US Counties

The json in this example is not projected, aka it uses a WGS84 spatial reference system and thus is measured in latitudes and longitudes and able to be projected by d3.projection.

The composite nature of the AlbersUsa projects Hawaii and Alaska differently than the rest of the country.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
<style>
	.boundary {
			fill:none;
			stroke: white;
			stroke-width: 1px;
		}
</style>	
	<script src="https://d3js.org/d3.v4.min.js"></script>
	<script src="https://d3js.org/d3-geo-projection.v1.min.js"></script>
	<script src="https://d3js.org/topojson.v1.min.js"></script>	
</head>
<body>
<script type="text/javascript">

var width = 960,
	height = 450;

var svg = d3.select("body").append("svg")
	.attr("width", width)
	.attr("height", height);

var projection = d3.geoAlbersUsa().scale(700);

var path = d3.geoPath().projection(projection);

var g = svg.append("g");
		
 d3.json("counties.json", function(error, counties) {

var land = g.insert("path", ".land")
		.datum(topojson.feature(counties, counties.objects.counties))
		.attr("class", "land")
		.attr("d", path);

g.insert("path", ".boundary")
		.datum(topojson.mesh(counties, counties.objects.counties, function(a, b) { return a !== b; }))
		.attr("class", "boundary")
		.attr("d", path);

});

</script>
</body>
</html>

https://d3js.org/d3.v4.min.js

https://d3js.org/d3-geo-projection.v1.min.js

https://d3js.org/topojson.v1.min.js