Mapbox.js choropleth example

<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>Choropleth</title>

<div id='map'></div>
<style>
.map-legend .swatch {
  width:20px;
  height:20px;
  float:left;
  margin-right:10px;
  }
.leaflet-popup-close-button {
  display: none;
  }
.leaflet-popup-content-wrapper {
  pointer-events: none;
  }
</style>
<script src='us-states.js'></script>
<script>
  var mapdataviz = L.mapbox.map('map', 'examples.map-9ijuk24y')
    .setView([37.8, -96], 4);

var popup = new L.Popup({ autoPan: false });

// statesData comes from the 'us-states.js' script included above
  var statesLayer = L.geoJson(statesData,  {
      style: getStyle,
      onEachFeature: onEachFeature
  }).addTo(mapdataviz);

function getStyle(feature) {
      return {
          weight: 2,
          opacity: 0.1,
          color: 'black',
          fillOpacity: 0.7,
          fillColor: getColor(feature.properties.density)
      };
  }

// get color depending on population density value
  function getColor(d) {
      return d > 1000 ? '#003E99' :
          d > 500  ? '#0058B3' :
          d > 200  ? '#0071CC' :
          d > 100  ? '#0A8BE6' :
          d > 50   ? '#23A4FF' :
          d > 20   ? '#3DBEFF' :
          d > 10   ? '#56D7FF' :
          '#70F1FF';
  }

function onEachFeature(feature, layer) {
      layer.on({
          mousemove: mousemove,
          mouseout: mouseout,
          click: zoomToFeature
      });
  }

var closeTooltip;

function mousemove(e) {
      var layer = e.target;

popup.setLatLng(e.latlng);
      popup.setContent('<div class="marker-title">' + layer.feature.properties.name + '</div>' +
          layer.feature.properties.density + ' people per square mile');

if (!popup._map) popup.openOn(mapdataviz);
      window.clearTimeout(closeTooltip);

// highlight feature
      layer.setStyle({
          weight: 3,
          opacity: 0.3,
          fillOpacity: 0.9
      });

if (!L.Browser.ie && !L.Browser.opera) {
          layer.bringToFront();
      }
  }

function mouseout(e) {
      statesLayer.resetStyle(e.target);
      closeTooltip = window.setTimeout(function() {
          mapdataviz.closePopup();
      }, 100);
  }

function zoomToFeature(e) {
      mapdataviz.fitBounds(e.target.getBounds());
  }

mapdataviz.legendControl.addLegend(getLegendHTML());

function getLegendHTML() {
    var grades = [0, 10, 20, 50, 100, 200, 500, 1000],
    labels = [],
    from, to;

for (var i = 0; i < grades.length; i++) {
      from = grades[i];
      to = grades[i + 1];

labels.push(
        '<li><span class="swatch" style="background:' + getColor(from + 1) + '"></span> ' +
        from + (to ? '&ndash;' + to : '+')) + '</li>';
    }

return '<span>People per square mile</span><ul>' + labels.join('') + '</ul>';
  }
</script>

</body>
</html>

https://api.tiles.mapbox.com/mapbox.js/v1.6.1/mapbox.js