diff --git a/src/ol/source/Stamen.js b/src/ol/source/Stamen.js
index 7434c70b47..d15cfe7933 100644
--- a/src/ol/source/Stamen.js
+++ b/src/ol/source/Stamen.js
@@ -5,58 +5,23 @@ import {inherits} from '../index.js';
 import OSM from '../source/OSM.js';
 import XYZ from '../source/XYZ.js';
 
-/**
- * @classdesc
- * Layer source for the Stamen tile server.
- *
- * @constructor
- * @extends {ol.source.XYZ}
- * @param {olx.source.StamenOptions} options Stamen options.
- * @api
- */
-const Stamen = function(options) {
-  const i = options.layer.indexOf('-');
-  const provider = i == -1 ? options.layer : options.layer.slice(0, i);
-  const providerConfig = Stamen.ProviderConfig[provider];
-
-  const layerConfig = Stamen.LayerConfig[options.layer];
-
-  const url = options.url !== undefined ? options.url :
-    'https://stamen-tiles-{a-d}.a.ssl.fastly.net/' + options.layer +
-      '/{z}/{x}/{y}.' + layerConfig.extension;
-
-  XYZ.call(this, {
-    attributions: Stamen.ATTRIBUTIONS,
-    cacheSize: options.cacheSize,
-    crossOrigin: 'anonymous',
-    maxZoom: options.maxZoom != undefined ? options.maxZoom : providerConfig.maxZoom,
-    minZoom: options.minZoom != undefined ? options.minZoom : providerConfig.minZoom,
-    opaque: layerConfig.opaque,
-    reprojectionErrorThreshold: options.reprojectionErrorThreshold,
-    tileLoadFunction: options.tileLoadFunction,
-    url: url,
-    wrapX: options.wrapX
-  });
-};
-
-inherits(Stamen, XYZ);
-
 
 /**
  * @const
  * @type {Array.<string>}
  */
-Stamen.ATTRIBUTIONS = [
+const ATTRIBUTIONS = [
   'Map tiles by <a href="https://stamen.com/">Stamen Design</a>, ' +
         'under <a href="https://creativecommons.org/licenses/by/3.0/">CC BY' +
         ' 3.0</a>.',
   OSM.ATTRIBUTION
 ];
 
+
 /**
  * @type {Object.<string, {extension: string, opaque: boolean}>}
  */
-Stamen.LayerConfig = {
+const LayerConfig = {
   'terrain': {
     extension: 'jpg',
     opaque: true
@@ -103,10 +68,11 @@ Stamen.LayerConfig = {
   }
 };
 
+
 /**
  * @type {Object.<string, {minZoom: number, maxZoom: number}>}
  */
-Stamen.ProviderConfig = {
+const ProviderConfig = {
   'terrain': {
     minZoom: 4,
     maxZoom: 18
@@ -120,4 +86,42 @@ Stamen.ProviderConfig = {
     maxZoom: 16
   }
 };
+
+
+/**
+ * @classdesc
+ * Layer source for the Stamen tile server.
+ *
+ * @constructor
+ * @extends {ol.source.XYZ}
+ * @param {olx.source.StamenOptions} options Stamen options.
+ * @api
+ */
+const Stamen = function(options) {
+  const i = options.layer.indexOf('-');
+  const provider = i == -1 ? options.layer : options.layer.slice(0, i);
+  const providerConfig = ProviderConfig[provider];
+
+  const layerConfig = LayerConfig[options.layer];
+
+  const url = options.url !== undefined ? options.url :
+    'https://stamen-tiles-{a-d}.a.ssl.fastly.net/' + options.layer +
+      '/{z}/{x}/{y}.' + layerConfig.extension;
+
+  XYZ.call(this, {
+    attributions: ATTRIBUTIONS,
+    cacheSize: options.cacheSize,
+    crossOrigin: 'anonymous',
+    maxZoom: options.maxZoom != undefined ? options.maxZoom : providerConfig.maxZoom,
+    minZoom: options.minZoom != undefined ? options.minZoom : providerConfig.minZoom,
+    opaque: layerConfig.opaque,
+    reprojectionErrorThreshold: options.reprojectionErrorThreshold,
+    tileLoadFunction: options.tileLoadFunction,
+    url: url,
+    wrapX: options.wrapX
+  });
+};
+
+inherits(Stamen, XYZ);
+
 export default Stamen;