Strongly-typed deep path

getPropValue.ts

type PropType<T, Path extends string> =
  string extends Path ? never :
  Path extends keyof T ? T[Path] :
  Path extends `${infer K}.${infer R}` ? K extends keyof T ? PropType<T[K], R> : never : never;

declare function getPropValue<T, P extends string>(obj: T, path: P): PropType<T, P>;

const obj = { 
  a: 
  { 
    b: 
    {
      c: 42, 
      d: 'hello' 
    }
  }
};

// number
const c = getPropValue(obj, 'a.b.c');  

// never
const xxx = getPropValue(obj, 'a.b.xxx');

propertyNameConverter.ts

interface Foo {
  bar(): void;
}

type AsyncType<T> = {
  [P in keyof T & string as `${P}Sync` | `${P}Async`]: T[P] 
};

var x: AsyncType<Foo>;

stringValidation.ts

type StringWithId<T extends string> = T extends `row_${infer _}` ? T : never;

declare function parseId<T extends string>(value: StringWithId<T>): void;

parseId("helloWorld");  // Error
parseId("row_one");  // Ok