Merging Counties by mbostock

index.html

<!DOCTYPE html>
<meta charset="utf-8">
<style>

path {
  stroke-linejoin: round;
  stroke-linecap: round;
}

.land {
  fill: #bbb;
}

.border {
  fill: none;
}

.border--static {
  stroke: #fff;
}

</style>
<body>
<script src="//d3js.org/d3.v3.min.js"></script>
<script src="//d3js.org/topojson.v1.min.js"></script>
<script>

var width = 960,
    height = 600;

var projection = d3.geo.albersUsa()
    .scale(1280)
    .translate([width / 2, height / 2]);

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

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

d3.json("/mbostock/raw/4090846/us.json", function(error, us) {
  if (error) throw error;

  var delay = 0;

  svg.append("path")
      .datum(topojson.merge(us, us.objects.counties.geometries))
      .attr("class", "land")
      .attr("d", path);

  var gStatic = svg.append("g")
      .attr("class", "border border--static");

  var gAnimate = svg.append("g")
      .attr("class", "border border--animate");

  gAnimate.selectAll("path")
      .data(d3.nest()
        .key(function(d) { return d.id / 1000 | 0; })
        .sortKeys(function(a, b) { return a - b; })
        .entries(us.objects.counties.geometries))
    .enter().append("path")
      .attr("d", function(d) { return path(topojson.merge(us, d.values)); })
      .each(function(d) { d.duration = (d.length = this.getTotalLength()) * 3; })
      .style("stroke-width", "1.5px")
      .style("stroke-dasharray", function(d) { return "0," + d.length; })
    .transition()
      .delay(function(d, i) { var delay1 = delay; delay += d.duration; return delay1; })
      .duration(function(d) { return d.duration; })
      .ease("linear")
      .each("start", function() { this.style.stroke = "#000"; })
      .style("stroke-dasharray", function(d) { return d.length + "," + d.length; })
      .each("end", function(d) {
        var id = +d.key;
        gStatic.append("path")
            .datum(topojson.mesh(us, us.objects.counties, function(a, b) { return a.id / 1000 ^ b.id / 1000 && ((a.id / 1000 | 0) === id || (b.id / 1000 | 0) === id); }))
            .attr("d", path);
      })
    .transition()
      .duration(750)
      .style("stroke-opacity", 0)
      .remove();
});

d3.select(self.frameElement).style("height", height + "px");

</script>