Underscore chaining. This example scans a collection, finds all items that match a condition, then returns a new collection from the matches with custom structure.

gistfile1.txt

var subscribersModel =  {
	users: [
	    {
	        'id': '1asdf',
	        'activeSubscriptions': ['monthly', 'gold'],
	        'gender': 'm',
	        'age': '31',
	        'username': 'CowboyWayne'
	    },
	    {
	        'id': '2df',
	        'activeSubscriptions': null,
	        'gender': 'f',
	        'age': '22',
	        'username': 'TanyaT'
	    },
	    {
	        'id': '3fda',
	        'activeSubscriptions': ['monthly'],
	        'gender': 'm',
	        'age': '48',
	        'username': 'BuckO'
	    }
    ]
};

var activeUsers = _.map(_.filter(subscribersModel.users, function(user) {
                return user.activeSubscriptions > 0;
            }), function(user) {
                return {
                    "id": user.id,
                    "subscriptions": user.activeSubscriptions
                };
            });

console.log(activeUsers);            
// logs:             
// [
//     {
//         id: '1asdf',
//         subscriptions: ['monthly', 'gold']
//     },
//     {
//         id: '3fda',
//         ['monthly']
//     }
// ]