FCC BAS #35 - Bonfire: Steamroller

function steamroller(arr) {
    var flat = [],
    i;
    for (i = 0; i < arguments.length; i++) {
        if (arguments[i] instanceof Array) {
            flat = flat.concat(steamroller.apply(null, arguments[i]));
        } else {
            flat.push(arguments[i]);
        }
    }
    
    return flat;
}

steamroller([1, [2], [3, [[4]]]]);