JS trick to mimic C# obj.onevent += delegate

index.js

const HandlerID = Symbol()

export function defineEvent(obj, name) {
	const handlers = new Set()
	const fn = function () {
		for (const handler of handlers) {
			try {
				handler()
			} catch (e) {
				Promise.reject(e)
			}
		}
	}
	fn[HandlerID] = 0

	Object.defineProperty(obj, name, {
		get() { return handlers.size ? fn : null },
		set(v) {
			if (v === null) {
				// console.log('=', v)
				handlers.clear()
			} else if (typeof v == "function") {
				// console.log('=', v)
				handlers.clear()
				handlers.add(v)
			} else if (v > 0) {
				// console.log('+=', v)
				v = handlerMaps.get(v)
				if (!v) throw new TypeError()
				handlers.add(v)
			} else if (v < 0) {
				// console.log('-=', v)
				v = handlerMaps.get(-v)
				if (!v) throw new TypeError()
				handlers.delete(v)
			} else {
				throw new TypeError()
			}
		},
	})
}

const handlerMaps = new Map()

Function.prototype.valueOf = function () {
	return this[HandlerID] ?? genHandlerID(this)
}

function genHandlerID(fn) {
	Promise.resolve(fn).then(disposeHandlerID)

	let id
	do { id = Math.random() }
	while (handlerMaps.has(id))

	handlerMaps.set(id, fn)
	fn[HandlerID] = id
	return id
}

function disposeHandlerID(fn) {
	handlerMaps.delete(fn[HandlerID])
	fn[HandlerID] = null
}

test.js

import { defineEvent } from "./index.js"
import assert from "node:assert/strict"

const x = {}
defineEvent(x, "onChange")

const handlers = [
	() => console.log(0),
	() => console.log(1),
	() => console.log(2),
]

assert.equal(x.onChange, null)
x.onChange += handlers[0]
assert.notEqual(x.onChange, null)

x.onChange() // output 0

x.onChange += handlers[1]
x.onChange += handlers[2]
x.onChange -= handlers[0]
x.onChange += handlers[1]

x.onChange() // output 1, 2

x.onChange -= handlers[1]
x.onChange -= handlers[2]
assert.equal(x.onChange, null)