mostCommon.js

function mostCommon (array) {
    return array
        .filter(function (value, index, array) { 
            return array.indexOf(value) === index;
        })
        .map(function(uniqueItem) {               
            var count = 0;

            array.forEach(function(item) {
                if (uniqueItem === item) {
                    count++;
                }
            });

            return {name: uniqueItem, count: count};
        })
        .sort(function(a, b) {
            return b.count - a.count; 
        })[0];
}