unflatten.js

function unflatten(list, count) {
    return _.map(_.range(Math.ceil(list.length/count)), function(mul) {
        return list.slice(mul*count, mul*count+count); 
    });
}
 
console.log(unflatten(_.range(1, 14), 3));
// [[1, 2, 3], [4, 5, 6], [7, 8, 9], [10, 11, 12], [13]]