text: function DockingStation () {
    this._rack = [];
    this._defaultCapacity = 20;
  
  };
  
  DockingStation.prototype.bikes = function() {
    return this._rack;
  };
  
  DockingStation.prototype.capacity = function () {
    return this._defaultCapacity;
  };
  
  DockingStation.prototype.release = function() {
    if(this._rack.length === 0) {
      throw new Error('cannot release a bike from an empty station');
    } else {
      var bike = this._rack.pop()
      return bike
    }
  };
  
  DockingStation.prototype.dock = function(bike) {
    if(this._rack.length === this._defaultCapacity) {
      throw new Error('cannot dock a bike to a full station');
    } else {
      this._rack.push(bike);
      return bike;
    }
  };
  