Please add typescript namespaces support

[vegansk]

Is your proposal related to a problem?

We heavily use typescript namespaces to group algebraic data types constructors, but CRA doesn't support it because of babel's default allowNamespaces setting value. And that's why we can't use CRA.

Describe the solution you'd like

Set allowNamespaces setting to true or allow to configure it somehow.

[heyimalex]

There are currently caveats to babel's namespace support, and we generally prefer to not enable features that may work in surprising ways. We're also pretty against adding configuration for niche features.

[vegansk]

But these caveats are well known for that who uses namespaces, and it's really works in babel. And here is a good description of the reasons of using namespaces:

Complex business fat clients like SPA may contain a lot types in different namespaces. ES module is not going to replace namespaces of TS, since module is 1 to 1 mapping with file. Manually avoiding circular references among files is a nightmare against application programming and overall productivity of app programmers. Namepaces is the best solution for code generation -- all generated by code generator stay in one file.

Our simplified example of using namespaces from real business application. Without namespaces it becomes a naming hell.

// ADT Route

export type Route = Route.Main | Route.Reports | Route.Report

export namespace Route {
  /* Главная страница */
  export interface Main {__tag: "Main"}

  /* Список отчетов */
  export interface Reports {__tag: "Reports"}

  /* Отчет */
  export interface Report {
    __tag: "Report"
    info: ReportInfo
  }
}

export namespace Route {
  export const Main = (): Route => {return {__tag: "Main"}}

  export const Reports = (): Route => {return {__tag: "Reports"}}

  export const Report = (info: ReportInfo): Route => {
    return {
      __tag: "Report",
      info
    }
  }
}

[heyimalex]

I think you could do that like this:

type Route = Route.Main | Route.Reports | Route.Report;

declare namespace Route {
  export interface Main {
    __tag: "Main";
  }
  export interface Reports {
    __tag: "Reports";
  }
  export interface Report {
    __tag: "Report";
    info: ReportInfo;
  }
}

const Route = {
  Main: (): Route => ({ __tag: "Main" }),
  Reports: (): Route => ({ __tag: "Reports" }),
  Report: (info: ReportInfo): Route => ({ __tag: "Report", info })
};

export default Route;

That being said, I'm kind of open to this? The problem I'm trying to prevent is code that appears to compile fine, but breaks at runtime. I'm honestly skeptical that the caveats are well known, but maybe isolatedModules plus uninitialized variable lints make this a non-issue. Will have to play around with it to see.

[vegansk]

I refactored our code generator to support suggested pattern, it works well. Thank you