_.map([1, 2, 3], function(num){ return num * 3; });
//=> [3, 6, 9]
_.map({one: 1, two: 2, three: 3}, function(num, key){ return num * 3; });
//=> [3, 6, 9]
//_.map([[1, 2], [3, 4]], _.first);
=> [1, 3]