Convert exported TypeScript types to undefined

[remcohaszing]

Bug Report

Current Behavior

When the code below is compiled using webpack and babel-loader, this triggers the following warning:

WARNING in ./src/Bar/index.tsx 1:0-42
"export 'BarProps' was not found in './Bar'
 @ ./src/index.tsx

The original issue is here: babel/babel-loader#769

However, I believe it can be fixed upstream (in @babel/plugin-transform-typescript).

Input Code

src/Bar/Bar.tsx

export interface BarProps {
  value: string;
}

export default function Bar(props?: BarProps) {}

src/Bar/index.tsx

export { default, BarProps } from './Bar'

src/index.tsx (Webpack entry point)

import Bar from './Bar';

console.log(Bar())

Expected behavior/code

There are no warnings

Babel Configuration (.babelrc, package.json, cli command)

module.exports = {
  plugins: ['@babel/plugin-transform-typescript']
}

Environment

• Babel version(s): 7.5.5
• Node/npm version: Node 10/npm 6
• OS: Ubuntu 18.04
• Monorepo: no
• How you are using Babel: loader

Possible Solution

Convert exported types and interfaces to undefined constants. This way, Webpack will find the constants and stop complaining and their types are still undefined when using typeof checks. A minifier should strip the constant, as it is unused in the final bundle.

I believe this should be an opt in solution on the Babel part. I.e. an option typeToConst: true for both @babel/plugin-transform-typescript and @babel/preset-typescript

Additional context/Screenshots

This webpack config can be used to verify behaviour.

module.exports = {
  devtool: 'source-map',
  resolve: {
    extensions: ['.tsx'],
  },
  module: {
    rules: [
      { 
        test: /\.tsx?$/, 
        use: 'babel-loader'
      }
    ]
  }
};

[babel-bot]

Hey @remcohaszing! We really appreciate you taking the time to report an issue. The collaborators on this project attempt to help as many people as possible, but we're a limited number of volunteers, so it's possible this won't be addressed swiftly.

If you need any help, or just have general Babel or JavaScript questions, we have a vibrant Slack community that typically always has someone willing to help. You can sign-up here for an invite.

[realduolaf]

#10220 How's it going?

[JLHwung]

@realduolaf #10220 fixes flow type export. It does not address this issue.

[realduolaf]

@JLHwung OK~ Thank you.

[jackmellis]

Is there any more news on this issue? This actually breaks builds using rollup and I’m struggling to find a workaround that doesn’t involve abandoning the Babel plugin

[nicolo-ribaudo]

In the next minor release, you'll be able to explicitly mark type-only exports using

export type { foo };