From https://youtu.be/wpgKd-rwnMw

JSON-stringify.ts

type JSONified<T> =
    JSONifiedValue<T extends { toJSON(): infer U } ? U : T>;

type JSONifiedValue<T> =
    T extends string | number | boolean | null ? T :
    T extends Function ? undefined :
    T extends Array<infer U> ? JSONifiedArray<U> :
    T extends object ? JSONifiedObject<T> :
    undefined;

type UndefinedAsNull<T> = T extends undefined ? null : T;

interface JSONifiedArray<T> extends Array<UndefinedAsNull<JSONified<T>>> { }

type JSONifiedObject<T> = {
    [P in keyof T]: JSONified<T[P]>
};

usage.ts

declare class Widget {
    toJSON(): { kind: 'Widget', date: Date };
}

type Item = {
    text: string;
    count: number;
    choice: 'yes' | 'no' | null;
    func: () => void;
    nested: {
        isSaved: boolean;
        data: [1, undefined, 2];
    };
    widget?: Widget;
    children: Item[];
};

declare let item: JSONified<Item>;