diff --git a/packages/babel-generator/src/index.js b/packages/babel-generator/src/index.js
index d16a06e299..711a335675 100644
--- a/packages/babel-generator/src/index.js
+++ b/packages/babel-generator/src/index.js
@@ -2,6 +2,7 @@ import detectIndent from "detect-indent";
 import SourceMap from "./source-map";
 import * as messages from "babel-messages";
 import Printer from "./printer";
+import type {Format} from "./printer";
 
 /**
  * Babel's code generator, turns an ast into code, maintaining sourcemaps,
@@ -20,23 +21,6 @@ class Generator extends Printer {
     this.ast = ast;
   }
 
-  format: {
-    shouldPrintComment: (comment: string) => boolean;
-    retainLines: boolean;
-    comments: boolean;
-    auxiliaryCommentBefore: string;
-    auxiliaryCommentAfter: string;
-    compact: boolean | "auto";
-    minified: boolean;
-    quotes: "single" | "double";
-    concise: boolean;
-    indent: {
-      adjustMultilineComment: boolean;
-      style: string;
-      base: number;
-    }
-  };
-
   ast: Object;
 
   /**
@@ -60,7 +44,7 @@ class Generator extends Printer {
  * - If `opts.compact = "auto"` and the code is over 100KB, `compact` will be set to `true`.
  */
 
-function normalizeOptions(code, opts, tokens) {
+function normalizeOptions(code, opts, tokens): Format {
   let style = "  ";
   if (code && typeof code === "string") {
     let indent = detectIndent(code).indent;
diff --git a/packages/babel-generator/src/printer.js b/packages/babel-generator/src/printer.js
index fd74a6da14..323342e54d 100644
--- a/packages/babel-generator/src/printer.js
+++ b/packages/babel-generator/src/printer.js
@@ -6,6 +6,23 @@ import * as n from "./node";
 import Whitespace from "./whitespace";
 import * as t from "babel-types";
 
+export type Format = {
+  shouldPrintComment: (comment: string) => boolean;
+  retainLines: boolean;
+  comments: boolean;
+  auxiliaryCommentBefore: string;
+  auxiliaryCommentAfter: string;
+  compact: boolean | "auto";
+  minified: boolean;
+  quotes: "single" | "double";
+  concise: boolean;
+  indent: {
+    adjustMultilineComment: boolean;
+    style: string;
+    base: number;
+  }
+};
+
 export default class Printer {
   constructor(format, map, tokens) {
     this.format = format || {};
@@ -13,6 +30,7 @@ export default class Printer {
     this._whitespace = tokens.length > 0 ? new Whitespace(tokens) : null;
   }
 
+  format: Format;
   insideAux: boolean = false;
   inForStatementInitCounter: number = 0;