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              <svg viewBox="0 0 960 600">
  <!--  we'll put SVG <path> elements here with D3js -->
</svg>

<p>
  Blog post:
  <a href="https://petrichor.studio/2018/05/21/get-started-creating-d3-maps/" target="_blank">
    "Get Started Creating D3 Maps"
  </a>
</p>

<p>
  Canvas version:
  <a href="https://codepen.io/jwasilgeo/pen/mLawGL" target="_blank">
    https://codepen.io/jwasilgeo/pen/mLawGL
  </a>
</p>

<script src="https://d3js.org/d3.v5.min.js"></script>
<script src="https://unpkg.com/topojson-client@3.0.0/dist/topojson-client.min.js"></script>
            
          
!
            
              body {
  margin: 5px;
  text-align: center;
  background-color: rgb(102, 102, 204);
}

svg {
  max-width: 960px;
  max-height: calc(80vh);
  outline: 0.75px solid white;
  stroke-linejoin: round;
  stroke-linecap: round;
}

path.feature {
  stroke-width: 0.75px;
  stroke: white;
  fill: none;
}

p, a {
  color: white;
  font-family: sans-serif;
  font-size: 0.95rem;
}

            
          
!
            
              // make D3 aware of the <svg> element in the HTML
var svg = d3.select("svg");

// get <svg> width and height from HTML instead of hard-coding values
var svgWidth = +svg.attr("viewBox").split(" ")[2],
  svgHeight = +svg.attr("viewBox").split(" ")[3];

// define the map projection
// we're going from spherical latitude/longitude coordinates to a flat map
var projection = d3
  // use Mercator for now, sigh, because of web map ubiquity...
  .geoMercator()
  // shift the projection translation offset to half of the parent <svg> width and height
  .translate([svgWidth / 2, svgHeight / 2])
  // [0, 0] is redundant, but you could set the initial center [longitude, latitude]
  .center([0, 0])
  // set an initial scale value
  .scale(svgWidth / (2 * Math.PI));

// NOTE: if you want to skip setting the translate and scale values
// and instead zoom out to the whole world, you can just use this shortcut:
// projection.fitSize([svgWidth, svgHeight], {
//   type: "Sphere"
// });

// "d3.geoPath" helps convert from geojson coordinates to <svg> or <canvas> drawing instructions
// it needs to know about the projection that was defined above

// I like to refer to it as a "geographic path generator" in the code
var geoPathGenerator = d3.geoPath().projection(projection);

// request publicly available and hosted world topojson data,
// convert it to geojson,
// and then draw it in the <svg>
d3
  .json("https://unpkg.com/world-atlas@1.1.4/world/50m.json")
  .then(function(loadedTopoJson) {
    // use the concept of a topojson "mesh" to convert topojson
    // to a single, efficient geojson geometry
    var geoJson = topojson.mesh(
      loadedTopoJson,
      loadedTopoJson.objects.countries
    );

    // and then draw the single geojson representing all country polygons
    // as a single <path> element within the <svg> parent element
    // (1 geojson geometry results in only 1 <path> element)
    svg
      .append("path")
      .attr("d", geoPathGenerator(geoJson))
      .attr("class", "feature");

    // IMPORTANT NOTE:
    // an alternate and very common approach is to convert the topojson to a geojson feature collection,
    // where each country polygon is represented by an individual geojson feature
    // and thuis each geojson feature is drawn as a unique <path> element within the <svg> parent element
    // (many geojson features result in many <path> elements)

    // WHY?
    // this is useful for thematic mapping

    // HOW?
    // you can uncomment the code below and try it out

    // var geoJson = topojson.feature(loadedTopoJson, loadedTopoJson.objects.land);

    // svg
    //   .selectAll("path")
    //   .data(geoJson.features)
    //   .enter()
    //   .append("path")
    //   .attr("d", geoPathGenerator)
    //   .attr("class", "feature");
  });

            
          
!
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