Force modify a URL to HTTPS if your website is under HTTPS

resolveHttps.js

/**
 * Force modify a URL to HTTPS if your website is under HTTPS
 *
 * @example resolveHttps('http://example.com')
 *          If you are under a HTTPS website, it'll return `https://example.com`,
 *          Else it'll return `http://example.com`
 *
 * @param {string} url
 * @return {string}
 */
function resolveHttps(url) {
  if (location.protocol === 'https:' && /^http:/i.test(url)) {
    return 'https:' + url.substr(5)
  }
  return url
}