diff --git a/background/background.js b/background/background.js
index dc7c295f..a05c6bdd 100644
--- a/background/background.js
+++ b/background/background.js
@@ -406,6 +406,7 @@ function onRuntimeMessage(msg, sender) {
   return fn.apply(context, msg.args);
 }
 
+// FIXME: popup.js also open editor but it doesn't use this API.
 function openEditor({id}) {
   let url = '/edit.html';
   if (id) {
diff --git a/js/prefs.js b/js/prefs.js
index cf0776ef..3e30d600 100644
--- a/js/prefs.js
+++ b/js/prefs.js
@@ -1,3 +1,4 @@
+/* global promisify */
 /* exported prefs */
 'use strict';
 
@@ -179,23 +180,6 @@ const prefs = (() => {
     },
   };
 
-  function promisify(fn) {
-    return (...args) =>
-      new Promise((resolve, reject) => {
-        fn(...args, (...result) => {
-          if (chrome.runtime.lastError) {
-            reject(chrome.runtime.lastError);
-          } else if (result.length === 0) {
-            resolve(undefined);
-          } else if (result.length === 1) {
-            resolve(result[0]);
-          } else {
-            resolve(result);
-          }
-        });
-      });
-  }
-
   function setAll(settings, synced) {
     for (const [key, value] of Object.entries(settings)) {
       set(key, value, synced);
diff --git a/popup.html b/popup.html
index 81070088..3becdeb1 100644
--- a/popup.html
+++ b/popup.html
@@ -151,11 +151,11 @@
   <link rel="stylesheet" href="manage/config-dialog.css">
   <script src="manage/config-dialog.js"></script>
 
+  <script src="js/promisify.js"></script>
   <script src="js/dom.js"></script>
   <script src="js/messaging.js"></script>
   <script src="js/localization.js"></script>
   <script src="js/prefs.js"></script>
-  <script src="js/promisify.js"></script>
   <script src="js/msg.js"></script>
   <script src="content/apply.js"></script>