[рони]

TrumpTower,

const cities = ['Москва'];
        let current = 'Москва';
        while (true) {
            lastLetter = current.match(/[^ЪЬЫ](?=[ЪЬЫ]*$)/i)[0];
            lastLetter = lastLetter.toUpperCase();
            let city = prompt(`${current}, введите город на букву ${lastLetter}`);
            if (!city) break;
            let includes = cities.includes(city);
            let firstLetter = city.startsWith(lastLetter)
            if (includes) {
                alert(`${city} такой город есть в списке`);
            }
            else if(firstLetter) {
               current = city;
               cities.push(city);
               alert(`${city} добавлен в список`);
            }

            else {
               alert(`${city} начинается не с ${lastLetter}`);
            }
        }