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

I need to get just the first item (actually, just the first key) off a rather large associative array in JavaScript. Here's how I'm doing it currently (using jQuery):

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

Just guessing, but I'd say there's got to be a better (read: more efficient) way of doing this. Any suggestions?

UPDATE: Thanks for the insightful answers and comments! I had forgotten my JavaScript 101, wherein the spec says you're not guaranteed a particular order in an associative array. It's interesting, though, that most browsers do implement it that way. I'd prefer not to sort the array before getting that first key, but it may be unavoidable given my use case.

Solution 1:

You can avoid having to create a function by referencing the first entry returned from Object.keys():

var firstKey = Object.keys(data)[0];

For the first entry from a sorted key list, simply add a call to the .sort() method:

var firstKey = Object.keys(data).sort()[0];

Solution 2:

There isn't really a first or last element in associative arrays (i.e. objects). The only order you can hope to acquire is the order the elements were saved by the parser -- and no guarantees for consistency with that.

But, if you want the first to come up, the classic manner might actually be a bit easier:

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

Want to avoid inheritance properties?

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

Solution 3:

In addition to Jonathan's solution, we can also extend the default array functionality:

Array.prototype.getKey = function() {
  for (var prop in this)
    if (this.propertyIsEnumerable(prop))
      return prop;
}