title: How to identify Leaflet's Marker during a `popupopen` event?
body: <p>when a marker is clicked, I need to execute some code that finds the <code>id</code> corresponding to the marker being clicked , retrieves data from backend API, then adds the newly retrieved data to the <code>content</code> of the popup that will open.</p>
  
  <p>The only way that is able to listen to a click event on the marker is </p>
  
  <pre><code>map.on('popupopen', function(e){
      // How to retrieve marker?
      // eg: Assign an id on creation, retrieve it now during popupopen
  };)
  </code></pre>
  
  <p>How can I find out which marker this is? Is it possible to add an <code>id</code> attribute to each marker, then retrieve this <code>id</code> during the <code>popupopen</code> event?</p>