events: support event handlers

lib/internal/event_target.js

@@ -445,10 +445,27 @@ function emitUnhandledRejectionOrErr(that, err, event) {
   process.emit('error', err, event);
 }
 
-// EventEmitter-ish API:
-
+function defineEventHandler(emitter, name) {
+  // 8.1.5.1 Event handlers - basically `on[eventName]` attributes
+  let eventHandlerValue;
+  Object.defineProperty(emitter, `on${name}`, {
+    get() {
+      return eventHandlerValue;
+    },
+    set(value) {
+      if (eventHandlerValue) {
+        emitter.removeEventListener(name, eventHandlerValue);
+      }
+      if (typeof value === 'function') {
+        emitter.addEventListener(name, value);
+      }
+      eventHandlerValue = value;
+    }
+  });
+}
 module.exports = {
   Event,
   EventTarget,
   NodeEventTarget,
+  defineEventHandler,
 };

test/parallel/test-eventtarget.js

@@ -6,6 +6,7 @@ const {
   Event,
   EventTarget,
   NodeEventTarget,
+  defineEventHandler
 } = require('internal/event_target');
 
 const {
@@ -439,3 +440,36 @@ ok(EventTarget);
   const event = new Event('');
   strictEqual(event.toString(), '[object Event]');
 }
+{
+  const target = new EventTarget();
+  defineEventHandler(target, 'foo');
+  target.onfoo = common.mustCall();
+  target.dispatchEvent(new Event('foo'));
+}
+{
+  const target = new EventTarget();
+  defineEventHandler(target, 'foo');
+  let count = 0;
+  target.onfoo = () => count++;
+  target.onfoo = common.mustCall(() => count++);
+  target.dispatchEvent(new Event('foo'));
+  strictEqual(count, 1);
+}
+{
+  const target = new EventTarget();
+  defineEventHandler(target, 'foo');
+  let count = 0;
+  target.addEventListener('foo', () => count++);
+  target.onfoo = common.mustCall(() => count++);
+  target.dispatchEvent(new Event('foo'));
+  strictEqual(count, 2);
+}
+{
+  const target = new EventTarget();
+  defineEventHandler(target, 'foo');
+  const fn = common.mustNotCall();
+  target.onfoo = fn;
+  strictEqual(target.onfoo, fn);
+  target.onfoo = null;
+  target.dispatchEvent(new Event('foo'));
+}