[plugin-transform-typescript] Default or named exported interface result in runtime error

[ThaNarie]

Input Code

interface IEvent {
  type: string;
}

export default IEvent;

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

{
  "presets": [
    ["@babel/env", {
      "targets": {
        "browsers": ["last 2 versions"],
        "node": "6"
      },
      "loose": true,
      "useBuiltIns": false
    }],
    "@babel/preset-stage-3",
    "@babel/typescript"
  ],
  "plugins": [
    ["@babel/plugin-transform-runtime", {
      "helpers": true,
      "polyfill": false,
      "regenerator": false,
      "moduleName": "@babel/runtime"
    }]
  ]
}

Expected Behavior

Either:

• Not referencing (thus exporting) IEvent at all
• Or, setting IEvent to undefined/null/anything so parsing the file doesn't result in a runtime error

Current Behavior

TS compilation results in:

"use strict";

exports.__esModule = true;
exports.default = void 0;
var _default = IEvent; // <<< IEvent doesn't exist!
exports.default = _default;

Which results in the runtime error:

Uncaught ReferenceError: IEvent is not defined

Possible Solution

When stripping interfaces (or probably any non-value types), either:

• leave a placeholder value for exports
• or, detect references in exports and remove those exports

Context

I'm using and re-exporting interfaces in my node modules' 'index.ts' file, so you can use them in other TypeScript projects, e.g.:

// index.ts
import { default as IEvent } from "./lib/IEvent";

// app.ts
import { IEvent } from "seng-event";

When publishing the package, the tsc also runs to generate d.ts files, which makes the code above work in a TS project (babel strips types and generates JS code, tsc generates the definitions).

Workaround
A workaround is to use named exports, since the faulty generated code only happens when using default exports.

export interface IEvent {
  type: string;
}

When using a named export, the export itself will be removed along with the interface, since it's a single statement. The (now empty) file will be imported somewhere, but will just result in an undefined value. This shouldn't matter, since Interfaces themselves are not used at runtime.

Your Environment

software	version(s)
Babel	7.0.0-beta.42
node	8.9.1
npm	5.5.1
Operating System	macOS 10.13.3

Related

Different but related to #7118 (where that's a syntax/parsing error, while this current issue is a runtime error)

---

Edit by @nicolo-ribaudo

This also doesn't work with export { interface } (#7641) - repl

[babel-bot]

Hey @ThaNarie! 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.

[nicolo-ribaudo]

Related: #7118

[ThaNarie]

@nicolo-ribaudo I noticed you changed the topic to include or named, but using named exports does work (and is a current workaround to keep using interfaces, I just rephrased my issue a bit) :)

[infctr]

btw this also doesn't work

export { IProps as IComponentProps }

[btmorex]

Not sure if what I'm seeing is related or a separate bug, but I experience the same behavior when there is anything else but an exported interface in the file (but I'm not re-exporting imported interfaces). For example:

interface A {}
export default A;

works fine, but:

export const a = () => {};
interface A {}
export default A;

gives me a runtime error.

Note: I haven't tested the above in particular, but my cases are just as trivial.

[leethree]

I'm getting this as a compile time error: SyntaxError: Export 'xxx' is not defined

[nicolo-ribaudo]

There is an open PR for that error.

[rifler]

I've updated

    "@babel/cli": "7.2.3",
    "@babel/core": "7.2.2",
    "@babel/plugin-proposal-class-properties": "7.3.0",
    "@babel/plugin-transform-runtime": "7.2.0",
    "@babel/preset-env": "7.3.1",
    "@babel/preset-react": "7.0.0",
    "@babel/preset-typescript": "7.1.0",

to the latest versions and code below stopped working with SyntaxError: xxx.ts: Export 'MyType' is not defined

type MyType = { field: string };

export { MyType };

There is an open PR for that error.

Where I can see it?

[rifler]

Oh, it breaks even if I do npm i instead of npm ci. Probably some indirect dependency updates in package-lock.json

[nicolo-ribaudo]

Where I can see it?

It's #9766. Sorry if I forgot to link it!

[leethree]

@nicolo-ribaudo The named exports are fixed but the default export is still broken in v7.4.5. Please reopen this issue.

REPL: https://babeljs.io/en/repl#?babili=false&browsers=&build=&builtIns=false&spec=false&loose=false&code_lz=JYOwLgpgTgZghgYwgAgJIFEBuFzIN4BQyyYAngA4QBcyAzmFKAOYDcBAvgQRAB7kD2UMMgAmEeAFcANsIzZwbIA&debug=false&forceAllTransforms=false&shippedProposals=false&circleciRepo=&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=true&presets=es2015%2Creact%2Cstage-2%2Ctypescript&prettier=false&targets=&version=7.4.5&externalPlugins=%40babel%2Fplugin-transform-typescript%407.4.5

[nicolo-ribaudo]

cc @airato do you want to take a look at this again?

[airato]

Yes, I'll take a look