How to use the mapnik.ProjTransform function in mapnik

function getCenterAndExtent(ds, projection) {
    var extent;
    // Convert datasource extent to lon/lat when saving
    var fromProj = new mapnik.Projection(projection);
    var toProj = new mapnik.Projection('+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs');
        
    if (fromProj === toProj) {
        try {
            extent = ds.extent();
        } catch (err) {
            return invalid('Error obtaining extent of Mapnik datasource.');
        }
    } else {
        try {
            var trans = new mapnik.ProjTransform(fromProj, toProj);
            //Bounding box
            extent = trans.forward(ds.extent());
        } catch (err) {
            return invalid('Error obtaining extent of Mapnik datasource.');
        }
    }
    
    //Center point of bounding box (extent)
    var center = [0.5 * (extent[0] + extent[2]), 0.5 * (extent[1] + extent[3])];
    var results = {
        extent: extent,
        center: center
    };
    return results;
}
/**

delete newdoc.bbox;
            }

            var geom = f.geometry();

            if (srs == '+init=epsg:4326') {
                geom.toJSON(function(err,json_string) {
                    newdoc.geometry = JSON.parse(json_string);
                    docs.push(newdoc);
                    i++;
                    immediate(feature);
                });
            } else {
                var from = new mapnik.Projection(srs);
                var to = new mapnik.Projection('+init=epsg:4326');
                var tr = new mapnik.ProjTransform(from,to);
                geom.toJSON({transform:tr},function(err,json_string) {
                    newdoc.geometry = JSON.parse(json_string);
                    docs.push(newdoc);
                    i++;
                    immediate(feature);
                });
            }
        }

Raster.prototype.getExtent = function(callback) {
  var current = new mapnik.Projection(this.projection);
  var wgs84 = new mapnik.Projection('+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs');
  var extent;
  var transform;

  try {
    extent = this.mapnikDatasource.extent();
  } catch (err) {
    return callback(invalid('Invalid raster: could not read extent'));
  }

  if (current !== wgs84) {
    try {
      transform = new mapnik.ProjTransform(current, wgs84);
      extent = transform.forward(extent);
    } catch (err) {
      return callback(invalid('Invalid raster: Unable to compute bounds. X or Y values exceed limits of provided CRS.'));
    }
  }

  return callback(null, extent);
};

this.getProjection(function(err, projection) {
    if (err) return callback(err);

    var current = new mapnik.Projection(projection);
    var wgs84 = new mapnik.Projection('+proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs');
    var transform;

    if (current !== wgs84) {
      transform = new mapnik.ProjTransform(current, wgs84);
      try {
        extent = transform.forward(extent);
      } catch (err) {
        return callback(invalid('Unable to get extent. Failed to forward project to WGS84 coordinates.'));
      }
    }

    _this._extent = extent;
    return callback(null, _this._extent);
  });
};