panda01

function unflatten(list, count) {
    var ret = [];
    // For every group
    _.range(Math.ceil(list.length/count)).forEach(function(mul) {
        ret.push(list.slice(mul*count, mul*count+count)); 
    });
    return ret;
}

console.log(unflatten(_.range(1, 13), 3));