circle.center()

circle.center([center])

If center is specified, sets the circle center to the specified point [longitude, latitude] in degrees, and returns this circle generator. The center may also be specified as a function; this function will be invoked whenever a circle is generated, being passed any arguments passed to the circle generator. If center is not specified, returns the current center accessor, which defaults to:

function center() {
  return [0, 0];
}