diff --git a/lib/babel-packages.js.flow b/lib/babel-packages.js.flow
index d6d45648e64e..a9629d16a354 100644
--- a/lib/babel-packages.js.flow
+++ b/lib/babel-packages.js.flow
@@ -133,3 +133,27 @@ declare module "@babel/helper-validator-identifier" {
   declare function isIdentifierChar(code: number): boolean;
   declare function isIdentifierName(name: string): boolean;
 }
+
+declare module "@babel/helper-validator-option" {
+  declare class OptionValidator {
+    descriptor: string;
+    constructor(descriptor: string): OptionValidator;
+    validateTopLevelOptions(options: Object, TopLevelOptionShape: Object): void;
+    validateBooleanOption<T>(
+      name: string,
+      value?: boolean,
+      defaultValue?: T
+    ): boolean | T;
+    validateStringOption<T>(
+      name: string,
+      value?: string,
+      defaultValue?: T
+    ): string | T;
+    invariant(condition: boolean, message: string): void;
+    formatMessage(message: string): string;
+  }
+  declare function findSuggestion(
+    str: string,
+    arr: $ReadonlyArray<string>
+  ): string;
+}
diff --git a/packages/babel-helper-validator-option/src/find-suggestion.js b/packages/babel-helper-validator-option/src/find-suggestion.ts
similarity index 89%
rename from packages/babel-helper-validator-option/src/find-suggestion.js
rename to packages/babel-helper-validator-option/src/find-suggestion.ts
index 57f14fc4a298..d418632ed307 100644
--- a/packages/babel-helper-validator-option/src/find-suggestion.js
+++ b/packages/babel-helper-validator-option/src/find-suggestion.ts
@@ -1,5 +1,3 @@
-// @flow
-
 const { min } = Math;
 
 // a minimal leven distance implementation
@@ -9,7 +7,7 @@ const { min } = Math;
 // that have less than 20 ASCII characters
 
 // https://rosettacode.org/wiki/Levenshtein_distance#ES5
-function levenshtein(a, b) {
+function levenshtein(a: string, b: string): number {
   let t = [],
     u = [],
     i,
@@ -44,7 +42,7 @@ function levenshtein(a, b) {
  * @param {string[]} arr
  * @returns {string}
  */
-export function findSuggestion(str: string, arr: string[]): string {
+export function findSuggestion(str: string, arr: readonly string[]): string {
   const distances = arr.map<number>(el => levenshtein(el, str));
   return arr[distances.indexOf(min(...distances))];
 }
diff --git a/packages/babel-helper-validator-option/src/index.js b/packages/babel-helper-validator-option/src/index.ts
similarity index 100%
rename from packages/babel-helper-validator-option/src/index.js
rename to packages/babel-helper-validator-option/src/index.ts
diff --git a/packages/babel-helper-validator-option/src/validator.js b/packages/babel-helper-validator-option/src/validator.ts
similarity index 90%
rename from packages/babel-helper-validator-option/src/validator.js
rename to packages/babel-helper-validator-option/src/validator.ts
index d899023c5dab..eff7165bfb0f 100644
--- a/packages/babel-helper-validator-option/src/validator.js
+++ b/packages/babel-helper-validator-option/src/validator.ts
@@ -1,4 +1,3 @@
-// @flow
 import { findSuggestion } from "./find-suggestion.js";
 
 export class OptionValidator {
@@ -30,13 +29,13 @@ export class OptionValidator {
 
   // note: we do not consider rewrite them to high order functions
   // until we have to support `validateNumberOption`.
-  validateBooleanOption(
+  validateBooleanOption<T>(
     name: string,
     value?: boolean,
-    defaultValue?: boolean,
-  ): boolean | void {
+    defaultValue?: T,
+  ): boolean | T {
     if (value === undefined) {
-      value = defaultValue;
+      return defaultValue;
     } else {
       this.invariant(
         typeof value === "boolean",
@@ -46,13 +45,13 @@ export class OptionValidator {
     return value;
   }
 
-  validateStringOption(
+  validateStringOption<T>(
     name: string,
     value?: string,
-    defaultValue?: string,
-  ): string | void {
+    defaultValue?: T,
+  ): string | T {
     if (value === undefined) {
-      value = defaultValue;
+      return defaultValue;
     } else {
       this.invariant(
         typeof value === "string",