Is this a bug report?

Yes

Environment

CRA ^2.0.6-next.c662dfb0 with TypeScript support

Steps to Reproduce

1. npx create-react-app --scripts-version @next myapp
2. cd myapp
3. yarn add typescript @types/react @types/react-dom @types/jest
4. mv src/index.{js,tsx}
5. Add src/setupProxy.js with following content:

const proxy = require('http-proxy-middleware')
    
module.exports = function(app) {
    app.use(proxy('/api', { target: 'http://localhost:5000/' }))
}

6. yarn run build

Expected Behavior

setupProxy.js should be compiled without any problems.

Actual Behavior

Behold the "Type error: Cannot compile namespaces when the '--isolatedModules' flag is provided. TS1208" error message.

Additionally: if one were to rewrite the setupProxy.js script with proper ES6 exports like this:

const proxy = require('http-proxy-middleware')
    
export const setupProxy = (app) => {
    app.use(proxy('/api', { target: 'http://localhost:5000/' }))
}

yarn run build compiles successfully, while yarn run start claims "unexpected token export".