lint-formatter.js

module.exports = results => {
  const byRuleId = results.reduce(
    (map, current) => {
      current.messages.forEach(({ ruleId, line, column }) => {
        if (!map[ruleId]) {
          map[ruleId] = [];
        }

        const occurrence = `${current.filePath}:${line}:${column}`;
        map[ruleId].push(occurrence);
      });
      return map;
    }, {}
  );

  return Object.entries(byRuleId)
    .map(([ruleId, occurrences]) => `${ruleId} (total: ${occurrences.length})\n${occurrences.join('\n')}`)
    .join('\n########################\n');
};