const q = "891811 de lundi";
const days = ['dimanche', 'lundi', 'mardi', 'mercredi', 'jeudi', 'vendredi', 'samedi'];
const today = new Date();
let fallbackDate = null;
const foundDay = days.find(d => q.includes(d));
if (foundDay) {
  const currentDay = today.getDay();
  const targetDay = days.indexOf(foundDay);
  let diff = targetDay - currentDay;
  if (diff < 0) diff += 7; // next occurrence of that day
  if (diff === 0 && q.includes('prochain')) diff += 7;
  const targetDate = new Date(today);
  targetDate.setDate(today.getDate() + diff);
  fallbackDate = targetDate.toISOString().split('T')[0];
}
console.log(fallbackDate);