gistfile1.js

for (i = 0; i<filterList.length; i++) {
    if (i === 0) {
        s = filterList[i];
    }
    else if (i === filterList.length - 1) {
        s = s + " and " + filterList[i];
    }
    else {
        s = s + ", " + filterList[i];
    }
}

gistfile2.js

if (filterList.length === 1) {
    return filterList[0];
}
else {
    [filterList.slice(0, -1).join(', '), filterList.pop()].join(' and ');
}