events: add CustomEvent

This implements the Web API `CustomEvent` in `internal/event_target`.

Signed-off-by: Daeyeon Jeong daeyeon.dev@gmail.com

PR-URL: nodejs/node#43514
Backport-PR-URL: nodejs/node#44082
Refs: nodejs/node#40678
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

doc/api/events.md

@@ -1978,6 +1978,31 @@ added: v14.5.0
 
 Removes the `listener` from the list of handlers for event `type`.
 
+### Class: `CustomEvent`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental.
+
+* Extends: {Event}
+
+The `CustomEvent` object is an adaptation of the [`CustomEvent` Web API][].
+Instances are created internally by Node.js.
+
+#### `event.detail`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+> Stability: 1 - Experimental.
+
+* Type: {any} Returns custom data passed when initializing.
+
+Read-only.
+
 ### Class: `NodeEventTarget`
 
 <!-- YAML
@@ -2115,6 +2140,7 @@ to the `EventTarget`.
 
 [WHATWG-EventTarget]: https://dom.spec.whatwg.org/#interface-eventtarget
 [`--trace-warnings`]: cli.md#--trace-warnings
+[`CustomEvent` Web API]: https://dom.spec.whatwg.org/#customevent
 [`EventTarget` Web API]: https://dom.spec.whatwg.org/#eventtarget
 [`EventTarget` error handling]: #eventtarget-error-handling
 [`Event` Web API]: https://dom.spec.whatwg.org/#event

lib/internal/event_target.js

@@ -76,6 +76,7 @@ const kTimestamp = Symbol('timestamp');
 const kBubbles = Symbol('bubbles');
 const kComposed = Symbol('composed');
 const kPropagationStopped = Symbol('propagationStopped');
+const kDetail = Symbol('detail');
 
 const isTrustedSet = new SafeWeakSet();
 const isTrusted = ObjectGetOwnPropertyDescriptor({
@@ -327,6 +328,49 @@ ObjectDefineProperties(
     stopPropagation: kEnumerableProperty,
   });
 
+function isCustomEvent(value) {
+  return isEvent(value) && (value?.[kDetail] !== undefined);
+}
+
+class CustomEvent extends Event {
+  /**
+   * @constructor
+   * @param {string} type
+   * @param {{
+   *   bubbles?: boolean,
+   *   cancelable?: boolean,
+   *   composed?: boolean,
+   *   detail?: any,
+   * }} [options]
+   */
+  constructor(type, options = kEmptyObject) {
+    if (arguments.length === 0)
+      throw new ERR_MISSING_ARGS('type');
+    super(type, options);
+    this[kDetail] = options?.detail ?? null;
+  }
+
+  /**
+   * @type {any}
+   */
+  get detail() {
+    if (!isCustomEvent(this))
+      throw new ERR_INVALID_THIS('CustomEvent');
+    return this[kDetail];
+  }
+}
+
+ObjectDefineProperties(CustomEvent.prototype, {
+  [SymbolToStringTag]: {
+    __proto__: null,
+    writable: false,
+    enumerable: false,
+    configurable: true,
+    value: 'CustomEvent',
+  },
+  detail: kEnumerableProperty,
+});
+
 class NodeCustomEvent extends Event {
   constructor(type, options) {
     super(type, options);
@@ -989,6 +1033,7 @@ const EventEmitterMixin = (Superclass) => {
 
 module.exports = {
   Event,
+  CustomEvent,
   EventEmitterMixin,
   EventTarget,
   NodeEventTarget,