function directionStr(direction) {
  if (direction === DIRECTION_DOWN) {
    return 'down';
  } else if (direction === DIRECTION_UP) {
    return 'up';
  } else if (direction === DIRECTION_LEFT) {
    return 'left';
  } else if (direction === DIRECTION_RIGHT) {
    return 'right';
  }
  return '';
}
@private
direction cons to string
@param {constant} direction
@returns {String}