jewelsAndStones.js

const numJewelsInStones = (jewels, stones) => {
    const cache = {};
    
    for (const j of jewels) {
        cache[j] = 0
    }
    
    for (const s of stones) {
        if (s in cache) {
            cache[s]++;
        }
    }
    
    return Object.keys(cache).reduce((ac, j) => {
        ac += cache[j];
        return ac;
    }, 0)
    
};