptions);
    const { assets } = await themeAssetsResponse.json();

    // Check if JSON template files exist for the template specified in APP_BLOCK_TEMPLATES
    const templateJSONFiles = assets.filter((file) => { return APP_BLOCK_TEMPLATES.some(template => file.key === `templates/${template}.json`); });

    if (templateJSONFiles.length > 0) {
        if (templateJSONFiles.length === APP_BLOCK_TEMPLATES.length) {
            console.log('All desired templates support sections everywhere!')
        } else if (templateJSONFiles.length) {
            console.log('Only some of the desired templates support sections everywhere.')
        }
    }

    // Retrieve the body of JSON templates and find what section is set as `main`
    const templateMainSections = (await Promise.all(templateJSONFiles.map(async (file, index) => {
        let themeAssetsResponse = await fetch(`https://${shop}/admin/api/${API_VERSION}/themes/${themeId}/assets.json?asset[key]=${file.key}`, getOptions);
        const { asset } = await themeAssetsResponse.json();

        const json = JSON.parse(asset.value)
        const main = Object.entries(json.sections).find(([id, section]) => id === 'main' || section.type.startsWith("main-"))
        if (main) {
            return assets.find((file) => file.key === `sections/${main[1].type}.liquid`);
        }
    }))).filter((value) => value)
    
    // Request the content of each section and check if it has a schema that contains a block of type '@app'
    const sectionsWithAppBlock = (await Promise.all(templateMainSections.map(async (file, index) => {
        let acceptsAppBlock = false;
        let themeAssetsResponse = await fetch(`https://${shop}/admin/api/${API_VERSION}/themes/${themeId}/assets.json?asset[key]=${file.key}`, getOptions);
        const { asset } = await themeAssetsResponse.json();
    
        const match = asset.value.match(/\{\%\s+schema\s+\%\}([\s\S]*?)\{\%\s+endschema\s+\%\}/m)
        const schema = JSON.parse(match[1]);
    
        if (schema && schema.blocks) {
            acceptsAppBlock = schema.blocks.some((b => b.type === '@app'));
        }
    
        return acceptsAppBlock ? file : null
    }))).filter((value) => value);

    if (templateJSONFiles.length > 0) {
        if (templateJSONFiles.length === sectionsWithAppBlock.length) {
            console.log('All desired templates have main sections that support app blocks!');
        } else if (sectionsWithAppBlock.length) {
            console.log('Only some of the desired templates support app blocks.');
        }
    } else {
        console.log("None of the desired templates support app blocks");
    }

} catch (error) {
    console.log(error);
}