I am modifying jPhysics for 3D resolution, but getting error in code that I can't figure? Any help appreciated.

jQuery.fn.physics = function(params) {
 this.each(function(i){
  //Defaults for first time (without physics properties) (when not in 'params')
  if (!this.position) {
   if (!params.position) this.position = new $.Vector(0,0,0);
   if (!params.velocity) this.velocity = new $.Vector(0,0,0);
   if (!params.acceleration) this.acceleration = new $.Vector(0,0,0);
   if (!params.minPosition) this.minPosition = new $.Vector(-Infinity,-Infinity,-Infinity);
   if (!params.maxPosition) this.maxPosition = new $.Vector(Infinity,Infinity,-Infinity);
   if (!params.minVelocity) this.minVelocity = new $.Vector(-Infinity,-Infinity,-Infinity);
   if (!params.maxVelocity) this.maxVelocity = new $.Vector(Infinity,Infinity,-Infinity);
   if (!params.minAcceleration) this.minAcceleration = new $.Vector(-Infinity,-Infinity,-Infinity);
   if (!params.maxAcceleration) this.maxAcceleration = new $.Vector(Infinity,Infinity,-Infinity);
   if (!params.mass) this.mass = 1;
   this.style.position = 'absolute';
  }
  //Use 'params' where possible
  if (params.position) this.position = params.position;
  if (params.velocity) this.velocity = params.velocity;
  if (params.acceleration) this.acceleration = params.acceleration;
  if (params.minPosition) this.minPosition = params.minPosition;
  if (params.maxPosition) this.maxPosition = params.maxPosition;
  if (params.minVelocity) this.minVelocity = params.minVelocity;
  if (params.maxVelocity) this.maxVelocity = params.maxVelocity;
  if (params.minAcceleration) this.minAcceleration = params.minAcceleration;
  if (params.maxAcceleration) this.maxAcceleration = params.maxAcceleration;
  if (params.mass) this.mass = params.mass;
  this.style.left = Math.round(this.position[0])+'px';
  this.style.top = Math.round(this.position[1])+'px';
  this.style.z-index = Math.round(this.position[2]);//this line throws "syntax error"
 });
 return this;
};

I am thinking the "-" in z-index is being parsed as an operator? If this is the case, how do I work around it?

this.style.z-index

Should be

this.style.zIndex

CSS properties that are split with a dash are always expressed in camelCase in JavaScript, since dashes are not allowed in property names, as they would indicate subtraction.

If Abel's suggestion doesn't work, try writing variables with a hyphen in bracket notation, like so:

this.style['z-index']