Typescript mapped type that will accept a nested object and a list of paths to make required.

recursive_require_with_path.ts

interface Phone {
  number: string
  extension?: string
}

interface Person {
  firstName?: string;
  lastName?: string;
  age?: number;
  address?: {
    street?: string;
    city?: string;
    zip?: number;
  },
  phones?: Phone[]
}

type Expand<T> = T extends ReadonlyArray<unknown> ? Expand<T[number]>[] : T extends object ? { [K in keyof T]: Expand<T[K]> } : T;

type TopLevelPath<T extends string> = T extends `${infer U}.${infer _}` ? U : T;

type PathsToTopLevel<T extends ReadonlyArray<string>> = {
    [K in keyof T]: TopLevelPath<T[K]>;
};

type RemoveTopLevelPath<T extends ReadonlyArray<string>> = {
    [K in keyof T]: T[K] extends `${infer _}.${infer V}` ? V : never;
};

type OmitProperties<T, K extends ReadonlyArray<string>> = {
    [P in keyof T as P extends K ? never : P]: T[P];
};

type RecursiveRequired<T, K extends ReadonlyArray<string>> = Omit<T, PathsToTopLevel<K>[number]> & {
  [P in PathsToTopLevel<K>[number]] : T[P & keyof T] extends Array<infer U>
    ? RecursiveRequired<U, RemoveTopLevelPath<K>>[]
    : T[P & keyof T] extends string
    ? string
    : T[P & keyof T] extends Object
    ? RecursiveRequired<T[P & keyof T], RemoveTopLevelPath<K>>
    : T[P & keyof T]
}

type PersonWithRequiredFields = Expand<RecursiveRequired<Person, ['firstName', 'phones', 'phones.extension', 'age']>>


// const p: PersonWithRequiredFields = {
//  firstName: 'Sanjit';    // Required
//  lastName: string;       // Optional
//  age: number;            // Optional
//  address?: {             // Optional
//   street?: string;       // Optional
//   city?: string;         // Optional
//   zip?: number;          // Optional
//  },         
//  phones: {               // Required
//    number: '123456789'   // Optional
//    extension: '123'      // Required
// }       
// }

I am not pro in typescript. Any suggestions to improve this code will be appreciated.

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