Counties File

<!DOCTYPE html>
<meta charset='utf-8'>
<title>US Counties</title>
<svg width="960" height="600" fill="none" stroke="#000" stroke-linejoin="round" stroke-linecap="round"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="https://unpkg.com/topojson-client@3"></script>
<script>

var svg = d3.select("svg");

var path = d3.geoPath();

d3.json("https://unpkg.com/us-atlas@1/us/10m.json", function(error, us) {
  if (error) throw error;

var counties = topojson.feature(us, us.objects.counties).features;

// Examine one of the county features in the developer console.
  // The "id" attribute is a 5-digit GEOID (state + county).
  // See: https://www.census.gov/geo/reference/geoidentifiers.html
  console.log(counties[0]);

svg.selectAll('path.county')
      .data( counties )
    .enter().append("path")
      .attr("d", path)
      .attr("stroke", "#aaa")
      .attr("stroke-width", 0.5);

svg.append("path")
      .attr("stroke-width", 0.5)
      .attr("d", path(topojson.mesh(us, us.objects.states, function(a, b) { return a !== b; })));

svg.append("path")
      .attr("d", path(topojson.feature(us, us.objects.nation)));
});

</script>

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

https://unpkg.com/topojson-client@3