diff --git a/frontend_tests/node_tests/topic_generator.js b/frontend_tests/node_tests/topic_generator.js
index 6902011bc8..d1af6c18d4 100644
--- a/frontend_tests/node_tests/topic_generator.js
+++ b/frontend_tests/node_tests/topic_generator.js
@@ -215,19 +215,19 @@ run_test('streams', () => {
 
 run_test('topics', () => {
     const streams = [1, 2, 3, 4];
-    const topics = {};
-
-    topics[1] = ['read', 'read', '1a', '1b', 'read', '1c'];
-    topics[2] = [];
-    topics[3] = ['3a', 'read', 'read', '3b', 'read'];
-    topics[4] = ['4a'];
+    const topics = new Map([
+        [1, ['read', 'read', '1a', '1b', 'read', '1c']],
+        [2, []],
+        [3, ['3a', 'read', 'read', '3b', 'read']],
+        [4, ['4a']],
+    ]);
 
     function has_unread_messages(stream, topic) {
         return topic !== 'read';
     }
 
     function get_topics(stream) {
-        return topics[stream];
+        return topics.get(stream);
     }
 
     function next_topic(curr_stream, curr_topic) {