diff --git a/static/js/index.es6 b/static/js/index.es6
index 50dbd74..d34a3c9 100644
--- a/static/js/index.es6
+++ b/static/js/index.es6
@@ -80,7 +80,7 @@ const vmMirList = new Vue({
 		},
 		refreshMirrorList() {
 			// do nothing if the tab is not visible
-			if (document.hidden ?? false) {
+			if (document.hidden === true) {
 				return;
 			}
 			const self = this;