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.

Start by tracing how esModuleInterop is represented in tsconfig.json and initialized by tsc --init, then assess the compiler and emitted-JavaScript consequences of changing its default. Done would require resolving the breaking-change implications and establishing how the opt-out behavior should work for existing projects.