D3 Examples - Utah - topojson merge

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<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
    <script src="https://d3js.org/d3-scale-chromatic.v1.min.js"></script>
  <script src="https://unpkg.com/topojson-server@3"></script>
  <script src="https://unpkg.com/topojson-client@3"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
  </style>
</head>
​
<body>
  <div id="choropleth"></div>
  <script>
    // Get target element's width and use aspect ratio to set height
var div_id  = "choropleth",
        width   = document.getElementById(div_id).clientWidth,
        height  = width*(2/3),
        center  = [-113.3010264, 39.7287941],
        // Set margins around rendered map
        margins = {"top": 0, "bottom": 0, "left": 0, "right": 0};
​
var projection = d3.geoMercator().scale(5000).center(center).translate([width/2.75, height/2.2]);
// Geo-paths take a GeoJSON geometry/feature object and generate an SVG path data string or render the path to a Canvas
var path = d3.geoPath().projection(projection);
​
function ready(error, utah, suicide_rates){
    if(error) console.error(error);
  
  var topology = topojson.topology({utah: utah});
  
  var districts = [
      "Bear River",
      "Central Utah",
      "Davis County",
      "Salt Lake County",
      "San Juan County",
      "Southeast",
      "Southwest",
      "Summit County",
      "Tooele County",
      "TriCounty",
      "Utah County",
      "Wasatch County",
      "Weber-Morgan"
    ]
  
  var districtsJSON = [];
  
  districts.forEach(function(district){
    console.log(district)
    let merged = topojson.merge(topology,           topology.objects.utah.geometries.filter(function(d) { 
      let countyData = suicide_rates.find(function(county){ return county.county == d.properties.name })
      let thisDistrict = countyData['health_district'];
      return thisDistrict == district; 
    })
  ); 
    districtsJSON.push(merged)
  })
​
​
    var min = d3.min(suicide_rates, function(d){ return +d.rate_per_100k; }),
            max = d3.max(suicide_rates, function(d){ return +d.rate_per_100k; });
​
    var color = d3.scaleQuantize()
                                .domain([min, max])
                                .range(d3.schemeReds[9]);
​
    function color_fill(data){
        var element = suicide_rates.find(function(element){ return element.county == data.properties.name; });
        data.rate = element.rate_per_100k;
        return color(data.rate);
    }
​
    // Select target element and attach <svg> and <g> elements
    var svg = d3.select("#" + div_id)
        .append("svg")
            // Set SVG element's top left corner and width/height attributes
            .attr("viewBox",margins.top + " " + margins.left + " " + (width - margins.right) + " " + (height - margins.bottom))
            // Supposed to make map responsive. Works sometimes.
            .attr("preserveAspectRatio", "xMidYMid meet")
            // Group together map paths and location markers    
        .append('g')
            .attr('class', div_id + "_group");
​
    // Group together country shape paths and enter data
    svg.append("g")
         .attr("class", div_id + "_counties")
         .selectAll("path")
         .data(utah.features)
         .enter()
         // Render and style map
         .append("path")
             .attr("d", path)
             .attr("stroke", "#fff")
             .attr("fill", function(d) { return color_fill(d); })
             .attr("stroke-width", "0.5px")
         .append("title")
             .text(function(d) { return d.rate + "%"; });
  
  svg.append("g")
         .attr("class", div_id + "_border")
         .selectAll("path")
         .data(districtsJSON)
        .enter()
        .append("path")
             .attr("d", path)
             .attr("stroke", "#fff")
             .attr("fill", "none")
             .attr("stroke-width", "3px")
   .attr("stroke", "black")
​
} // Close function ready(error, utah, suicide_rates){...
​
d3.queue()
    .defer(d3.json, "https://rawgit.com/becausealice2/FA-viz/master/suicide_rates/utah-counties.json")
    .defer(d3.csv, "https://rawgit.com/becausealice2/FA-viz/master/suicide_rates/suicide-by-health-district.csv")
.await(ready);
    
    
​
  </script>
</body>
​
    