Extend cleanly any JS class

Coffeescript's class syntax is compatible with JavaScript's prototype inheritance solutions. For example you can extend Google Maps classes using concise and DRY:

class ns.CustomMarker extends google.maps.OverlayView
  constructor: (@map, @param) ->
    this.setMap(map)

  draw: -> # (...)

Instead of doing it the official, dirty way ;)

ns.CustomOverlay = function(map, param) {
  this.param = param;
  this.map = map;
  this.setMap(map);
}

ns.CustomOverlay.prototype = new google.maps.OverlayView();

ns.CustomOverlay.prototype.draw = function() {
  # (...)
}