TypeScript: make subclass with writable properties

Question

I am trying to define a type T where all properties of T are non-readonly (writable), but still assignable to T. However, this does not seem to work when there are private/protected properties in T:

type Writable<T> = {
    -readonly [P in keyof T]: T[P];
}

class HttpClient {
    private handler: any;

    readonly foo: any;

    get(): void {
      // ...
    }
}

 // Error: property 'handler' is missing in type 'Writable<HttpClient>'
 // but required in 'HttpClient'
const httpClient: HttpClient = {} as Writable<HttpClient>; 

Is there a way to get this working?

Answer 1

A mapped type, such as Writable can't map over private fields (they are not part of what keyof returns). This is by design, after all a private field should not be accessible from outside.

Also if a class has a private field, an object literal (even if it contains the private field) can't by definition satisfy the type of the class.

 // Property 'handler' is private in type 'HttpClient' but not in type '{ handler: string; foo: string; get(): void; }'.
const httpClient: HttpClient = { // not assignable
  handler: "",
  foo: "",
  get () {}
}

Even another class with a private field with the same name can't satisfy the original class.

// Type '(Anonymous class)' is not assignable to type 'HttpClient'.
//   Types have separate declarations of a private property 'handler'.
const httpClient2: HttpClient = new class { // not assignable
  private handler: any
  foo: any
  get () {}
} 

The only way to make this work is if the private field is optional:

class HttpClient {
    private handler?: any;

    readonly foo: any;

    get(): void {
      // ...
    }
}

const httpClient: HttpClient = {
  foo: "",
  get () {}
}
console.log(httpClient instanceof HttpClient) // false 

Play

Also since an object with writable properties is assignable to a reference with a readonly property there is no need for Writable at all.

Note: Even though you can assign an object literal to the class it will not be an instance of the class, so httpClient instanceof HttpClient is false.