tilemap in "Acts.prototype.SetX = function (tilemap, zX)" is an "object type", not an "object instance".
the first parameter of "cr.plugins_.Tilemap.prototype.Exps.PositionToTileX.call(tilemap , fake_ret , zX);" should be "object instance".
So you could try
Acts.prototype.SetX = function (tilemap, zX)
{
var tilemap_inst = tilemap.getFirstPicked();
cr.plugins_.Tilemap.prototype.Exps.PositionToTileX.call(tilemap_inst , fake_ret , zX);
this.testing = fake_ret.value;
};