Convert an array of jsons into a json of arrays (and vice versa)

function rmp_transpose(my_object) {
    if (my_object instanceof Array) {
        //we try to convert array of json to json of arrays
        var a_keys = new Array();
        var my_output = {}

        if (my_object.length > 0) {
            for (my_key in my_object[0]) {
                a_keys.push(my_key);
            }
        }
        for (var j = 0; j < a_keys.length; j++) {
            var a_cell = new Array();
            for (var i = 0; i < my_object.length; i++) {
                if (my_object[i][a_keys[j]] != undefined) {
                    a_cell.push(my_object[i][a_keys[j]]);
                } else {
                    a_cell.push("");
                }
            }
            my_output[a_keys[j]] = a_cell;
        }
        return my_output;
    } else {
        //we try to convert json of arrays into an array of json
        var my_output = new Array();
        var a_keys = new Array();
        for (my_key in my_object) {
            a_keys.push(my_key);
        }
        if (a_keys.length > 0) {
            for (var i = 0; i < my_object[a_keys[0]].length; i++) {
                var my_json = {};
                for (var j = 0; j < a_keys.length; j++) {
                    my_json[a_keys[j]] = my_object[a_keys[j]][i];
                }
                my_output.push(my_json);
            }
        }
        return my_output;
    }
}

Example

// if my_object is an array of jsons to transpose
var my_object = [
        { "firstname":"Tony", "lastname":"Boy" },
        { "firstname":"John", "lastname":"Smith" }
    ];

JSON.stringify(rmp_transpose(my_object));

// will return
/*
{
    "firstname":["Tony","John"],
    "lastname":["Boy","Smith"]
}
*/

JSON.stringify(rmp_transpose(rmp_transpose(my_object)));

//will return the original my_object
/*
[
    { "firstname":"Tony", "lastname":"Boy" },
    { "firstname":"John", "lastname":"Smith" }
]
*/