Set esModuleInterop to "true" by default

Search Terms

• esModuleInterop

Suggestion

Change esModuleInterop from being an opt-in to instead be an opt-out, taking one step further towards deprecating the non-esModuleInterop mode and make esModuleInterop the only supported mode going forward (which has to be the long term goal).

Use Cases

When Typescript 2.7 introduced esModuleInterop almost 3 years ago it emphasized clearly that:

We highly recommend applying it both to new and existing projects.

esModuleInterop is also a recommended option to set in tsconfig.json and when one runs tsc --init it gets set to true automatically.

However, the fact that it still is opt-in makes that non-esModuleInterop mode is still very prevalent, which kind of works against the very problems it was meant to fix – as rather than having just the pre-esModuleInterop functionality, modules now have to try to make things work across both options – and on top of that increasingly have to work with TS-validated JS.

All in all, that causes problems: https://github.com/fastify/env-schema/pull/17

And is hard to wrap ones head around: https://github.com/fox1t/modules-playground

Even if one really tries: https://github.com/fox1t/modules-playground/pull/3

Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript/JavaScript code
  • It would for all code which haven’t yet adopted esModuleInterop, but it would make it easier to get TypeScript and JavaScript code to stay compatible with one another
• This wouldn’t change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
  • It would emit different JS for all code which haven’t yet adopted esModuleInterop, but it would make it easier to get TypeScript and JavaScript code to stay compatible with one another
• This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript’s Design Goals.
  • Apart from Do not cause substantial breaking changes from TypeScript 1.0.

[bmeck]

@voxpelli

My impression was that esModuleInterop was meant to increase compatibility?

It kind of does, but not enough to be completely compatible. Particularly how "default" works still tries to do some magic and things start to get incompatible because of it.

Given:

// ./entry.ts
import * as asNamespace from './dep.js';
import asDefault from './dep.js';

console.log({
  asDefault,
  asNamespace
});

// ./dep.js
module.exports = {
  __esModule: true,
  default: 'prop named default'
};

Compiling to CommonJS will result in the following outputs:

Node (after renaming .ts to .mjs):

$ node ./entry.mjs
{
  asDefault: { __esModule: true, default: 'prop named default' },
  asNamespace: [Module: null prototype] {
    __esModule: true,
    default: { __esModule: true, default: 'prop named default' }
  }
}

After TS:

$ node ./out/entry.js
{ asDefault: 'prop named default',
  asNamespace: { __esModule: true, default: 'prop named default' } }

Namely, even with esModuleInterop it tries to do some hoisting of the default property if __esModule:true exists that Node won’t ever do.

[bmeck]

I’m a bit wary of using esModuleInterop since how it interacts with node’s ESM implementation of default isn’t quite compatible. This leads to various conversations about what to do usually ending up with “don’t use default exports” as my go to response. Enabling it by default would remove some issues but it seems like a second migration would have to happen afterward if node compatibility was another configuration option.