Infer type of object of classes

Question

Building a form config builder, want to infer types dynamically, here's the minimal reproduction code

interface Meta {
    optional: boolean
}

class Base<Type = any> {
    readonly _type!: Type
    baseMeta: Meta

    constructor() {
        this.baseMeta = {
            optional: false,
        }
    }

    optional() {
        this.baseMeta.optional = true
        return this;
    }
}

class FieldText extends Base<string> {

}

class FieldNumber extends Base<number> {

}


class Field {
    static text() {
        return new FieldText();
    }

    static number() {
        return new FieldNumber();
    }
}

const fields = {
    fieldText: Field.text(),
    fieldNumber: Field.number(),
    fieldTextOptional: Field.text().optional(),
    fieldNumberOptional: Field.number().optional(),
}

Given code above I want to achieve one of the following types, doesn't matter which one

type ExpectedType = {
    fieldText: string;
    fieldNumber: number;
    fieldTextOptional: number | undefined;
    fieldNumberOptional: number | undefined;
}

Also fine

type ExpectedTypeWithOptionalKeys = {
    fieldText: string;
    fieldNumber: number;
    fieldTextOptional?: number;
    fieldNumberOptional?: number;
}

I've gotten to infering types, but can't figure out why the optional property is not taken in consideration

type ExtractFieldType<O, T> = O extends true ? T | undefined : T;

type SchemaType<T extends Record<string, Base>> = {
    [Property in keyof T]: ExtractFieldType<T[Property]['baseMeta']['optional'], T[Property]['_type']>;
};

type Schema = SchemaType<typeof fields>;

type Schema results in:

type Schema = {
    fieldText: string;
    fieldNumber: number;
    fieldTextOptional: string;
    fieldNumberOptional: number;
}

this O extends true? T | undefined: T O extends true? T | undefined: T O extends true? T | undefined: T always returns T, doesn't take actual value of optional into consideration

EDIT:

after a bit of fumbling around with @kelly 's answer for providing a value to the optional function here's the end result:

    optional<T extends boolean>(value?: T): Base<Type, T> {
        const optional = typeof value === 'boolean' ? value : true;
        this.baseMeta.optional = optional as Optional;
        return this as unknown as Base<Type, T>;
    }

first you can't have a default value as then Typescript get's angry with:

Type 'boolean' is not assignable to type 'T'.
  'boolean' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'boolean'.(2322)

So we simply make value optional and then the rest works pretty much as Kelly explained

except for return this as unknown as Base<Type, T>;

if you remove the as unknown part you get the following error:

Conversion of type 'this' to type 'Base<Type, T>' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
  Type 'Base<Type, Optional>' is not comparable to type 'Base<Type, T>'.
    Type 'Optional' is not comparable to type 'T'.
      'Optional' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'boolean'.(2352)

Here's the final working in Typescript Playground

Answer 1

The type of optional in Meta is always boolean . You need to store the value in a generic to use later:

interface Meta<Optional extends boolean> {
    optional: Optional;
}

class Base<Type = any, Optional extends boolean = false> {
    readonly _type!: Type
    baseMeta: Meta<Optional>

    constructor() {
        this.baseMeta = {
            optional: false as Optional,
        }
    }

    optional(): Base<Type, true> {
        this.baseMeta.optional = true as Optional; // needs unsafe assert, or alternatively //@ts-ignore
        return this as Base<Type, true>;
    }
}

Then when you change it with optional , trick TypeScript into believing you are returning Base<Type, true> instead.

You only need to change this. Your original solution now works.

---

Addressing the followup in the comments:

optional<T extends boolean = true>(value: T = true): Base<Type, T> {
    this.baseMeta.optional = value as Optional;
    return this as Base<Type, T>;
}