GeoJSON.js

var GeoJSON = function( geojson, options ){

	var _geometryToGoogleMaps = function( geojsonGeometry, opts, geojsonProperties ){
		
		var googleObj;
		
		switch ( geojsonGeometry.type ){
			case "Point":
				opts.position = new google.maps.LatLng(geojsonGeometry.coordinates[1], geojsonGeometry.coordinates[0]);
				googleObj = new google.maps.Marker(opts);
				if (geojsonProperties) {
					googleObj.set("geojsonProperties", geojsonProperties);
				}
				break;
				
			case "MultiPoint":
				googleObj = [];
				for (var i = 0; i < geojsonGeometry.coordinates.length; i++){
					opts.position = new google.maps.LatLng(geojsonGeometry.coordinates[i][1], geojsonGeometry.coordinates[i][0]);
					googleObj.push(new google.maps.Marker(opts));
				}
				if (geojsonProperties) {
					for (var k = 0; k < googleObj.length; k++){
						googleObj[k].set("geojsonProperties", geojsonProperties);
					}
				}
				break;
				
			case "LineString":
				var path = [];
				for (var i = 0; i < geojsonGeometry.coordinates.length; i++){
					var coord = geojsonGeometry.coordinates[i];
					var ll = new google.maps.LatLng(coord[1], coord[0]);
					path.push(ll);
				}
				opts.path = path;
				googleObj = new google.maps.Polyline(opts);
				if (geojsonProperties) {
					googleObj.set("geojsonProperties", geojsonProperties);
				}
				break;
				
			case "MultiLineString":
				googleObj = [];
				for (var i = 0; i < geojsonGeometry.coordinates.length; i++){
					var path = [];
					for (var j = 0; j < geojsonGeometry.coordinates[i].length; j++){
						var coord = geojsonGeometry.coordinates[i][j];
						var ll = new google.maps.LatLng(coord[1], coord[0]);
						path.push(ll);
					}
					opts.path = path;
					googleObj.push(new google.maps.Polyline(opts));
				}
				if (geojsonProperties) {
					for (var k = 0; k < googleObj.length; k++){
						googleObj[k].set("geojsonProperties", geojsonProperties);
					}
				}
				break;
				
			case "Polygon":
				var paths = [];
				var exteriorDirection;
				var interiorDirection;
				for (var i = 0; i < geojsonGeometry.coordinates.length; i++){
					var path = [];
					for (var j = 0; j < geojsonGeometry.coordinates[i].length; j++){
						var ll = new google.maps.LatLng(geojsonGeometry.coordinates[i][j][1], geojsonGeometry.coordinates[i][j][0]);
						path.push(ll);
					}
					if(!i){
						exteriorDirection = _ccw(path);
						paths.push(path);
					}else if(i == 1){
						interiorDirection = _ccw(path);
						if(exteriorDirection == interiorDirection){
							paths.push(path.reverse());
						}else{
							paths.push(path);
						}
					}else{
						if(exteriorDirection == interiorDirection){
							paths.push(path.reverse());
						}else{
							paths.push(path);
						}
					}
				}
				opts.paths = paths;
				googleObj = new google.maps.Polygon(opts);
				if (geojsonProperties) {
					googleObj.set("geojsonProperties", geojsonProperties);
				}
				break;
				
			case "MultiPolygon":
				googleObj = [];
				for (var i = 0; i < geojsonGeometry.coordinates.length; i++){
					var paths = [];
					var exteriorDirection;
					var interiorDirection;
					for (var j = 0; j < geojsonGeometry.coordinates[i].length; j++){
						var path = [];
						for (var k = 0; k < geojsonGeometry.coordinates[i][j].length; k++){
							var ll = new google.maps.LatLng(geojsonGeometry.coordinates[i][j][k][1], geojsonGeometry.coordinates[i][j][k][0]);
							path.push(ll);
						}
						if(!j){
							exteriorDirection = _ccw(path);
							paths.push(path);
						}else if(j == 1){
							interiorDirection = _ccw(path);
							if(exteriorDirection == interiorDirection){
								paths.push(path.reverse());
							}else{
								paths.push(path);
							}
						}else{
							if(exteriorDirection == interiorDirection){
								paths.push(path.reverse());
							}else{
								paths.push(path);
							}
						}
					}
					opts.paths = paths;
					googleObj.push(new google.maps.Polygon(opts));
				}
				if (geojsonProperties) {
					for (var k = 0; k < googleObj.length; k++){
						googleObj[k].set("geojsonProperties", geojsonProperties);
					}
				}
				break;
				
			case "GeometryCollection":
				googleObj = [];
				if (!geojsonGeometry.geometries){
					googleObj = _error("Invalid GeoJSON object: GeometryCollection object missing \"geometries\" member.");
				}else{
					for (var i = 0; i < geojsonGeometry.geometries.length; i++){
						googleObj.push(_geometryToGoogleMaps(geojsonGeometry.geometries[i], opts, geojsonProperties || null));
					}
				}
				break;
				
			default:
				googleObj = _error("Invalid GeoJSON object: Geometry object must be one of \"Point\", \"LineString\", \"Polygon\" or \"MultiPolygon\".");
		}
		
		return googleObj;
		
	};
	
	var _error = function( message ){
	
		return {
			type: "Error",
			message: message
		};
	
	};

	var _ccw = function( path ){
		var isCCW;
		var a = 0;
		for (var i = 0; i < path.length-2; i++){
			a += ((path[i+1].lat() - path[i].lat()) * (path[i+2].lng() - path[i].lng()) - (path[i+2].lat() - path[i].lat()) * (path[i+1].lng() - path[i].lng()));
		}
		if(a > 0){
			isCCW = true;
		}
		else{
			isCCW = false;
		}
		return isCCW;
	};
		
	var obj;
	
	var opts = options || {};
	
	switch ( geojson.type ){
	
		case "FeatureCollection":
			if (!geojson.features){
				obj = _error("Invalid GeoJSON object: FeatureCollection object missing \"features\" member.");
			}else{
				obj = [];
				for (var i = 0; i < geojson.features.length; i++){
					obj.push(_geometryToGoogleMaps(geojson.features[i].geometry, opts, geojson.features[i].properties));
				}
			}
			break;
		
		case "GeometryCollection":
			if (!geojson.geometries){
				obj = _error("Invalid GeoJSON object: GeometryCollection object missing \"geometries\" member.");
			}else{
				obj = [];
				for (var i = 0; i < geojson.geometries.length; i++){
					obj.push(_geometryToGoogleMaps(geojson.geometries[i], opts));
				}
			}
			break;
		
		case "Feature":
			if (!( geojson.properties && geojson.geometry )){
				obj = _error("Invalid GeoJSON object: Feature object missing \"properties\" or \"geometry\" member.");
			}else{
				obj = _geometryToGoogleMaps(geojson.geometry, opts, geojson.properties);
			}
			break;
		
		case "Point": case "MultiPoint": case "LineString": case "MultiLineString": case "Polygon": case "MultiPolygon":
			obj = geojson.coordinates
				? obj = _geometryToGoogleMaps(geojson, opts)
				: _error("Invalid GeoJSON object: Geometry object missing \"coordinates\" member.");
			break;
		
		default:
			obj = _error("Invalid GeoJSON object: GeoJSON object must be one of \"Point\", \"LineString\", \"Polygon\", \"MultiPolygon\", \"Feature\", \"FeatureCollection\" or \"GeometryCollection\".");
	
	}
	
	return obj;
	
};

index.html

<!DOCTYPE html>
<html>
  <head>
    <style type="text/css">
      html, body, #map_canvas { height: 100%; margin: 0;}
    </style>
    <script type="text/javascript" src="https://maps.googleapis.com/maps/api/js?sensor=false"></script>
    <script src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.1/jquery.min.js"></script>
    <script src="http://d3js.org/topojson.v1.min.js"></script> 
    <script src="GeoJSON.js"></script>
    <script>

      initialize = function(){
        var mapOptions = {
          center: new google.maps.LatLng(35, -105),
          zoom: 4,
          mapTypeId: google.maps.MapTypeId.ROADMAP
        };

        var map = new google.maps.Map(document.getElementById("map_canvas"),
            mapOptions);

	$.getJSON('http://bl.ocks.org/d/4554134/us_counties.json', function (data) {
	    var county_geojson = topojson.feature(data, data.objects.counties),
		state_geojson = topojson.feature(data, data.objects.states);

	    var style = {
		strokeColor: "#FFFF00",
		strokeWeight: 1,
		strokeOpacity: 0.45,
		fillOpacity: 0.1,
	    };
	    var counties = new GeoJSON(county_geojson, style);

            //restyle for states
            style.strokeWeight = 2;
            style.strokeColor = 'black';
	    var states = new GeoJSON(state_geojson, style);

            addGeometries(counties);
            addGeometries(states);
	});

        addGeometries = function(overlays, style){
          $.each(overlays, function(i, overlay){
            if (!overlay.length) {
		overlay.setMap(map);
            } else {
              // recurse for multigeoms
	      addGeometries(overlay)
            }
          })
        }
      }
    </script>
  </head>
  <body onload="initialize()">
    <div id="map_canvas" style="width:100%; height:100%"></div>
  </body>
</html>