ArcGIS API Javascript Server creating a dropdown menu

I am trying to rebuild this ESRI example with my own feature layers: https://developers.arcgis.com/javascript/jssamples/dataadapterfeaturelayer.html

Since I just want a dropdown menu existing out of the 5 layers I assume my script is going to be a simplified version of the example I have given above. I do succeed in visualizing my data and making the dropdown menu. I however do not succeed in making the dropdown menu active. I do not know where I am going wrong, I think I do not have enough understanding of the fundamentels of Javascript to put this together. Therefore I am asking it here.

Where am I going wrong? Or do you have any tips for me to continue in a more effective way?

What I made so far:

var map;
require(["esri/map","esri/layers/FeatureLayer", "dojo/domReady!", "esri/InfoTemplate"],
    function(Map, FeatureLayer) { 
        map = new Map("mapDiv", {
        center: [ 5.5, 52.485],
        zoom: 08,
        basemap: "satellite"});  

        map.on("load", function() {
           query("#lplist").on("change", function(e) {
             var value = e.currentTarget.value;

             // Remove previously added layer
             if (layer) {map.removeLayer(layer);
                layer = null;
                 }

        switch (value) {
          case "Nederland":
          layer = "http://.../MapServer/0";
          case "Provincies":
          layer = "http://.../MapServer/1";
          case "Gemeenten":
          layer = "http://.../MapServer/2";
          case "Wijken":
          layer = "http://.../MapServer/3";
          case "Buurten":
          layer = "http://.../MapServer/4";
          break;
        }))

        if (layer) {
          featureLayer = new FeatureLayer(layer,{
          mode: FeatureLayer.MODE_ONDEMAND,
          outFields: ["*"],
          });
          map.addLayer(featureLayer);
          map.infoWindow.resize(155,75);
        };

        }}}); 

Underneath I am calling the above script (which is saved as a .js file) from a HTML file. I am under the understanding nothing is going wrong here, since it is exactly the same code as in the example.

<div id="map" data-dojo-type="dijit.layout.ContentPane" data-dojo-pros="region:'center'">
<div id="lppanel" class="roundedCorners">
  <table>
    <tr>
      <td style="padding:5px;">
        <div style="font-size: 16pt; font-weight:bold;">
         Voeg het gewenste schaalniveau toe aan de kaart:
        </div>
        <div style="padding:10px;">
          <select id="lplist">
            <option value="choose" selected="selected">(Selecteer een optie)</option>
            <option value="Nederland">Nederland</option>
            <option value="Provincies">Provincies</option>
            <option value="Gemeenten">Gemeenten</option>
            <option value="Wijken">Wijken</option>
            <option value="Buurten">Buurten</option>
          </select>
        </div>
      </td>
    </tr>
  </table>
</div>

  <script src=map_.js>
  </script>