Exporting of TypeScript types separate from their declarations doesn't get erased

[pelotom]

Hi, I just tried out the new babel-preset-typescript in version 7.0.0-alpha.19 with much anticipation. Unfortunately I discovered that it doesn't erase types that are exported separate from their declaration:

type Foo = number
export { Foo }

gets transformed to

export { Foo }

which of course generates an error if you try to run it: ReferenceError: Foo is not defined. It correctly deletes the export if you write

export type Foo = number

[babel-bot]

Hey @pelotom! 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.

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[ghost]

Hi @pelotom, we haven't written the guide for this yet, but this plugin needs to be used with the --isolatedModules option. If you have that enabled (and a recent enough version of the typescript package), you should get a compile error, because we don't support re-exports of types in that mode. The reason being that in general, in order to determine whether something is a type, we need information about other modules.

import { T } from "./other-module";
export { T }; // Is this a type export?

In babel we transpile files independently, so we can't access the information we would need there.

[Jessidhia]

@andy-ms is something like import { T as ImportedT } from './other-module'; export type T = ImportedT (intended to be) supported?

[ghost]

Yes, that should work since we can use local information to determine that it's a type-only import.

[rodrigopivi]

Experiencing the same error:

export type IExecuteGQL = <T>({ persistedQueryId, variables }: IGQLPersistedQuery<T>) => Promise<T>;

Valid typescript that does not work with current babel-preset-typescript 7.0.0-beta.3

[ghost]

Hi @rodrigopivi, see #6667. In the meanwhile you could replace the binding pattern { persistedQueryId, variables } with a single name, e.g. <T>(query: IGQLPersistedQuery<T>) => Promise<T>;

[alangpierce]

@andy-ms it seems like the original code should be reasonable to handle even in isolatedModules mode:

type Foo = number
export { Foo }

I realize that isolatedModules currently gives an error, but seems like that error is overly aggressive here. This isn't really a re-export, it's just an exported type across two statements.

Just like how Babel (and TS) will remove imported identifiers that are only used as types in the file, couldn't it also remove exported identifiers that are only declared as types in the file? It looks to me like the TypeScript compiler already does this with transpileModule, so seems like a case where Babel/isolatedModules is more limited than it needs to be.