index.js

module.exports = {
    algo(input) {
        const groups = [];
        let currentGroup = '';

        for (const letter of input.split('')) {
            if (letter !== currentGroup.charAt(currentGroup.length - 1)) {
                groups.push(currentGroup);
                currentGroup = '';
            }
            
            currentGroup += letter;
        }

        groups.push(currentGroup);

        groups.sort((groupA, groupB) => {
            if (groupB.length < groupA.length) {
                return -1;
            } else if (groupB.length > groupA.length) {
                return 1;
            } else {
                return 0;
            }
        });

        return groups[0];
    }
};