Typescript type definition nested namespace with interfaces

Question

I would like to define typing for a Javascript project. Let's consider a simplified version:

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

There is a folder 'a', and inside it there is folder 'b', and I would like to use it as:

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

The typing that I ideally would like to have looks like 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, this gives me the error Cannot use namespace 'a' as a value.

I noticed if I change interfaces to classes, the problem resolves. Can someone please shed some light why it is like that? Is there a way to get such definitions with interfaces?

Adrian Leonhard · Accepted Answer · 2017-10-03 08:56:30Z

3

https://github.com/Microsoft/TypeScript/issues/17530

A namespace that contains no values or statements is considered an uninstantiated namespace, and has no runtime value.

You can prevent the error by doing:

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

However, unless the namespaces a and a.b are actually available as globals at runtime, you should write:

declare module "a" {
    export interface A {
        foo: string
    }
}

declare module "a/b" {
    import {A} from 'a'
    export interface B {
        bar: A
    }
}

answered Oct 3, 2017 at 8:56

Adrian Leonhard

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2 Comments

Wickoo Over a year ago

What do you mean by "unless the namespaces a and a.b are actually available as globals at runtime"?

Adrian Leonhard Over a year ago

@Wickoo i.e. you can access them without importing them, like setTimeout

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