diff --git a/.flowconfig b/.flowconfig
index b34b11e181..bd13cc7965 100644
--- a/.flowconfig
+++ b/.flowconfig
@@ -5,3 +5,6 @@
 .*node_modules/config-chain.*
 .*firefox/.*
 .*flow-coverage-report/.*
+
+[libs]
+./packages/devtools-config/flow-interface.js
diff --git a/packages/devtools-config/flow-interface.js b/packages/devtools-config/flow-interface.js
new file mode 100644
index 0000000000..34eb071a33
--- /dev/null
+++ b/packages/devtools-config/flow-interface.js
@@ -0,0 +1,12 @@
+declare module "devtools-config" {
+  declare module.exports: {
+    isDevelopment: () => boolean;
+    getValue: (key: string) => any;
+    isEnabled: () => boolean;
+    isTesting: () => boolean;
+    isFirefoxPanel: () => boolean;
+    isFirefox: () => boolean;
+    setConfig: (value: Object) => void;
+    getConfig: () => Object;
+  }
+}
diff --git a/public/js/utils/defer.js b/public/js/utils/defer.js
index c8d2651eb8..b24bc4ad43 100644
--- a/public/js/utils/defer.js
+++ b/public/js/utils/defer.js
@@ -1,12 +1,26 @@
-module.exports = function defer() {
-  let resolve, reject;
-  let promise = new Promise(function() {
-    resolve = arguments[0];
-    reject = arguments[1];
+/* flow */
+
+export type deferred = {
+  resolve: (result: any) => void,
+  reject: (result: any) => void,
+  promise: Promise
+};
+
+function defer(): deferred {
+  let resolve: (result: any) => void;
+  let reject: (result: any) => void;
+  let promise = new Promise(function(
+    innerResolve: (result: any) => void,
+    innerReject: (result: any) => void
+  ) {
+    resolve = innerResolve;
+    reject = innerReject;
   });
   return {
     resolve: resolve,
     reject: reject,
     promise: promise
   };
-};
+}
+
+module.exports = defer;
diff --git a/public/js/utils/log.js b/public/js/utils/log.js
index 4395787acd..444f0c3c63 100644
--- a/public/js/utils/log.js
+++ b/public/js/utils/log.js
@@ -1,11 +1,13 @@
+/* @flow */
+
 const { isDevelopment } = require("devtools-config");
 
-function log() {
+function log(...args: any[]) {
   if (!isDevelopment()) {
     return;
   }
 
-  console.log.apply(console, ["[log]", ...arguments]);
+  console.log.apply(console, ["[log]", ...args]);
 }
 
 module.exports = log;