Support CustomEvent

[alshdavid]

Is your feature request related to a problem? Please describe.
When trying to mimic browser APIs surrounding EventTarget, we frequently use CustomEvent which is currently unavailable in Node.

It's useful in that it allows us to dispatch events through an EventTarget and include a custom data payload.

In a some way, this is the closest we have to a standardised Observable API (specifically Subject) and it's very useful.

Describe the solution you'd like
CustomEvent exists

Describe alternatives you've considered
Writing a polyfill

class CustomEvent extends Event { 
  constructor(message, data) {
    super(message, data)
    this.detail = data.detail
  }
}

const et = new EventTarget()
et.addEventListener('message', ev => console.log(ev.detail))
et.dispatchEvent(new CustomEvent('message', { detail: 'foo' }))

[github-actions]

There has been no activity on this feature request for 5 months and it is unlikely to be implemented. It will be closed 6 months after the last non-automated comment.

For more information on how the project manages feature requests, please consult the feature request management document.

[regseb]

Yet another polyfill proposal that:

• handles no options;
• uses null for the default value of detail;
• define detail readonly.

class CustomEvent extends Event {
    #detail;

    constructor(type, options) {
        super(type, options);
        this.#detail = options?.detail ?? null;
    }

    get detail() {
        return this.#detail;
    }
}

[bnoordhuis]

For anyone wanting to work on this, the steps to take are:

1. Enable the dom/events WPT tests in test/wpt
2. Make them pass for regular Events/EventTargets
3. Then start working on CustomEvent

W.r.t. (1):

diff --git a/test/wpt/status/dom/events.json b/test/wpt/status/dom/events.json
new file mode 100644
index 00000000000..0967ef424bc
--- /dev/null
+++ b/test/wpt/status/dom/events.json
@@ -0,0 +1 @@
+{}
diff --git a/test/wpt/test-events.js b/test/wpt/test-events.js
new file mode 100644
index 00000000000..5040d56d6a2
--- /dev/null
+++ b/test/wpt/test-events.js
@@ -0,0 +1,7 @@
+'use strict';
+require('../common');
+const { WPTRunner } = require('../common/wpt');
+
+const runner = new WPTRunner('dom/events');
+
+runner.runJsTests();

Where dom/events is https://github.com/web-platform-tests/wpt/tree/master/dom/events