Typescript: TS4023: Exported variable 'X' has or is using name 'Y' from external module 'a/file/path' but cannot be named

[janakerman]

I'm attempting to create a set of external TypeScript modules as follows:

// ClassA.ts
export default class ClassA {
  public method() { return true; } 
}

// ModuleB.ts
import ClassA from './ClassA';
var moduleB = {
  ClassA: ClassA
};
export default moduleB;

// ModuleA.ts
import moduleB from './ModuleB';
var moduleA = {
  moduleB: moduleB
}
export default moduleA;

The idea is that this will be compiled down and supplied as a library. The plan is that the consumer of the library will instantiate an instance of ClassA as follows:

import moduleA from './ModuleA';
var anInstance = moduleA.moduleB.ClassA();

This looks as though it is compiling down to the expected JS, but I'm getting an error compiler error which I'm struggling to find further information about.

Using tsc v1.6.2
.../src/ModuleA.ts(3,5): error TS4023: Exported variable 'moduleA' has or is using name 'ClassA' from external module ".../src/ClassA" but cannot be named.

Can anyone here shed any light on this issue? Does what I'm trying to do make sense?

[janakerman]

Oh - if this isn't a suitable place to ask this question. Please let me know and I'd be happy to post it on another medium.

[RyanCavanaugh]

FWIW you'll usually get faster answers on Stack Overflow, but we do field well-phrased questions here.

The problem here is that you're using the --declaration flag, but have not provided a way for the compiler to do its job.

When trying to emit ModuleA.d.ts, the compiler needs to write an object type literal (e.g. { moduleB: { classA: *mumble?* } }) representing the shape of the module. But there isn't a name in scope that refers directly to classA, so it the type "cannot be named" and there's an error.

If you add an import of ClassA to ModuleA.ts, the error should go away.

[janakerman]

Hi Ryan - thanks for the advice RE stack overflow and the advice - adding the import to ModuleA got things compiling as expected.

A little context here - my goal here is indeed to generate declarations for my external modules. (Something I believe isn't quite working yet? #5039?) This import may be completely necessary but It feels a little strange for ModuleA to need to import the symbols that moduleB already exports. The result is that every time I add to ModuleB's object export, I'll need to add the import to ModuleA - either directly:

import {moduleB} from './moduleB';
import {ClassA} from './ClassA';

Or via ModuleB (in an effort to reduce specifying paths for ClassA in both places):

import {moduleB, ClassA} from './moduleB';

I know very little about the TS compiler - so maybe what I'm saying is completely unrealistic, but at first thought it feels like the compiler could deduce that ModuleB is exporting an object with N symbols on it, so ModuleA needs those symbols? The import information for ModuleB is already there - could ModuleA use that? Or would that be impossible because the import for ClassA is completely internal to ModuleB, so there's no way that the compiler can deduce the type for ClassA when creating definitions for ModuleA?

Thank you again for taking the time to answer. I've sorted my problem now so the above is mainly curiosity - there's no rush to answer.

[RyanCavanaugh]

I'm not sure exactly what you're saying. What should ModuleA.d.ts look like in this proposal?

[mhegazy]

The compiler will not add dependencies (i.e. import statements) that did not exist in the user code when it emits declarations. the error you are getting means that the compiler is trying to write a type annotation for an exported declaration but could not. this can have one of two reasons, either the name is not accessible, i.e. not imported in the current module, or there is a declaration that is shadowing the original declaration.

in both cases, your work around is to add explicit type annotation, if you add any type annotation, it will be emitted verbatim in the output; the other option is to make sure the name is accessible, i.e. you have an import to the module, and you understand what that means for your users that are importing your module.

[cwalv]

@mhegazy said:

The compiler will not add dependencies (i.e. import statements) that did not exist in the user code when it emits declarations.

The problem is that I don't always need the import statements in the code (obviously, since it works without --declarations), and including them is noisy, causing things like tslint to complain about the "unused import". I can see why you wouldn't want the compiler to add dependencies to emitted javascript, but what's the problem with adding them to emitting declarations?

[mhegazy]

feel free to log an issue to track this suggestion. the rational was imports are a declaration of dependency, and the compiler should not create one for you unless you instruct it to do.

[Offirmo]

I have the same problem described by @unional

[frederickfogerty]

Please reopen this issue, the issues outlined by @unional are very realistic and this makes it very hard to add any intermediate/helper libs on top of libraries while using the same types as the original module we are re-exporting.

[mhegazy]

Issue https://github.com/Microsoft/TypeScript/issues/9944 tracks adding the import at the declaration emit phase.

[unional]

Thank!

[felixfbecker]

The problem with adding the import is that with noUnusedLocals the compiler will complain about the type not being used. I could add an explicit type annotation, but then I don't get inference. Example:

class Whatever {
  fetch(uri: string): Promise<void> { }
  ensureFetched = MemoizedFunction<(uri: string) => Promise<void>> = memoize((uri: string) => this.fetch(uri))
}

I would like to omit type annotation for ensureFetched

[rohmanhm]

https://github.com/styled-components/styled-components/issues/973

[salim7]

I found a workaround for this:
in tsconfig: include: [ ..., "node_modules/@your_scope/your_library" ]
good luck and have fun :smiley:

[pelotom]

@salim7 this slows down your compilation times (because you're recompiling a library that should've already been compiled) and forces you to use the least common denominator of strictness settings with the target library.

[PFight]

@mhegazy the problem has reproduced in next scenario:

import * as Foo from "./Foo";

export class Bar {
    baz = new Foo.Baz(); // Compiler forgot "Foo." prefix in the type, and throws this error, because "Baz" without perfix is not imported.
    getBaz() { // All the same
       return new Foo.Baz();
    }
}

Solution is specify type explicit:

import * as Foo from "./Foo";

export class Bar {
    baz: Foo.Baz = new Foo.Baz(); // ok
    getBaz(): Foo.Baz { // ok
       return new Foo.Baz();
    }
}

[mhegazy]

I can not get this to reproduce using the sample above. please file a new bug, and provide more context to be able to reproduce the issue.

[mohyeid]

@PFight Thank you! That was it for me!

[brunolemos]

I have just faced this issue when using:

export { IMyInterface } from './file'
````

The solution was to do this:
```ts
import { IMyInterface } from './file'
export { IMyInterface }

But this really shouldn't be necessary tbh.

[yordis]

Did anyone figured out how to deal with noUnusedLocals ?

[pelotom]

@yordis // @ts-ignore

[yordis]

@pelotom yeah this is completely my fault,

I was thinking on one thing and I wrote something else.

Did Typescript fixed the issue between noUnusedLocals and the declarations?

[AnyhowStep]

My current workaround, which feels like a total pain to me,

import {SomeInterface} from "./SomeFile";

const _dummySomeInterface : undefined|SomeInterface = undefined;
_dummySomeInterface;

//Code that implicitly uses SomeInterface

Avoids noUnusedLocals, allows type inference for generic interfaces, too, where possible.