Why is it not possible to declare an interface or type within a TypeScript class?

I am struggling to define interface | type within a TypeScript class. Here is the code snippet:

class MyClass {
    interface IClass {
        name: string,
        id: string
    }
}

However, I keep encountering this error:

Unexpected token. A constructor, method, accessor, or property was expected.

My Desired Outcome:

I am developing a framework where users can extend the base class Randoms and override certain methods, but the child class does not provide any type intelligence.
Here is an example of the issue in the code:

abstract class RandomsRoute {
   public get (req:Resquest, res:Response): Promise <void> { res.send ('') }
}

// client side

import RandomsRoute, { Request, Response } from '@my-pkg'

class Client extends RandomsRoute {
   // public get (req, res) {res.send('client side')} // error here
   public get (req: Request, res: Response): Promise <void> { res.send ('') }
}

The problematic part seems to be:

{ Request, Response } from '@my-pkg'

I hope to simplify the import process for users and potentially offer better APIs. Any suggestions on how to achieve this?

Answer №1

In the current state, it is not feasible to directly declare types or interfaces within a class body. Although there is a pending feature request for this at microsoft/TypeScript#7061, it has not been integrated into the language yet. Even if implemented, exporting these types out of the class could pose challenges. For instance, while you can define types within functions, those definitions remain hidden outside the function scope:

function foo() {
  interface Foo {
    a: string,
    b: number;
  }
}

const f: Foo = {a: "", b: 1}; // error!
// ----> ~~~ cannot find name Foo

const g: foo.Foo = {a: "", b: 1}; // error!
// ----> ~~~ cannot find namespace foo

function foo() {
  export interface Foo { // error!
  //~~~~ <-- modifiers cannot appear here.
    a: string,
    b: number;
  }
}

Hence, it might not be ideal to define your interface within the class itself.


A recommended alternative, as discussed in this comment on microsoft/TypeScript#7061, is to merge the class with a namespace from which you can subsequently export your type:

export class MyClass {

}
export declare namespace MyClass {
  export interface IClass {
    name: string,
    id: string
  }
}

Then, in another file, you can import MyClass and access both the class and the namespace:

import { MyClass } from '@my-pkg'
const c: MyClass = new MyClass();
const i: MyClass.IClass = { name: "", id: "" };

Playground link to code

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