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Non-exclusive markers

<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>Non-exclusive markers</title>
  <script src='http://api.tiles.mapbox.com/mapbox.js/v0.6.7/mapbox.js'></script>
  <link href='http://api.tiles.mapbox.com/mapbox.js/v0.6.7/mapbox.css' rel='stylesheet' />
  <style>
    body { margin:0; padding:0; }
    #map { position:absolute; top:0; bottom:0; width:100%; }
  </style>
</head>
<body>
<style>
 #filters {
    position:absolute;
    top:10px;left:10px;
    z-index:100;
    padding:5px;
    background:#fff;
    }
#filters ul {
    list-style-type:none;
    margin:0;
    padding:0;
}
</style>
<div id='map-ui'>
  <form id='filters'>
    <ul id='options'>
        <li><input type='checkbox' checked=checked class='filter'
            name='filter' id='fast-food' value='fast-food' /><label for='fast-food'>fast-food</label></li>
        <li><input type='checkbox' checked=checked class='filter'
            name='filter' id='bicycle' value='bicycle' /><label for='bicycle'>bicycle</label></li>
        <li><input type='checkbox' checked=checked class='filter'
            name='filter' id='bar' value='bar' /><label for='bar'>bar</label></li>
    </ul>
  </form>
</div>
<div id='map'></div>
<script>
  var map = mapbox.map('map');

  map.addLayer(mapbox.layer().id('examples.map-zr0njcqy'));

  var markersLayer = mapbox.markers.layer().id('examples.map-zr0njcqy', function() {

      // This code is in the second callback to id because it depends
      // on the features being loaded asynchronously. If it were put right
      // after the call to .id(), the markers layer wouldn't contain any
      // data - it would still be on its way from MapBox to the browser.

      // Cinch the map display to show all markers
      map.extent(markersLayer.extent());

      var filters = document.getElementById('filters');
      var checkboxes = document.getElementsByClassName('filter');

      function change(e) {
          // Find all checkboxes that are checked and build a list of their values
          var on = [];
          for (var i = 0; i < checkboxes.length; i++) {
              if (checkboxes[i].checked) on.push(checkboxes[i].value);
          }
          // The filter function takes a GeoJSON feature object
          // and returns true to show it or false to hide it.
          markersLayer.filter(function(f) {
              // check each marker's symbol to see if its value is in the list
              // of symbols that should be on, stored in the 'on' array
              return on.indexOf(f.properties['marker-symbol']) !== -1;
          });
          return false;
      }

      // When the form is touched, re-filter markers
      filters.onchange = change;

      // Initially filter the markers
      change();

  });

  map.addLayer(markersLayer);

  // Attribute map
  map.ui.attribution.add()
      .content('<a href="http://mapbox.com/about/maps">Terms &amp; Feedback</a>');
</script>
The code and documentation to mapbox.js is hosted on GitHub where you can contribute changes and improvements.