Class 1

#earthquakes {
    marker-width: 3;
    ...
    [zoom >= 10] {
        marker-width: 8;
    }
    [zoom >= 16] {
        marker-width: 13;
    }
}

Class 2

$.getJSON(url)
  .done(function (data) {
    // Do something with the data
  });

Class 3

Class 4

Class 5

Class 6

Class 7

Class 8

Class 9

Class 10

Class 11

Class 12