setInterval(function(){
                  context.rotate( 15 );
                  
                  context.fillStyle = "rgba(0,0,0,0.05)";
                  context.fillRect(0, 0, elem.width, elem.height);
                  
                  context.fillStyle = "rgba(255, 0, 0, 1)";
                  context.fillRect(pos, pos, 20, 20);
                  
                  pos += dir;
                  
                  if ( pos > elem.width ) {
                          dir = -1;
                  } else if ( pos + 20 < 0 ) {
                          dir = 1;
                  }
          }, 10);
  };