Add on signatures for BaseObject and Observable.

src/ol/Object.js

@@ -87,6 +87,12 @@ class BaseObject extends Observable {
   constructor(opt_values) {
     super();
 
+    /***
+     * @type {import("./Observable").OnSignature<import("./Observable").EventTypes, import("./events/Event.js").default> &
+     *    import("./Observable").OnSignature<import("./ObjectEventType").Types, ObjectEvent>}
+     */
+    this.on;
+
     // Call {@link module:ol/util.getUid} to ensure that the order of objects' ids is
     // the same as the order in which they were created.  This also helps to
     // ensure that object properties are always added in the same order, which

src/ol/ObjectEventType.js

@@ -13,3 +13,7 @@ export default {
    */
   PROPERTYCHANGE: 'propertychange',
 };
+
+/**
+ * @typedef {'propertychange'} Types
+ */

src/ol/Observable.js

@@ -5,13 +5,16 @@ import EventTarget from './events/Target.js';
 import EventType from './events/EventType.js';
 import {listen, listenOnce, unlistenByKey} from './events.js';
 
-
 /***
  * @template {string} Type
  * @template {Event|import("./events/Event.js").default} EventClass
  * @typedef {(type: Type|Type[], listener: (event: EventClass) => (void|boolean)) => import("./events.js").EventsKey|Array<import("./events.js").EventsKey>} OnSignature
  */
 
+/**
+ * @typedef {'change'|'error'} EventTypes
+ */
+
 /**
  * @classdesc
  * Abstract base class; normally only used for creating subclasses and not
@@ -60,7 +63,7 @@ class Observable extends EventTarget {
    * @return {import("./events.js").EventsKey|Array<import("./events.js").EventsKey>} Unique key for the listener. If
    *     called with an array of event types as the first argument, the return
    *     will be an array of keys.
-   * @protected
+   * @api
    */
   on(type, listener) {
     if (Array.isArray(type)) {