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@babel/plugin-transform-flow-comments

You should be able to use this plugin instead of @babel/plugin-flow-strip-types to preserve the /* @flow */ directive and still use flow.

Flow Comments Blog Post

Example​

In

JavaScript
function foo(bar?) {}
function foo2(bar?: string) {}
function foo(x: number): string {}
type B = {
  name: string,
};
export type GraphQLFormattedError = number;
import type A, { B, C } from "./types";
import typeof D, { E, F } from "./types";

Out

JavaScript
"use strict";

function foo(bar /*:: ?*/) {}
function foo2(bar /*:: ?: string*/) {}
function foo(x /*: number*/) /*: string*/ {}
/*:: type B = {
  name: string;
};*/
/*:: export type GraphQLFormattedError = number;*/
/*:: import type A, { B, C } from './types';*/
/*:: import typeof D, { E, F } from './types';*/

Installation​

npm install --save-dev @babel/plugin-transform-flow-comments

Usage​

babel.config.json
{
  "plugins": ["@babel/plugin-transform-flow-comments"]
}

Via CLI​

Shell
babel --plugins @babel/plugin-transform-flow-comments script.js

Via Node API​

JavaScript
require("@babel/core").transformSync("code", {
  plugins: ["@babel/plugin-transform-flow-comments"],
});