What is the most efficient way to get the first item from an associative array in JavaScript?

getKey = function (data) {
    var firstKey;
    $.each(data, function (key, val) {
        firstKey = key;
        return false;
    });
    return firstKey;
};

function getKey(data) {
  for (var prop in data)
    return prop;
}

function getKey(data) {
  for (var prop in data)
    if (data.propertyIsEnumerable(prop))
      return prop;
}