Search Terms

isolated modules emit declaration

Suggestion

In the blog post introducing Babel 7, it's recommended to use --isolatedModules in the "Caveats" section (https://blogs.msdn.microsoft.com/typescript/2018/08/27/typescript-and-babel-7/).

We use Babel to transpile our TS to better work with other tools in the ecosystem (such as Emotion or Jest), but we'd also still like to generate declaration files so consumers of our libraries can enjoy the type information.

However, when trying to follow the advice of adding isolatedModules to our config, we get the following warning:

tsconfig.json:5:5 - error TS5053: Option 'declaration' cannot be specified with option 'isolatedModules'.
5     "declaration": true,
      ~~~~~~~~~~~~~

Use Cases

As mentioned, I'd like to create declaration files for libraries who are transpiled using babel. I'd also like to make sure that whatever guarantees isolatedModules adds to allowed syntax are present, outside of the type information.

Babel itself will complain if we use e.g. const enum, but that only happens at compile time - isolatedModules will also show a warning in the IDE while writing the code.

Examples

Same as we currently get with {"declaration": true, "emitDeclarationOnly": true}, but with added {"isolatedModules": true}.

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• 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
• 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.