diff --git a/src/ol/source/TileDebug.js b/src/ol/source/TileDebug.js
index c9424f29a5..ba10e7dce7 100644
--- a/src/ol/source/TileDebug.js
+++ b/src/ol/source/TileDebug.js
@@ -9,6 +9,72 @@ import {toSize} from '../size.js';
 import TileSource from '../source/Tile.js';
 import _ol_tilecoord_ from '../tilecoord.js';
 
+
+/**
+ * @constructor
+ * @extends {ol.Tile}
+ * @param {ol.TileCoord} tileCoord Tile coordinate.
+ * @param {ol.Size} tileSize Tile size.
+ * @param {string} text Text.
+ */
+const LabeledTile = function(tileCoord, tileSize, text) {
+
+  Tile.call(this, tileCoord, TileState.LOADED);
+
+  /**
+   * @private
+   * @type {ol.Size}
+   */
+  this.tileSize_ = tileSize;
+
+  /**
+   * @private
+   * @type {string}
+   */
+  this.text_ = text;
+
+  /**
+   * @private
+   * @type {HTMLCanvasElement}
+   */
+  this.canvas_ = null;
+
+};
+inherits(LabeledTile, Tile);
+
+
+/**
+ * Get the image element for this tile.
+ * @return {HTMLCanvasElement} Image.
+ */
+LabeledTile.prototype.getImage = function() {
+  if (this.canvas_) {
+    return this.canvas_;
+  } else {
+    const tileSize = this.tileSize_;
+    const context = createCanvasContext2D(tileSize[0], tileSize[1]);
+
+    context.strokeStyle = 'black';
+    context.strokeRect(0.5, 0.5, tileSize[0] + 0.5, tileSize[1] + 0.5);
+
+    context.fillStyle = 'black';
+    context.textAlign = 'center';
+    context.textBaseline = 'middle';
+    context.font = '24px sans-serif';
+    context.fillText(this.text_, tileSize[0] / 2, tileSize[1] / 2);
+
+    this.canvas_ = context.canvas;
+    return context.canvas;
+  }
+};
+
+
+/**
+ * @override
+ */
+LabeledTile.prototype.load = function() {};
+
+
 /**
  * @classdesc
  * A pseudo tile source, which does not fetch tiles from a server, but renders
@@ -42,82 +108,18 @@ inherits(TileDebug, TileSource);
 TileDebug.prototype.getTile = function(z, x, y) {
   const tileCoordKey = _ol_tilecoord_.getKeyZXY(z, x, y);
   if (this.tileCache.containsKey(tileCoordKey)) {
-    return /** @type {!ol.source.TileDebug.Tile_} */ (this.tileCache.get(tileCoordKey));
+    return /** @type {!ol.source.LabeledTile} */ (this.tileCache.get(tileCoordKey));
   } else {
     const tileSize = toSize(this.tileGrid.getTileSize(z));
     const tileCoord = [z, x, y];
     const textTileCoord = this.getTileCoordForTileUrlFunction(tileCoord);
     const text = !textTileCoord ? '' :
       this.getTileCoordForTileUrlFunction(textTileCoord).toString();
-    const tile = new TileDebug.Tile_(tileCoord, tileSize, text);
+    const tile = new LabeledTile(tileCoord, tileSize, text);
     this.tileCache.set(tileCoordKey, tile);
     return tile;
   }
 };
 
 
-/**
- * @constructor
- * @extends {ol.Tile}
- * @param {ol.TileCoord} tileCoord Tile coordinate.
- * @param {ol.Size} tileSize Tile size.
- * @param {string} text Text.
- * @private
- */
-TileDebug.Tile_ = function(tileCoord, tileSize, text) {
-
-  Tile.call(this, tileCoord, TileState.LOADED);
-
-  /**
-   * @private
-   * @type {ol.Size}
-   */
-  this.tileSize_ = tileSize;
-
-  /**
-   * @private
-   * @type {string}
-   */
-  this.text_ = text;
-
-  /**
-   * @private
-   * @type {HTMLCanvasElement}
-   */
-  this.canvas_ = null;
-
-};
-inherits(TileDebug.Tile_, Tile);
-
-
-/**
- * Get the image element for this tile.
- * @return {HTMLCanvasElement} Image.
- */
-TileDebug.Tile_.prototype.getImage = function() {
-  if (this.canvas_) {
-    return this.canvas_;
-  } else {
-    const tileSize = this.tileSize_;
-    const context = createCanvasContext2D(tileSize[0], tileSize[1]);
-
-    context.strokeStyle = 'black';
-    context.strokeRect(0.5, 0.5, tileSize[0] + 0.5, tileSize[1] + 0.5);
-
-    context.fillStyle = 'black';
-    context.textAlign = 'center';
-    context.textBaseline = 'middle';
-    context.font = '24px sans-serif';
-    context.fillText(this.text_, tileSize[0] / 2, tileSize[1] / 2);
-
-    this.canvas_ = context.canvas;
-    return context.canvas;
-  }
-};
-
-
-/**
- * @override
- */
-TileDebug.Tile_.prototype.load = function() {};
 export default TileDebug;