add entity metadata.

js/entity.mjs

@@ -52,6 +52,17 @@ export function is_visible (entity) {
 
 
 
+const metadata = new Map();
+
+export function get_meta (entity) {
+	if (!metadata.has(entity.id)) {
+		metadata.set(entity.id, new Map());
+	}
+	return metadata.get(entity.id);
+}
+
+
+
 // TESTS
 
 assert("entity creation works.", _ => {
@@ -123,3 +134,10 @@ assert("creating entities with preset visibility works.", _ => {
 	});
 	return e2.visible;
 });
+
+assert("entity `get_meta` works.", _ => {
+	const e = create();
+	const meta = get_meta(e);
+	meta.set("test", "gay");
+	return get_meta(e).get("test") === "gay";
+});