events: add once method to use promises with EventEmitter

This change adds a EventEmitter.once() method that wraps ee.once in a
promise.

Co-authored-by: David Mark Clements <[email protected]>

PR-URL: #26078
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Ruben Bridgewater <[email protected]>
Reviewed-By: Gus Caplan <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>
Reviewed-By: Anatoli Papirovski <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Сковорода Никита Андреевич <[email protected]>

doc/api/events.md

@@ -653,6 +653,47 @@ newListeners[0]();
 emitter.emit('log');
 ```
 
+## events.once(emitter, name)
+<!-- YAML
+added: REPLACEME
+-->
+* `emitter` {EventEmitter}
+* `name` {string}
+* Returns: {Promise}
+
+Creates a `Promise` that is resolved 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.
+
+```js
+const { once, EventEmitter } = require('events');
+
+async function run() {
+  const ee = new EventEmitter();
+
+  process.nextTick(() => {
+    ee.emit('myevent', 42);
+  });
+
+  const [value] = await once(ee, 'myevent');
+  console.log(value);
+
+  const err = new Error('kaboom');
+  process.nextTick(() => {
+    ee.emit('error', err);
+  });
+
+  try {
+    await once(ee, 'myevent');
+  } catch (err) {
+    console.log('error happened', err);
+  }
+}
+
+run();
+```
+
 [`--trace-warnings`]: cli.html#cli_trace_warnings
 [`EventEmitter.defaultMaxListeners`]: #events_eventemitter_defaultmaxlisteners
 [`domain`]: domain.html

lib/events.js

@@ -27,6 +27,7 @@ function EventEmitter() {
   EventEmitter.init.call(this);
 }
 module.exports = EventEmitter;
+module.exports.once = once;
 
 // Backwards-compat with node 0.10.x
 EventEmitter.EventEmitter = EventEmitter;
@@ -485,3 +486,32 @@ function unwrapListeners(arr) {
   }
   return ret;
 }
+
+function once(emitter, name) {
+  return new Promise((resolve, reject) => {
+    const eventListener = (...args) => {
+      if (errorListener !== undefined) {
+        emitter.removeListener('error', errorListener);
+      }
+      resolve(args);
+    };
+    let errorListener;
+
+    // Adding an error listener is not optional because
+    // if an error is thrown on an event emitter we cannot
+    // guarantee that the actual event we are waiting will
+    // be fired. The result could be a silent way to create
+    // memory or file descriptor leaks, which is something
+    // we should avoid.
+    if (name !== 'error') {
+      errorListener = (err) => {
+        emitter.removeListener(name, eventListener);
+        reject(err);
+      };
+
+      emitter.once('error', errorListener);
+    }
+
+    emitter.once(name, eventListener);
+  });
+}

test/parallel/test-events-once.js

@@ -0,0 +1,93 @@
+'use strict';
+
+const common = require('../common');
+const { once, EventEmitter } = require('events');
+const { strictEqual, deepStrictEqual } = require('assert');
+
+async function onceAnEvent() {
+  const ee = new EventEmitter();
+
+  process.nextTick(() => {
+    ee.emit('myevent', 42);
+  });
+
+  const [value] = await once(ee, 'myevent');
+  strictEqual(value, 42);
+  strictEqual(ee.listenerCount('error'), 0);
+  strictEqual(ee.listenerCount('myevent'), 0);
+}
+
+async function onceAnEventWithTwoArgs() {
+  const ee = new EventEmitter();
+
+  process.nextTick(() => {
+    ee.emit('myevent', 42, 24);
+  });
+
+  const value = await once(ee, 'myevent');
+  deepStrictEqual(value, [42, 24]);
+}
+
+async function catchesErrors() {
+  const ee = new EventEmitter();
+
+  const expected = new Error('kaboom');
+  let err;
+  process.nextTick(() => {
+    ee.emit('error', expected);
+  });
+
+  try {
+    await once(ee, 'myevent');
+  } catch (_e) {
+    err = _e;
+  }
+  strictEqual(err, expected);
+  strictEqual(ee.listenerCount('error'), 0);
+  strictEqual(ee.listenerCount('myevent'), 0);
+}
+
+async function stopListeningAfterCatchingError() {
+  const ee = new EventEmitter();
+
+  const expected = new Error('kaboom');
+  let err;
+  process.nextTick(() => {
+    ee.emit('error', expected);
+    ee.emit('myevent', 42, 24);
+  });
+
+  process.on('multipleResolves', common.mustNotCall());
+
+  try {
+    await once(ee, 'myevent');
+  } catch (_e) {
+    err = _e;
+  }
+  process.removeAllListeners('multipleResolves');
+  strictEqual(err, expected);
+  strictEqual(ee.listenerCount('error'), 0);
+  strictEqual(ee.listenerCount('myevent'), 0);
+}
+
+async function onceError() {
+  const ee = new EventEmitter();
+
+  const expected = new Error('kaboom');
+  process.nextTick(() => {
+    ee.emit('error', expected);
+  });
+
+  const [err] = await once(ee, 'error');
+  strictEqual(err, expected);
+  strictEqual(ee.listenerCount('error'), 0);
+  strictEqual(ee.listenerCount('myevent'), 0);
+}
+
+Promise.all([
+  onceAnEvent(),
+  onceAnEventWithTwoArgs(),
+  catchesErrors(),
+  stopListeningAfterCatchingError(),
+  onceError()
+]);