[fix] Return null if the event handler is not set

Make the `onclose`, `onerror`, `onmessage`, and `onopen` getters return
`null` instead of `undefined` if the event handler is not set.

Author: lpinca

lib/websocket.js

@@ -124,31 +124,31 @@ class WebSocket extends EventEmitter {
    */
   /* istanbul ignore next */
   get onclose() {
-    return undefined;
+    return null;
   }
 
   /**
    * @type {Function}
    */
   /* istanbul ignore next */
   get onerror() {
-    return undefined;
+    return null;
   }
 
   /**
    * @type {Function}
    */
   /* istanbul ignore next */
   get onopen() {
-    return undefined;
+    return null;
   }
 
   /**
    * @type {Function}
    */
   /* istanbul ignore next */
   get onmessage() {
-    return undefined;
+    return null;
   }
 
   /**
@@ -528,7 +528,7 @@ Object.defineProperty(WebSocket.prototype, 'CLOSED', {
         if (listener[kForOnEventAttribute]) return listener[kListener];
       }
 
-      return undefined;
+      return null;
     },
     set(handler) {
       for (const listener of this.listeners(method)) {

test/websocket.test.js

@@ -2097,10 +2097,10 @@ describe('WebSocket', () => {
 
       const ws = new WebSocket('ws://localhost', { agent: new CustomAgent() });
 
-      assert.strictEqual(ws.onmessage, undefined);
-      assert.strictEqual(ws.onclose, undefined);
-      assert.strictEqual(ws.onerror, undefined);
-      assert.strictEqual(ws.onopen, undefined);
+      assert.strictEqual(ws.onmessage, null);
+      assert.strictEqual(ws.onclose, null);
+      assert.strictEqual(ws.onerror, null);
+      assert.strictEqual(ws.onopen, null);
 
       ws.onmessage = NOOP;
       ws.onerror = NOOP;
@@ -2114,7 +2114,7 @@ describe('WebSocket', () => {
 
       ws.onmessage = 'foo';
 
-      assert.strictEqual(ws.onmessage, undefined);
+      assert.strictEqual(ws.onmessage, null);
       assert.strictEqual(ws.listenerCount('onmessage'), 0);
     });
 
@@ -2149,7 +2149,7 @@ describe('WebSocket', () => {
       ws.on('open', NOOP);
 
       assert.deepStrictEqual(ws.listeners('open'), [NOOP]);
-      assert.strictEqual(ws.onopen, undefined);
+      assert.strictEqual(ws.onopen, null);
     });
 
     it("doesn't remove listeners added with `on`", () => {
@@ -2214,7 +2214,7 @@ describe('WebSocket', () => {
       assert.strictEqual(listeners.length, 1);
       assert.strictEqual(listeners[0][kListener], NOOP);
 
-      assert.strictEqual(ws.onopen, undefined);
+      assert.strictEqual(ws.onopen, null);
     });
 
     it("doesn't remove listeners added with `addEventListener`", () => {