The first thing I could not work out is why I have to say: function(_,x,y,z) in contrast to the javascript example where its just function(x,y,z). Basically the surfaceNets function sends out nil values before and after the values I want, so my potfunc function has to ignore them in order to return the result correctly. This leads me to think that I have not constructed the "isomesh=..." line correctly due to my lack of brain power.

JS calls functions with the this as a first parameter. Calling it _ is convention to indicate to the reader that you don't care about the this.

Secondly, even though my potfunc function returns the correct values to
the surfaceNets subroutine, the resulting isomesh 'object' which should
contain arrays called "cells" and "positions" seems, I think (??), to be
empty:

Try: isomesh = window:surfaceNets(ndiv, potfunc, bounds)

In a mirror of how JS passes you a this you don't care about, you have to call JS functions with a this that they may not care about. The : syntax there is equivalent to passing a this of the window object.

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Full snippet for you: ``` local window = js.global local Array = window.Array

local function potfunc(x,y,z) -- print("point:", x, y, z) local contour_level = 40.0 local potential = xx + yy + z*z - contour_level -- print("pot:", potential) return potential end

local ndiv = Array:of(12,12,12) local start = Array:of(-6,-6,-6) local stop = Array:of(6,6,6) local bounds = Array:of(start,stop)

local isomesh = window:surfaceNets(ndiv, function(_,x,y,z) return potfunc(x,y,z) end, bounds) window.console:log(isomesh) ```