leaflet markercluster doesn't display

Question

I've made a markeckuster to show some address points on my leaflet map and make them cluster:

var addressPoints = [
      [-37.8210922667, 175.2209316333, "2"],
      [-37.8210819833, 175.2213903167, "3"],
      [-37.8210881833, 175.2215004833, "3A"],
      [-37.8211946833, 175.2213655333, "1"],
      [-37.8209458667, 175.2214051333, "5"],
      [-37.8208292333, 175.2214374833, "7"],
      [-37.8325816, 175.2238798667, "537"],
      [-37.8315855167, 175.2279767, "454"],
      [-37.8096336833, 175.2223743833, "176"],
      [-37.80970685, 175.2221815833, "178"],
      [-37.8102146667, 175.2211562833, "190"],
      [-37.8088037167, 175.2242227, "156"],
      [-37.8112330167, 175.2193425667, "210"],
      [-37.8116368667, 175.2193005167, "212"],
      [-37.80812645, 175.2255449333, "146"],
      [-37.8080231333, 175.2286383167, "125"],
      [-37.8089538667, 175.2222222333, "174"],
      [-37.8080905833, 175.2275400667, "129"]
    ]
    for (var i=0; i<addressPoints.length ; i++){
      var a = addressPoints[i];
      var marker = L.marker(new L.latLng( a[0], a[1]));
    }
    var markerClusters = L.markerClusterGroup();
    markerClusters.addLayer(marker);
    map.addLayer(markerClusters);

and for the cdn, I added these:

<link rel="stylesheet" href="https://unpkg.com/leaflet@1.3.1/dist/leaflet.css" integrity="sha512-Rksm5RenBEKSKFjgI3a41vrjkw4EVPlJ3+OiI65vTjIdo9brlAacEuKOiQ5OFh7cOI1bkDwLqdLw3Zg0cRJAAQ==" crossorigin="" />
<link rel="stylesheet" href="https://unpkg.com/leaflet.markercluster@1.3.0/dist/MarkerCluster.css" />
<link rel="stylesheet" href="https://unpkg.com/leaflet.markercluster@1.3.0/dist/MarkerCluster.Default.css" />

<script src="https://unpkg.com/leaflet@1.3.1/dist/leaflet.js" integrity="sha512-/Nsx9X4HebavoBvEBuyp3I7od5tA0UzAxs+j83KgC8PU0kgB4XiK4Lfe4y4cgBtaRJQEIFCW+oC506aPT2L1zw==" crossorigin=""></script>
<script src="https://unpkg.com/leaflet.markercluster@1.3.0/dist/leaflet.markercluster.js"></script>

but it still won't work... there's nothing displaying on my map

Answer 1

You need to add every marker to the group and not only the last one:

var markerClusters = L.markerClusterGroup().addTo(map);
for (var i=0; i<addressPoints.length ; i++){
      var a = addressPoints[i];
      var marker = L.marker(new L.latLng( a[0], a[1])).addTo(markerClusters);
}

Answer 2

 var addressPoints = [ [-37.8210922667, 175.2209316333, "2"], [-37.8210819833, 175.2213903167, "3"], [-37.8210881833, 175.2215004833, "3A"], [-37.8211946833, 175.2213655333, "1"], [-37.8209458667, 175.2214051333, "5"], [-37.8208292333, 175.2214374833, "7"], [-37.8325816, 175.2238798667, "537"], [-37.8315855167, 175.2279767, "454"], [-37.8096336833, 175.2223743833, "176"], [-37.80970685, 175.2221815833, "178"], [-37.8102146667, 175.2211562833, "190"], [-37.8088037167, 175.2242227, "156"], [-37.8112330167, 175.2193425667, "210"], [-37.8116368667, 175.2193005167, "212"], [-37.80812645, 175.2255449333, "146"], [-37.8080231333, 175.2286383167, "125"], [-37.8089538667, 175.2222222333, "174"], [-37.8080905833, 175.2275400667, "129"] ] var tiles = L.tileLayer('//{s}.tile.openstreetmap.org/{z}/{x}/{y}.png'); var map = L.map('map', { center: L.latLng(-37.8080, 175.2275), zoom: 7, layers: [tiles] }); var mcg = L.markerClusterGroup(); for (var i = 0; i < addressPoints.length; i++) { var a = addressPoints[i]; var title = a[2]; var marker = L.marker([a[0], a[1]]); mcg.addLayer(marker); } map.addLayer(mcg);

 #map { height: 250px; }

 <link href="https://unpkg.com/leaflet.markercluster@1.3.0/dist/MarkerCluster.Default.css" rel="stylesheet"/> <link href="https://unpkg.com/leaflet.markercluster@1.3.0/dist/MarkerCluster.css" rel="stylesheet"/> <link href="https://unpkg.com/leaflet@1.3.1/dist/leaflet.css" rel="stylesheet"/> <script src="https://unpkg.com/leaflet@1.3.1/dist/leaflet.js"></script> <script src="https://unpkg.com/leaflet.markercluster@1.3.0/dist/leaflet.markercluster.js"></script> <div id="map"></div>

Here is the working snippet with your code. You have to add every marker to the cluster group.