Creating nested namespaces with interfaces in Typescript type definitions

In my Javascript project, I am trying to define typing for a specific structure. Consider the following simplified example:

a
|
+ A.js
+ b
  |
  + B.js 

Here we have a folder 'a', and inside it there is another folder 'b'. My goal is to import modules like this:

import { A } from "a"
import { B } from "a/b" 

The ideal typing that I want to implement would look something like this:

declare namespace a {
    interface A { }

    namespace b {
        interface B { }
    }
}

declare module "a" {
    export = a
}

declare module "a/b" {
    export = a.b
}

However, when I try to use this setup, I encounter an error stating

Cannot use namespace 'a' as a value
.

I found that changing the interfaces to classes resolves the issue. Can anyone explain why this is the case? Is there a way to achieve the desired definitions using interfaces?

Answer №1

https://github.com/Microsoft/TypeScript/issues/17531

If a namespace does not contain any values or statements, it is considered an uninstantiated namespace and holds no runtime value.

To avoid the error, you can use this code:

declare module "x/y" {
    import y = x.y
    export = y
}

However, if the namespaces x and x.y are not available as globals during runtime, it is recommended to use:

declare module "x" {
    export interface X {
        bar: number
    }
}

declare module "x/y" {
    import {X} from 'x'
    export interface Y {
        foo: X
    }
}

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