multimode.js

function multimode(arr) {
    const counts = new Map();
    for (let i = 0; i < arr.length; i++) {
        counts.set(arr[i], (counts.get(arr[i]) || 0) + 1);
    }
    let maxCount, maxElem;
    counts.forEach((count, index) => {
        if (maxCount === undefined || count > maxCount) {
            maxCount = count;
            maxElem = index;
        } else if (count === maxCount) {
            maxElem = Array.isArray(maxElem) ? maxElem.concat(index) : [maxElem, index];
        }
    });
    return maxElem;
}

multimode([1,2,3,3]); // 3
multimode([1,2,2,3,3]); // [2, 3]
multimode([1,2,3]); // [1, 2, 3]