events: Handle a range of this values for dispatchEvent

On the web, dispatchEvent is finicky about its `this` value. An exception is
thrown for `this` values which are not an EventTarget.

Author: Zirak

lib/internal/event_target.js

@@ -8,6 +8,7 @@ const {
   Set,
   Symbol,
   NumberIsNaN,
+  SymbolFor,
   SymbolToStringTag,
 } = primordials;
 
@@ -16,13 +17,16 @@ const {
     ERR_INVALID_ARG_TYPE,
     ERR_EVENT_RECURSION,
     ERR_OUT_OF_RANGE,
-    ERR_MISSING_ARGS
+    ERR_MISSING_ARGS,
+    ERR_INVALID_THIS,
   }
 } = require('internal/errors');
 
 const { customInspectSymbol } = require('internal/util');
 const { inspect } = require('util');
 
+const kIsEventTarget = SymbolFor('nodejs.event_target');
+
 const kEvents = Symbol('kEvents');
 const kStop = Symbol('kStop');
 const kTarget = Symbol('kTarget');
@@ -185,6 +189,10 @@ class Listener {
 }
 
 class EventTarget {
+  // Used in checking whether an object is an EventTarget. This is a well-known
+  // symbol as EventTarget may be used cross-realm. See discussion in #33661.
+  static [kIsEventTarget] = true;
+
   [kEvents] = new Map();
   #emitting = new Set();
 
@@ -257,6 +265,10 @@ class EventTarget {
       throw new ERR_INVALID_ARG_TYPE('event', 'Event', event);
     }
 
+    if (!isEventTarget(this)) {
+      throw new ERR_INVALID_THIS('EventTarget');
+    }
+
     if (this.#emitting.has(event.type) ||
         event[kTarget] !== null) {
       throw new ERR_EVENT_RECURSION(event.type);
@@ -447,6 +459,15 @@ function validateEventListenerOptions(options) {
   };
 }
 
+// Test whether the argument is an event object. This is far from a fool-proof
+// test, for example this input will result in a false positive:
+// > isEventTarget({ constructor: EventTarget })
+// It stands in its current implementation as a compromise. For the relevant
+// discussion, see #33661.
+function isEventTarget(obj) {
+  return obj && obj.constructor && obj.constructor[kIsEventTarget];
+}
+
 function addCatch(that, promise, event) {
   const then = promise.then;
   if (typeof then === 'function') {

test/parallel/test-eventtarget.js

@@ -439,3 +439,28 @@ ok(EventTarget);
   const event = new Event('');
   strictEqual(event.toString(), '[object Event]');
 }
+
+{
+  // `this` value of dispatchEvent
+  const target = new EventTarget();
+  const target2 = new EventTarget();
+  const event = new Event('foo');
+
+  ok(target.dispatchEvent.call(target2, event));
+
+  [
+    'foo',
+    {},
+    [],
+    1,
+    null,
+    undefined,
+    false,
+    Symbol(),
+    /a/
+  ].forEach((i) => {
+    throws(() => target.dispatchEvent.call(i, event), {
+      code: 'ERR_INVALID_THIS'
+    });
+  });
+}