Q:

Sorting nested arrays of objects by date

I'm trying to sort an array that looks like this:
var dateGroups = [
  [
     {age:20, date: Fri Feb 03 2012 14:30:00 GMT+1100 (EST)}, 
     {age:12, date: Fri Feb 03 2012 18:20:00 GMT+1100 (EST)}, 
     {age:18, date: Fri Feb 03 2012 21:43:00 GMT+1100 (EST)}
  ],
  [
     {age:32, date: Fri Feb 01 2012 10:54:00 GMT+1100 (EST)}, 
     {age:44, date: Fri Feb 01 2012 11:45:00 GMT+1100 (EST)}, 
  ],
  [
     {age:22, date: Fri Feb 05 2012 10:54:00 GMT+1100 (EST)}, 
     {age:22, date: Fri Feb 05 2012 18:22:00 GMT+1100 (EST)},
  ]
]

The objects inside dateGroups' nested arrays are already sorted in ascending order, but I also want to sort the arrays themselves based on the grouped dates.
In this case the array should then look like this:
var dateGroups = [
  [
     {age:32, date: Fri Feb 01 2012 10:54:00 GMT+1100 (EST)}, 
     {age:44, date: Fri Feb 01 2012 11:45:00 GMT+1100 (EST)}, 
  ],
  [
     {age:20, date: Fri Feb 03 2012 14:30:00 GMT+1100 (EST)}, 
     {age:12, date: Fri Feb 03 2012 18:20:00 GMT+1100 (EST)}, 
     {age:18, date: Fri Feb 03 2012 21:43:00 GMT+1100 (EST)}
  ],
  [
     {age:22, date: Fri Feb 05 2012 10:54:00 GMT+1100 (EST)}, 
     {age:22, date: Fri Feb 05 2012 18:22:00 GMT+1100 (EST)},
  ]
]

The function used to sort should also return the new sorted version of dateGroups.
I've tried using Underscore.js's sortBy() function but I can't figure out how to sort the arrays based on the value of a property inside one of the objects. Is there a specific way to sort Date objects? Or are they sorted in the same way as numbers or letters?

A:

According to Underscore.js documentation, you should simply write your own iterator for that cause. Something like this:
_.sortBy(dateGroups, function(arrayElement) {
    //element will be each array, so we just return a date from first element in it
    return arrayElement[0].date.getTime();
});

A:

You can sort them by passing a custom sort function to Array.sort.  
dateGroups.sort(function(a, b) {
    return b[0].date.getTime() - a[0].date.getTime();
});

The custom function needs to return a number less than zero (a comes before b), greater than zero (a comes after b) or zero (a and b are equal). 

