Merging TopoJSON states d3.v4 topojson 2.2

This demonstrates using topojson.merge, a new feature in TopoJSON 2.2, to merge multiple polygons into a single polygon whilst removing interior borders. (An alternative approach is to draw the polygons twice.)

Unlike mbostock's original example, this uses a pre-projected topojson file and a null projection.

forked from almccon's block: Merging TopoJSON states

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

.state {
  fill: #ccc;
}

.state-boundary {
  fill: none;
  stroke: #fff;
}

.state.selected {
  fill: orange;
  stroke: #000;
}

</style>
<body>
<script src="https://d3js.org/d3.v4.min.js"></script>
<script src="https://d3js.org/topojson.v2.min.js"></script>
<script>

var width = 1024,
    height = 768;

var path = d3.geoPath()
    .projection(null);  // topojson file is already projected

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

var selected = d3.set([
  'AK', 'YT', 'BC', 'WA', 'OR', 'CA', 'NY'
]);

d3.json("https://gist.githubusercontent.com/almccon/fb125b016b5c9afad99b/raw/23f4acccdf42487c99a10f8fa21e8be70ae05979/ne_50m_usa_canada.topojson", function(error, us) {

console.log(us);

svg.append("path")
      .datum(topojson.feature(us, us.objects.usa_canada))
      .attr("class", "state")
      .attr("d", path);

svg.append("path")
      .datum(topojson.mesh(us, us.objects.usa_canada, function(a, b) { return a !== b; }))
      .attr("class", "state-boundary")
      .attr("d", path);

svg.append("path")
      .datum(topojson.merge(us, us.objects.usa_canada.geometries.filter(function(d) { return selected.has(d.id); })))
      .attr("class", "state selected")
      .attr("d", path);
});

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

</script>
<script>
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  m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m)
  })(window,document,'script','https://www.google-analytics.com/analytics.js','ga');

ga('create', 'UA-93898246-1', 'auto');
  ga('send', 'pageview');

</script>
</body>

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

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