init.

code was extracted from another project, so it's already in a somewhat
"complete" state.

export.mjs

@@ -0,0 +1 @@
+export * from "./src/core.mjs";

src/core.mjs

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+const registered_listeners = new Map();
+
+export function trigger (event_name, data) {
+	for (const callback of listeners(event_name)) {
+		callback(data);
+	}
+}
+
+export function listen (event_name, callback) {
+	listeners(event_name).push(callback);
+	// to find an event later, for deletion or other things.
+	return [event_name, listeners(event_name).length - 1];
+}
+
+// makes a callback function stop being used to listen to an event, and returns that callback function.
+export function ignore (event_id) {
+	const [event_name, index] = event_id;
+	return listeners(event_name).splice(index, 1)[0];
+}
+
+function listeners (event_name) {
+	if (!registered_listeners.has(event_name))
+		registered_listeners.set(event_name, []);
+
+	return registered_listeners.get(event_name);
+}
+
+
+
+export function attach_listener (target, event_name, callback) {
+	return listen(event_name, data => {
+		if (data.target === target) {
+			callback(data);
+		}
+	});
+}
+
+export function create_alias (event_name, alias) {
+	// note that aliases are one-directional; `event_name` isn't triggered when `alias` is.
+	return listen(event_name, data => trigger(alias, data));
+}
+
+export function create_group (group_name, members) {
+	// TODO: return list of event id:s so groups can be `ignore`:d.
+	for (const event_name of members) {
+		create_alias(event_name, group_name);
+	}
+}