Code to transform traefik's acme.json into certificate for node

getCertificate.js

/**
 * Extract specific key and certificate from traefik acme.json file
 */
const fs = require('fs/promises');

/**
 * Decodes found certificate
 * @param {{certificate: string, key: string}} data Data to decode
 * @returns {{certificate: string, key: string}} decoded data
 */
const decode = ({ certificate, key }) => ({
  certificate: Buffer.from(certificate, 'base64').toString('ascii'),
  key: Buffer.from(key, 'base64').toString('ascii'),
});

/**
 * Locate best matching certificate in the supplied file.
 * @param {string} acmeFile file to search
 * @param {string} domain domain to search for
 * @returns {undefined | {certificate: string, key: string}}
 * @throws on file or parse error.
 */
const find = async (acmeFile, domain) => {
  // Build lookup to map domain to it's certificate.  Expects JSON :
  // {
  //   "provider1": {
  //     "Certificates": [
  //       {
  //         "certificate": "ZHoG3nz3coraa8A...",
  //         "key": "JQkFBS0NBBOUZWUzN...",
  //         "domain": {
  //           "main": "example.com",
  //           "sans": ["other.example.com"]
  //         }
  //       },
  //       {...}
  //     ]
  //   },
  //   "provider2": { ... }
  // }
  const text = await fs.readFile(acmeFile, 'utf-8');
  const certs = Object.values(JSON.parse(text))
    .reduce((out, d) => out.concat(d.Certificates), [])
    .reduce((out, d) => {
      const {
        certificate,
        domain: { main, sans = [] },
        key,
      } = d;
      // Add main certificate to lookup.
      out[main] = { certificate, key };
      // Add alternate names to lookup.
      sans.forEach((san) => (out[san] = out[main]));
      return out;
    }, []);

  // Use lookup to locate best match.
  if (certs[domain]) {
    // Exact Match
    return decode(certs[domain]);
  }
  // Look for wildcard match on parent domain.
  domain = `*.${domain.split('.').slice(1).join('.')}`;
  if (certs[domain]) {
    // Wildcard match
    return decode(certs[domain]);
  }
  // Wildcards only work one level deep, therefore not found
};

/**
 * Search supplied acme.json file for domain certificate with wildcard
 * fallback.
 * @param {string} acmeFile qualified path to file
 * @param {string} domain domain to search for
 * @returns { key: string, certificate: string } if found, otherwise undefine
 */
export const getCertificate = async (acmeFile, domain) => {
  return find(acmeFile, domain);
};

// getCertificate('./acme.json', 'my.example.com').then(console.debug);