TypeScript .d.ts syntax - export and declare

Question

I need help trying to understand what is the correct way to create a .d.ts file.

What threw me is that some people use this syntax:

// lib-a.d.ts
namespace My.Foo.Bar {
  interface IFoo {}
  interface IBar {}
}

vs.

// lib-b.d.ts
declare namespace My.Foo.Bar {
  interface IFoo {}
  interface IBar {}
}

vs.

// lib-c.d.ts
namespace My.Foo.Bar {
  export interface IFoo {}
  export interface IBar {}
}

vs.

// lib-d.d.ts
declare namespace My.Foo.Bar {
  export interface IFoo {}
  export interface IBar {}
}

vs.

// lib-e.d.ts
declare module My.Foo.Bar {
  export interface IFoo {}
  export interface IBar {}
}

Which one is correct? What is declare used for? What is export used for? When to use namespace vs. module?

Answer 1

The correct way is:

declare namespace NS {
    interface InterfaceTest {
        myProp: string;
    }

    class Test implements InterfaceTest {
        myProp: string;
        myFunction(): string;
    }
}

You always can check the correct signature by writing some .ts file and compiling it with the --declaration option (tsc test.ts --declaration). This will generate a d.ts file with the correct typings.

For example the above declaration file was generated from the following code:

namespace NS {
    export interface InterfaceTest {
        myProp: string;
    }

    export class Test implements InterfaceTest {
        public myProp: string = 'yay';

        public myFunction() {
            return this.myProp;
        }
    }   

    class PrivateTest implements InterfaceTest {
        public myPrivateProp: string = 'yay';

        public myPrivateFunction() {
            return this.myProp;
        }
    }
}