diff --git a/ygo-json-loader.mjs b/ygo-json-loader.mjs
index fa9f94c..b6dbd8c 100644
--- a/ygo-json-loader.mjs
+++ b/ygo-json-loader.mjs
@@ -56,9 +56,9 @@ export const md_table = Object.create(null);
 md_table['en'] = md_en_table;
 md_table['ja'] = md_jp_table;
 
-export const pre_release = new Map(Object.entries(pre_table));
+export const pre_release = pre_table;
 export const id_to_cid = inverse_mapping(cid_table);
-const pack_id_table = inverse_mapping(pre_release);
+const pack_id_table = Object.fromEntries(Object.entries(pre_release).map(([k, v]) => [v, k]));
 
 export const official_name = {
 	__proto__: null,
@@ -174,7 +174,7 @@ export function get_pack_name(id) {
 	if (id <= MAX_CARD_ID)
 		return '';
 	const pack_id = id - id % 1000;
-	const pack_name = pack_id_table.get(pack_id);
+	const pack_name = pack_id_table[pack_id];
 	if (!pack_name)
 		return '';
 	if (pack_name.charAt(0) === '_')
diff --git a/ygo-query.mjs b/ygo-query.mjs
index f762b8f..6af6a18 100644
--- a/ygo-query.mjs
+++ b/ygo-query.mjs
@@ -315,10 +315,10 @@ export function generate_condition(params, id_list) {
 		const pack = pack_list[params.pack].filter(x => Number.isSafeInteger(x) && x > 0);
 		qstr += ` AND ${list_condition('id', 'pack', pack, arg)}`;
 	}
-	else if (typeof params.pack === 'string' && pre_release.has(params.pack)) {
+	else if (typeof params.pack === 'string' && Object.hasOwn(pre_release, params.pack)) {
 		qstr += " AND (id BETWEEN $pack_begin AND $pack_end)";
-		arg.$pack_begin = pre_release.get(params.pack);
-		arg.$pack_end = pre_release.get(params.pack) + 500;
+		arg.$pack_begin = pre_release[params.pack];
+		arg.$pack_end = pre_release[params.pack] + 500;
 	}
 	else if (Number.isSafeInteger(params.limit) && params.limit > 0) {
 		arg.$limit = params.limit;
@@ -721,7 +721,7 @@ export function query_card(params) {
 		is_sorted = true;
 		meta.pack = params.pack;
 	}
-	else if (typeof params.pack === 'string' && pre_release.has(params.pack)) {
+	else if (typeof params.pack === 'string' && Object.hasOwn(pre_release, params.pack)) {
 		is_sorted = true;
 		meta.pack = params.pack;
 	}