events: add support for EventTarget in once

PR-URL: #29498
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>

doc/api/events.md

@@ -703,11 +703,15 @@ added: v11.13.0
 * `name` {string}
 * Returns: {Promise}
 
-Creates a `Promise` that is resolved when the `EventEmitter` emits the given
+Creates a `Promise` that is fulfilled when the `EventEmitter` emits the given
 event or that is rejected when the `EventEmitter` emits `'error'`.
 The `Promise` will resolve with an array of all the arguments emitted to the
 given event.
 
+This method is intentionally generic and works with the web platform
+[EventTarget](WHATWG-EventTarget) interface, which has no special
+`'error'` event semantics and does not listen to the `'error'` event.
+
 ```js
 const { once, EventEmitter } = require('events');
 
@@ -735,7 +739,7 @@ async function run() {
 
 run();
 ```
-
+[WHATWG-EventTarget](https://dom.spec.whatwg.org/#interface-eventtarget)
 [`--trace-warnings`]: cli.html#cli_trace_warnings
 [`EventEmitter.defaultMaxListeners`]: #events_eventemitter_defaultmaxlisteners
 [`domain`]: domain.html

lib/events.js

@@ -497,6 +497,17 @@ function unwrapListeners(arr) {
 
 function once(emitter, name) {
   return new Promise((resolve, reject) => {
+    if (typeof emitter.addEventListener === 'function') {
+      // EventTarget does not have `error` event semantics like Node
+      // EventEmitters, we do not listen to `error` events here.
+      emitter.addEventListener(
+        name,
+        (...args) => { resolve(args); },
+        { once: true }
+      );
+      return;
+    }
+
     const eventListener = (...args) => {
       if (errorListener !== undefined) {
         emitter.removeListener('error', errorListener);

test/parallel/test-events-once.js

@@ -4,6 +4,53 @@ const common = require('../common');
 const { once, EventEmitter } = require('events');
 const { strictEqual, deepStrictEqual } = require('assert');
 
+class EventTargetMock {
+  constructor() {
+    this.events = {};
+  }
+
+  addEventListener = common.mustCall(function(name, listener, options) {
+    if (!(name in this.events)) {
+      this.events[name] = { listeners: [], options };
+    }
+    this.events[name].listeners.push(listener);
+  });
+
+  removeEventListener = common.mustCall(function(name, callback) {
+    if (!(name in this.events)) {
+      return;
+    }
+    const event = this.events[name];
+    const stack = event.listeners;
+
+    for (let i = 0, l = stack.length; i < l; i++) {
+      if (stack[i] === callback) {
+        stack.splice(i, 1);
+        if (stack.length === 0) {
+          Reflect.deleteProperty(this.events, name);
+        }
+        return;
+      }
+    }
+  });
+
+  dispatchEvent = function(name, ...arg) {
+    if (!(name in this.events)) {
+      return true;
+    }
+    const event = this.events[name];
+    const stack = event.listeners.slice();
+
+    for (let i = 0, l = stack.length; i < l; i++) {
+      stack[i].apply(this, arg);
+      if (event.options.once) {
+        this.removeEventListener(name, stack[i]);
+      }
+    }
+    return !name.defaultPrevented;
+  };
+}
+
 async function onceAnEvent() {
   const ee = new EventEmitter();
 
@@ -84,10 +131,48 @@ async function onceError() {
   strictEqual(ee.listenerCount('myevent'), 0);
 }
 
+async function onceWithEventTarget() {
+  const et = new EventTargetMock();
+
+  process.nextTick(() => {
+    et.dispatchEvent('myevent', 42);
+  });
+  const [ value ] = await once(et, 'myevent');
+  strictEqual(value, 42);
+  strictEqual(Reflect.has(et.events, 'myevent'), false);
+}
+
+async function onceWithEventTargetTwoArgs() {
+  const et = new EventTargetMock();
+
+  process.nextTick(() => {
+    et.dispatchEvent('myevent', 42, 24);
+  });
+
+  const value = await once(et, 'myevent');
+  deepStrictEqual(value, [42, 24]);
+}
+
+async function onceWithEventTargetError() {
+  const et = new EventTargetMock();
+
+  const expected = new Error('kaboom');
+  process.nextTick(() => {
+    et.dispatchEvent('error', expected);
+  });
+
+  const [err] = await once(et, 'error');
+  strictEqual(err, expected);
+  strictEqual(Reflect.has(et.events, 'error'), false);
+}
+
 Promise.all([
   onceAnEvent(),
   onceAnEventWithTwoArgs(),
   catchesErrors(),
   stopListeningAfterCatchingError(),
-  onceError()
+  onceError(),
+  onceWithEventTarget(),
+  onceWithEventTargetTwoArgs(),
+  onceWithEventTargetError(),
 ]).then(common.mustCall());