diff --git a/packages/babylon/src/options.js b/packages/babylon/src/options.js
index ecac819f46..1456be6360 100755
--- a/packages/babylon/src/options.js
+++ b/packages/babylon/src/options.js
@@ -1,7 +1,15 @@
+/* @flow */
 // A second optional argument can be given to further configure
 // the parser process. These options are recognized:
 
-export const defaultOptions = {
+export const defaultOptions: {
+  sourceType: string,
+  allowReturnOutsideFunction: boolean,
+  allowImportExportEverywhere: boolean,
+  allowSuperOutsideMethod: boolean,
+  plugins: Array<string>,
+  strictMode: any
+} = {
   // Source type ("script" or "module") for different semantics
   sourceType: "script",
   // When enabled, a return at the top level is not considered an