Zoom Behavior (Scale)

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1">

<title>Zoom Behavior</title>

svg {
    background-color: #A7DBD8;
  }

.sphere {
    fill: none;
    stroke: #79A09E;
  }

.land {
    fill: #E0E4CC;
    stroke: #ACAF9F;
    stroke-width: 1;
  }

.graticule {
    fill: none;
    stroke: #79A09E;
    stroke-width: 1;
    stroke-dasharray: 1,2;
  }

.overlay {
    fill-opacity: 0;
  }
  </style>

// Set the dimensions of the map
    var width  = 960,
        height = 480;

// Create a selection for the container div and append the svg element
    var div = d3.select('#map-container'),
        svg = div.append('svg');

// Set the size of the SVG element
    svg.attr('width', width).attr('height', height);

// Create and configure a geographic projection
    var projection = d3.geo.equirectangular()
      .translate([width / 2, height / 2]);

// Create and configure a path generator
    var pathGenerator = d3.geo.path()
      .projection(projection);

// Create and configure the graticule generator (one line every 20 degrees)
    var graticule = d3.geo.graticule();

// Retrieve the geographic data asynchronously
    d3.json('land.geojson', function(err, data) {

// Throw errors on getting or parsing the file
      if (err) { throw err; }

// Create a group to hold the map-related features
      var grp = svg.selectAll('g.container').data([data]);

grp.enter().append('g').classed('container', true);
      grp.exit().remove();

// Shpere (map boundaries)
      var sphere = grp.selectAll('path.sphere').data([{type: 'Sphere'}]);

sphere.enter().append('path').classed('sphere', true);
      sphere.attr('d', pathGenerator);
      sphere.exit().remove();

// Graticule lines (behind the land)
      var lines = grp.selectAll('path.graticule').data([graticule()]);

lines.enter().append('path').classed('graticule', true);
      lines.attr('d', pathGenerator);
      lines.exit().remove();

// Land
      var land = grp.selectAll('path.land').data([data]);

land.enter().append('path').classed('land', true);
      land.attr('d', pathGenerator);
      land.exit().remove();

var strokeScale = d3.scale.linear()
        .domain([1, 8])
        .range([1, 1/8]);

// Create and configure the zoom behaviour
      var zoomBehavior = d3.behavior.zoom()
        .scaleExtent([1, 8])
        .on('zoom', function() {

// Get the zoom event scale and translate vector
          var scale = d3.event.scale,
              translate = d3.event.translate;

// Translate and scale the group
          grp.attr('transform', function() {

var s = 'scale(' + scale + ')',
                t = 'translate(' + translate + ')';

return t + s;
          });

// Adapts the stroke width to the zoom level
          svg.selectAll('path')
            .style('stroke-width', strokeScale(scale));
        });

// Bind the zoom behaviour to the SVG container
      svg.call(zoomBehavior);

});
  </script>

</body>
</html>

https://cdnjs.cloudflare.com/ajax/libs/d3/3.5.6/d3.min.js