leetcode-819.js

var mostCommonWord = function(paragraph, banned) {
    const words = paragraph.toLowerCase().split(/\W+/)
    const freqs = frequencies(words, banned)
    return findMaxWord(freqs)
};

function frequencies(words, banned) {
    const freqs = {}
    for (let w of words) {
        if (w && !banned.includes(w)) {
            freqs[w] = freqs[w] ? freqs[w] + 1 : 1
        }
    }
    return freqs
}

function findMaxWord(freqs) {
    let max = 0
    let maxWord = ""
    for (let w in freqs) {
        if (freqs[w] > max) {
            max = freqs[w]
            maxWord = w
        }
    }
    return maxWord
}