Border Mesh

This example uses topojson.mesh to highlight California’s borders with its three neighboring states.

<!DOCTYPE html>
<svg width="960" height="600" fill="none" stroke-linejoin="round" stroke-linecap="round"></svg>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="https://d3js.org/topojson.v2.min.js"></script>
<script>

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

var path = d3.geoPath();

d3.json("https://d3js.org/us-10m.v1.json", function(error, us) {
  if (error) throw error;

svg.append("g")
      .attr("fill", "#000")
    .selectAll("path")
    .data(topojson.feature(us, us.objects.states).features)
    .enter().append("path")
      .attr("d", path)
    .append("title")
      .text(function(d) { return d.id; });

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

// California-Nevada border
  svg.append("path")
      .attr("stroke", "cyan")
      .attr("stroke-width", 2.5)
      .attr("d", path(topojson.mesh(us, us.objects.states, border("06", "32"))));

// California-Arizona border
  svg.append("path")
      .attr("stroke", "magenta")
      .attr("stroke-width", 2.5)
      .attr("d", path(topojson.mesh(us, us.objects.states, border("06", "04"))));

// California-Oregon border
  svg.append("path")
      .attr("stroke", "yellow")
      .attr("stroke-width", 2.5)
      .attr("d", path(topojson.mesh(us, us.objects.states, border("06", "41"))));
});

function border(id0, id1) {
  return function(a, b) {
    return a.id === id0 && b.id === id1
        || a.id === id1 && b.id === id0;
  };
}

</script>

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

https://d3js.org/topojson.v2.min.js