3 files changed, 60 insertions, 18 deletions

diff --git a/cli/rt/01_web_util.js b/cli/rt/01_web_util.js
index 4a3df65ce..c843ac086 100644
--- a/cli/rt/01_web_util.js
+++ b/cli/rt/01_web_util.js
@@ -113,9 +113,44 @@
     }
   }
 
+  const handlerSymbol = Symbol("eventHandlers");
+  function makeWrappedHandler(handler) {
+    function wrappedHandler(...args) {
+      if (typeof wrappedHandler.handler !== "function") {
+        return;
+      }
+      return wrappedHandler.handler.call(this, ...args);
+    }
+    wrappedHandler.handler = handler;
+    return wrappedHandler;
+  }
+  function defineEventHandler(emitter, name) {
+    // HTML specification section 8.1.5.1
+    Object.defineProperty(emitter, `on${name}`, {
+      get() {
+        return this[handlerSymbol]?.get(name)?.handler;
+      },
+      set(value) {
+        if (!this[handlerSymbol]) {
+          this[handlerSymbol] = new Map();
+        }
+        let handlerWrapper = this[handlerSymbol]?.get(name);
+        if (handlerWrapper) {
+          handlerWrapper.handler = value;
+        } else {
+          handlerWrapper = makeWrappedHandler(value);
+          this.addEventListener(name, handlerWrapper);
+        }
+        this[handlerSymbol].set(name, handlerWrapper);
+      },
+      configurable: true,
+      enumerable: true,
+    });
+  }
   window.__bootstrap.webUtil = {
     illegalConstructorKey,
     requiredArguments,
+    defineEventHandler,
     cloneValue,
   };
 })(this);
diff --git a/cli/rt/27_websocket.js b/cli/rt/27_websocket.js
index c0790d14d..675c1e836 100644
--- a/cli/rt/27_websocket.js
+++ b/cli/rt/27_websocket.js
@@ -2,7 +2,7 @@
 
 ((window) => {
   const core = window.Deno.core;
-  const { requiredArguments } = window.__bootstrap.webUtil;
+  const { requiredArguments, defineEventHandler } = window.__bootstrap.webUtil;
   const CONNECTING = 0;
   const OPEN = 1;
   const CLOSING = 2;
@@ -63,12 +63,10 @@
 
               const errEvent = new ErrorEvent("error");
               errEvent.target = this;
-              this.onerror?.(errEvent);
               this.dispatchEvent(errEvent);
 
               const event = new CloseEvent("close");
               event.target = this;
-              this.onclose?.(event);
               this.dispatchEvent(event);
               core.close(this.#rid);
             });
@@ -76,7 +74,6 @@
             this.#readyState = OPEN;
             const event = new Event("open");
             event.target = this;
-            this.onopen?.(event);
             this.dispatchEvent(event);
 
             this.#eventLoop();
@@ -86,12 +83,10 @@
 
           const errEvent = new ErrorEvent("error");
           errEvent.target = this;
-          this.onerror?.(errEvent);
           this.dispatchEvent(errEvent);
 
           const closeEvent = new CloseEvent("close");
           closeEvent.target = this;
-          this.onclose?.(closeEvent);
           this.dispatchEvent(closeEvent);
         }
       }).catch((err) => {
@@ -102,12 +97,10 @@
           { error: err, message: err.toString() },
         );
         errorEv.target = this;
-        this.onerror?.(errorEv);
         this.dispatchEvent(errorEv);
 
         const closeEv = new CloseEvent("close");
         closeEv.target = this;
-        this.onclose?.(closeEv);
         this.dispatchEvent(closeEv);
       });
     }
@@ -159,11 +152,6 @@
       return this.#url;
     }
 
-    onopen = () => {};
-    onerror = () => {};
-    onclose = () => {};
-    onmessage = () => {};
-
     send(data) {
       requiredArguments("WebSocket.send", arguments.length, 1);
 
@@ -241,7 +229,6 @@
             reason,
           });
           event.target = this;
-          this.onclose?.(event);
           this.dispatchEvent(event);
           core.close(this.#rid);
         });
@@ -272,7 +259,6 @@
             origin: this.#url,
           });
           event.target = this;
-          this.onmessage?.(event);
           this.dispatchEvent(event);
 
           this.#eventLoop();
@@ -284,20 +270,17 @@
             reason: message.reason,
           });
           event.target = this;
-          this.onclose?.(event);
           this.dispatchEvent(event);
         } else if (message.type === "error") {
           this.#readyState = CLOSED;
 
           const errorEv = new ErrorEvent("error");
           errorEv.target = this;
-          this.onerror?.(errorEv);
           this.dispatchEvent(errorEv);
 
           this.#readyState = CLOSED;
           const closeEv = new CloseEvent("close");
           closeEv.target = this;
-          this.onclose?.(closeEv);
           this.dispatchEvent(closeEv);
         }
       }
@@ -319,6 +302,10 @@
     },
   });
 
+  defineEventHandler(WebSocket.prototype, "message");
+  defineEventHandler(WebSocket.prototype, "error");
+  defineEventHandler(WebSocket.prototype, "close");
+  defineEventHandler(WebSocket.prototype, "open");
   window.__bootstrap.webSocket = {
     WebSocket,
   };
diff --git a/cli/tests/websocket_test.ts b/cli/tests/websocket_test.ts
index 93fddf446..0a91ac308 100644
--- a/cli/tests/websocket_test.ts
+++ b/cli/tests/websocket_test.ts
@@ -273,3 +273,23 @@ Deno.test("echo arraybuffer with binaryType arraybuffer", async () => {
   };
   await promise;
 });
+
+Deno.test("Event Handlers order", async () => {
+  const promise = createResolvable();
+  const ws = new WebSocket("ws://localhost:4242");
+  const arr: number[] = [];
+  ws.onerror = (): void => fail();
+  ws.addEventListener("message", () => arr.push(1));
+  ws.onmessage = () => fail();
+  ws.addEventListener("message", () => {
+    arr.push(3);
+    ws.close();
+    assertEquals(arr, [1, 2, 3]);
+  });
+  ws.onmessage = () => arr.push(2);
+  ws.onopen = (): void => ws.send("Echo");
+  ws.onclose = (): void => {
+    promise.resolve();
+  };
+  await promise;
+});