Using D3, topojson and node.js to calculate the centroid of topojson features

centroid.js

/* A node.js script that uses D3 and topojson to calculate and output the lat/long
 * coordinates of the centroid for a collection of topojson boundary features.
 */

// install using npm install d3
var d3 = require("d3");
// download from http://d3js.org/topojson.v1.min.js
var topojson = require("./topojson.v1.min.js");

// load json
var subplaces = require("./subplaces.topo.json");
subplaces = topojson.feature(subplaces, subplaces.objects.test);

var projection = d3.geo.mercator();
var path = d3.geo.path().projection(projection);

var features = subplaces.features;
for (var i = 0; i < features.length; i++) {
    var feat = features[i];
    
    // calculate the centroid (which is in pixels) and then invert back to lat/long
    var centroid = projection.invert(path.centroid(feat));

    // output: id, long, lat
    console.log(feat.properties.SP_CODE + "," + centroid[1] + "," + centroid[0]);
}