URL host validation utility

host-validator.ts

/**
 * Validate URL host
 *
 * This supports domain names, IDN domain names, IPv4, and IPv6 addresses.
 *
 * Intentional spec incompatibilities:
 * - Blank hosts ('') and blank FQDN hosts ('.') are considered invalid.
 *
 * @param host - Host to validate
 * @returns true if host is valid and doesn't need additional encoding
 */
const isValidHost = (host: string): boolean => {
  const url = new globalThis.URL('https://-');
  url.host = host;
  const parsed = url.host;
  // If input host value is empty or contains [/\], or the processed
  // host contains more '%' characters, then the input is an invalid
  // or an improperly-encoded host.
  return (
    host.length > 0 &&
    parsed.length > 0 &&
    parsed.length >= host.length &&
    // host does not include whitespace
    !/\s/.test(host) &&
    // host includes at least one 'letter' or 0-9 digit character
    // (required reading: https://unicode.org/reports/tr18/#property_syntax )
    /[\p{L}\p{Nd}]/u.test(host) &&
    !/[/\\]/.test(host) &&
    (host === '-' || parsed !== '-') &&
    (parsed === host.toLowerCase() ||
      (parsed.length > host.length &&
        [...parsed.matchAll(/%/g)].length === [...host.matchAll(/%/g)].length))
  );
}