let planets = [
                { name: 'mars', diameter: 6779 },
              ]
            

              let planets = [
                { name: 'mars', diameter: 6779 },
                { name: 'earth', diameter: 12742 },
                { name: 'jupiter', diameter: 139820 }
              ]
            

              let circumferenceReducer = ( c, planet ) => {
                return c + planet.diameter * Math.PI;
              }
          
              let planets = [
                { name: 'mars', diameter: 6779 },
                { name: 'earth', diameter: 12742 },
                { name: 'jupiter', diameter: 139820 }
              ]
          
              let c = planets.reduce( circumferenceReducer, 0 )