Most Common Word

let mostCommonWord = function(paragraph, banned) {
    let wordMap = paragraph.toLowerCase()
        .replace(/\W+/g, ' ')
        .trim()
        .split(' ')
        .filter(word => banned.indexOf(word) < 0)
        .reduce((map, y) => {
            if (map.has(y)) {
                map.set(y, map.get(y) + 1);
            }
            else {
                map.set(y, 1);
            }
            return map;
        }, new Map()); //?


    let word = null, count = 0;
    for (let [key, val] of wordMap) {
        if (val > count) {
            count = val;
            word = key;
        }
    }

    return word;
};