buildArrayOfClasses.js

function countClasses(array) {
    var object = {}
        ;

    array.forEach(function(element) {
        if (object[element]) {
            object[element]++;
        }
        else {
            object[element] = 1;
        }
    })

    return object;
}

function buildArray(object) {
    var array = []
        , maxProperty
        ;

    while (Object.keys(object).length) {
        maxProperty = findMaxProperty(object);
        array.push(maxProperty);
        delete object[maxProperty];
    }

    return array;
}

function findMaxProperty(object) {
    var max = 0
        , maxProperty
        , property
        ;

    for (property in object) {
        if (object[property] > max) {
            max = object[property];
            maxProperty = property;
        }
    }

    return maxProperty;
}

var arr = ['xxx','bad','asd','asd', 'asd', 'bad', 'bad'];
buildArray(countClasses(arr))