how to get distinct values from json in jquery

I'm not sure how performant this will be, but basically I'm using an object as a key/value dictionary. I haven't tested this, but this should be sorted in the loop.

function(data) {
    var birthDates = {};
    var param = "birthDate"
    $.each(data.people, function() {
        if (!birthDates[this[param]])
            birthDates[this[param]] = [];   
        birthDates[this[param]].push(this);
    });

    for(var d in birthDates) {
        // add d to array here
        // or do something with d
        // birthDates[d] is the array of people
    }
}

function(data){
    var arr = new Array();
    $.each(data.people, function(i, person){
        if (jQuery.inArray(person.birthDate, arr) === -1) {
            alert(person.birthDate);
            arr.push(person.birthDate);
        }
    });
}

Here's my take:

function getUniqueBirthdays(data){
    var birthdays = [];
    $.each(data.people, function(){
        if ($.inArray(this.birthDate,birthdays) === -1) {
            birthdays.push(this.birthDate);
        }
    });
    return birthdays.sort();
}