1 files changed, 35 insertions, 0 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);