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 '';
	}
direction cons to string
@param {Const} direction
@returns {String}