From 91b6c093b120d68f544ddc4c72911a9fc1c28386 Mon Sep 17 00:00:00 2001
From: Benjamin Gruenbaum <benjamingr@gmail.com>
Date: Sat, 30 May 2020 00:51:02 +0300
Subject: [PATCH] events: support event handlers

PR-URL: https://github.com/nodejs/node/pull/34015
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
---
 lib/internal/event_target.js      | 21 +++++++++++++++++--
 test/parallel/test-eventtarget.js | 34 +++++++++++++++++++++++++++++++
 2 files changed, 53 insertions(+), 2 deletions(-)

diff --git a/lib/internal/event_target.js b/lib/internal/event_target.js
index 7c34acd5c5dbe1..9fef64aaba2490 100644
--- a/lib/internal/event_target.js
+++ b/lib/internal/event_target.js
@@ -479,10 +479,27 @@ 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,
 };
diff --git a/test/parallel/test-eventtarget.js b/test/parallel/test-eventtarget.js
index 947477de5a7768..743221316382b8 100644
--- a/test/parallel/test-eventtarget.js
+++ b/test/parallel/test-eventtarget.js
@@ -6,6 +6,7 @@ const {
   Event,
   EventTarget,
   NodeEventTarget,
+  defineEventHandler
 } = require('internal/event_target');
 
 const {
@@ -438,6 +439,39 @@ 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'));
+}
 
 {
   // `this` value of dispatchEvent