code: function(joint){
          if(joint.parent){
              throw new Error("It is not possible to be added to more than one world one of the joint");
          }
          if(this.joints!=null)(this.joints.prev=joint).next=this.joints;
          this.joints=joint;
          joint.parent=this;
          this.numJoints++;
          joint.awake();
          joint.attach();
      }
docs: I'll add a joint to the world.
  Joint that has been added will be the operands of each step.
  @param  shape Joint to be added