TypeScript Generic Constraints in Depth: `extends`, `keyof`, and the Patterns That Prevent Runtime Errors - snippet-4.ts

snippet-4.ts

type PathsToStringProps<T> = {
  [K in keyof T]: T[K] extends string
    ? K
    : T[K] extends object
    ? `${K & string}.${PathsToStringProps<T[K]> & string}`
    : never;
}[keyof T];

function getNestedString<T, P extends PathsToStringProps<T>>(
  obj: T,
  path: P
): string {
  const keys = (path as string).split(".");
  let value: any = obj;
  for (const key of keys) {
    value = value[key];
  }
  return value;
}

const data = {
  user: {
    profile: {
      email: "alice@example.com",
      age: 30,
    },
  },
};

// Valid: path resolves to string property
const email = getNestedString(data, "user.profile.email");

// Compiler error: path does not resolve to string property
const age = getNestedString(data, "user.profile.age");