I want to map all optional fields of an object to required fields with undefined as option.
From

{ isOptional?: string, required: number }

To

{ isOptional: string| undefined, required: number}

I can’t find the reason why this won’t work. Playground

type IsOptional<T extends {}, K extends keyof T> =
  { [P in keyof T]?: T[P] } extends Pick<T, K> ? true : false;

type OptionalInfo<T extends {}> = { [P in keyof T]-?: IsOptional<T, P> };

type OptionalToUndefined<T extends {}> = {
  [P in keyof Required<T>]: OptionalInfo<T>[P] extends true
    ? T[P] | undefined
    : T[P];
};

type test1 = OptionalInfo<{ isOptional?: string; required: number }>;
// {isOptional: true, required: false}

type test2 = OptionalToUndefined<{ isOptional?: string, required: number }>;
// {isOptional: string, required: number}