title: Leaflet: TypeError L.markerClusterGroup is not a function
body: <p>I found this problem on stackoverflow but answers don't work in my application. </p>
  
  <p>If I tried <code>var cluster = L.markerClusterGroup()</code> I had error </p>
  
  <blockquote>
    <p><code>L.markerClusterGroup()</code> is not a function </p>
  </blockquote>
  
  <p>or <code>var cluster = new L.markerClusterGroup()</code> I had error </p>
  
  <blockquote>
    <p><code>L.markerClusterGroup()</code> is not a constructor</p>
  </blockquote>
  
  <p>I include css and script in my index.html file</p>
  
  <pre><code>&lt;link rel="stylesheet" href="https://unpkg.com/leaflet@1.5.1/dist/leaflet.css" integrity="sha512-xwE/Az9zrjBIphAcBb3F6JVqxf46+CDLwfLMHloNu6KEQCAWi6HcDUbeOfBIptF7tcCzusKFjFw2yuvEpDL9wQ==" crossorigin=""/&gt;
  &lt;script type="text/javascript" src="https://unpkg.com/leaflet@1.5.1/dist/leaflet.js" integrity="sha512-GffPMF3RvMeYyc1LWMHtK8EbPv0iNZ8/oTtHPx9/cc2ILxQ+u905qIwdpULaqDkyBKgOaB57QTMg7ztg8Jm2Og==" crossorigin=""&gt;&lt;/script&gt;
  
  &lt;!-- Cluster --&gt;
  &lt;link rel="stylesheet" href="https://unpkg.com/leaflet.markercluster@1.4.1/dist/MarkerCluster.css" /&gt;
  &lt;link rel="stylesheet" href="https://unpkg.com/leaflet.markercluster@1.4.1/dist/MarkerCluster.Default.css" /&gt;
  &lt;script type="text/javascript" src="https://unpkg.com/leaflet.markercluster@1.4.1/dist/leaflet.markercluster.js"&gt;&lt;/script&gt;
  </code></pre>