player presses space;and you have the current direction to throw the rope.

set up ray: new Ray(player.position, direction, start_distance);

each-frame after pressing space {

   ray.position = player.position;
   var result = space.rayCast(ray);

   if(result!=null) {
      //yay rope hit something
      create rope joint with player position, and ray.at(result.distance)
   }else {
      //still not hit anything
      ray.maxDistance += distance_increment;
   }
}


------------------------------------------ SAME AS ABOVE BUT WITH GRAPHICS ----------------

player presses space;and you have the current direction to throw the rope.

set up ray: new Ray(player.position, direction, start_distance);

each-frame after pressing space {

   ray.position = player.position;
   var result = space.rayCast(ray);

   if(result!=null) {
      //yay rope hit something
      create rope joint with player position, and ray.at(result.distance)
      start rendering rope as normal based on rope anchor points.
   }else {
      //still not hit anything
      ray.maxDistance += distance_increment;
      render rope from player.position up to ray.at(ray.maxDistance)
   }
}