[Feature Request] fieldof similar to keyof but include private and protected field

[jonlepage]

Suggestion

🔍 Search Terms

fieldof , mapped type, mapped private protected
Can maybe related: #35416 #4822

✅ Viability Checklist

My suggestion meets these guidelines:

• ✅ This wouldn't be a breaking change in existing TypeScript/JavaScript code
• ✅ This wouldn't change the runtime behavior of existing JavaScript code
• ✅ This could be implemented without emitting different JS based on the types of the expressions
• [ ?] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• [ ?] This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

add a new utility fieldof for map easily all keys field include, private and protected

📃 Motivating Example

i have similar issue here where some pattern make complications !

type Writable<T> = { -readonly [P in keyof T]: T[P] }; // make writable A.parent for Childrable


class A {
  public readonly parent?: A
  protected Renderer() { };
}

// in this context, let say is a component added to A (plugin), with some method authorized to write on A.
class Childrable {
  public entity!: A;
  public readonly children: A[] = [];

  public addChild(...children: Writable<A>[]) {
    if (children.length > 1) {
      for (const child of children) this.addChild(child);
    } else {
      const child = children[0];

      if (child) {
        // So Writable<A> allow replace A.parent
        child.parent = this.entity;
        // But Writable<A> seem remove protected.Renderer prototype and now they are not compatible
        // I need allow replace A.parent and add A in Childrable.children in the same scope !
        this.children.push(child);
      }
    }

    return this;
  }
}

playground

It would be great to see a easy way to handle more patterns with protected, private fields when we map types for specific case.
In my upper example, Childrable is a component where you can attach to Entity A , and Childrable have some method where allowed to write in some readOnly field from A.
But map to readOnly will remove private and protected field and will create issue, in this case fieldof will fix the issue, becaue we want allow Writable, in context of component have method for mutate entities.

💻 Use Cases

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

So same as keyof, but include all private and protected fields for specific pattern.
This will also maybe unlock more powerful features and patterns, but let just talk about this specific case for now.

[tadhgmister]

The only cases I could imagine this being potentially useful are to allow Required<X> and Writable<X> to be assignable to X for any class X with protected fields. In any other case this would break the most basic use case of protected methods since protected fields are checked for collision between classes not assignability.

As for your case you declare A.parent to be readonly which means it shouldn't be assigned outside the constructor and you are asking for a feature to literally allow you to break that assurance, so I'm not convinced even for the Writable case it is a good idea.

I'm pretty sure you just want the argument to be A not Writable<A> so that it is expected for external users to pass actual instances and then use a type assert inside the method to allow you to modify the property despite that not being allowed according to the declaration of A. playground

class Childrable {
  public entity!: A;
  public readonly children: A[] = [];

  public addChild(...children: A[]) {
    // ...
      const child = children[0];
      if (child) {
        // use type assertion here to indicate we are breaking the declared type in A 
        (child as Writable<A>).parent = this.entity;
        this.children.push(child);
      }
    return this;
  }
}

[jonlepage]

I found a way by abstract class with a method useWritable()
But it would still be interesting to be able to extract all the fields.
Am not fan of type assertion expression for readability.

export abstract class Token extends Entity {
	public readonly parent?: Token;

	isContainer(): this is Container {
		return false;
	}

	isPrimitive(): this is Primitive {
		return false;
	}

	useWritable(): Writable<this> {
		return this;
	}
}

export class Container extends Token {
	declare public readonly parent?: Container;
	public readonly children: Token[] = [];

	public override isContainer(){
		return true;
	}

	protected override get Renderer() {
		return Renderer;
	}

	public addChild( ...children: Token[] ) {
		if ( children.length > 1 ) {
			for ( const child of children ) this.addChild( child );
		} else {
			const child = children[0];

			if ( child ) {
				if ( child.parent?.isContainer() ) child.parent.removeChild( child );
				child.useWritable().parent = this;
				this.children.push( child );
			}
		}

		return this;
	}
}

[katz12]

I have a use case for this wherein I'm using branded types to create a branded class (similar to what https://www.npmjs.com/package/@prosopo/ts-brand does) and the class that I am branding contains a private field.

class Foo {
  #privateField: string
}

const BrandedFoo = brandedClass(Foo, 'brand')
const instance = new BrandedFoo()

So the instance I create is actually a Foo (and should have the private member) but it is not assignable to Foo because the type mapping that creates the branded class uses keyof:

export type Resolve<T> = T extends Function ? T : { [K in keyof T]: T[K] };
export const brandKey = Symbol("brand");
export type Brand<T, U> = Resolve<
	T & {
		[brandKey]: U;
	}
>;

EDIT: So as not to waste your time, I want you to know that I'm not currently blocked on this. I ended up removing that Resolve type and T & { [brandKey]: U } works just fine with private members.

I don't really see the point in handling Function in branding anyway.