declaration: Namespace which exports class can't be merged with variable

[ikokostya]

TypeScript Version: 2.2.0-dev.20170117

I try to write declaration for got module.

Code

// Use interface, because `got` function has `delete` property.
interface GotApi {
    (): any;
    delete(): any;
}

declare var got: GotApi;

declare namespace got {
    export class HTTPError extends Error {}
}

export = got;

Expected behavior:

No errors. HttpError class is available using got exported variable:

import * as got from 'got';

// let error = ...
if (error instanceof got.HTTPError) {
}

Actual behavior:

I get compiler errors for declaration file:

index.d.ts(6,13): error TS2300: Duplicate identifier 'got'.
index.d.ts(8,19): error TS2300: Duplicate identifier 'got'.

Used tsconfig.json:

{
    "compilerOptions": {
        "module": "commonjs",
        "moduleResolution": "node",
        "target": "es2015",
        "noImplicitAny": true,
        "strictNullChecks": true,
        "outDir": "build"
    }
}

Related issue: #1784

[aluanhaddad]

Note that the GotApi interface is not exported

[ikokostya]

Note that the GotApi interface is not exported

How interface should be exported? This code (similar to request declaration) also isn't compiled with the same error:

declare namespace got {
    export interface GotApi {
        (): any;
        delete(): any;
    }

    export class HTTPError {}
}

declare var got: got.GotApi;
export = got;

If remove class export from namespace, compilation is completed:

interface GotApi {
    (): any;
    delete(): any;
}

declare var got: GotApi;

declare namespace got {
    // export class HTTPError extends Error {} <--- Comment this line
}

export = got;

[mhegazy]

Ideally this would be rewitten as function+namespace but it can not, #1784 (and #9846) track that.

Instantiated namespaces (i.e. ones that has a value in them) can not merge with variables, since one of them will override the other at run-time.

you can write your declaration as such:

declare class HTTPError { }

interface GotApi {
    (): any;
    delete(): any;
    HTTPError: typeof HTTPError;
}

declare var got: GotApi;
export = got;

[ikokostya]

Instantiated namespaces (i.e. ones that has a value in them) can not merge with variables, since one of them will override the other at run-time.

As I understand from documentation about declaration merging function and variable are value creating declarations, i.e. code

interface GotApi {
    (): any;
}
declare var got: GotApi;
declare namespace got {
    export class HTTPError {}
}
export = got;

and

declare var got: () => any;
declare namespace got {
    export class HTTPError {}
}
export = got;

fails to compile. But function declaration is compiled:

declare function got(): any;
declare namespace got {
    export class HTTPError {}
}
export = got;

Why this happen?

Also, I propose to make error message is more detailed, because current message error TS2300: Duplicate identifier 'got'. is cryptic at first glance. For example, namespace and variable declaration can't be merged.

[mhegazy]

var got: ()=> any is treated as a variable, and not as a function declaration. the reason is a variable can be assigned to later on where as a function can not.

[ikokostya]

@mhegazy Thanks for explanation.

[BendingBender]

@mhegazy So why is it then not allowed to merge a declare const with a declare namespace? AFAIK, const is also not assignable more than once.