Challenge 1: Build a Generic hasProperties — Possible Solution
====================================================================

function hasProperties<K extends PropertyKey>(
  obj: unknown,
  ...keys: K[]
): obj is Record<K, unknown> {
  if (typeof obj !== "object" || obj === null) {
    return false;
  }
  return keys.every((key) => key in obj);
}

// --- Usage ---

function handle(value: unknown) {
  if (hasProperties(value, "id", "email", "createdAt")) {
    // value is narrowed to Record<"id" | "email" | "createdAt", unknown>
    console.log(value.id, value.email, value.createdAt);
  } else {
    console.log("Missing one or more required fields");
  }
}

handle({ id: 1, email: "a@b.com", createdAt: "2026-01-01" }); // logs all three
handle({ id: 1, email: "a@b.com" });                          // logs the else branch — missing createdAt
handle(null);                                                  // logs the else branch — not an object at all

// A single-key call still works, since ...keys accepts any number of arguments:
if (hasProperties(value, "id")) {
  console.log(value.id);
}

WHY THIS WORKS
--------------
- `...keys: K[]` accepts any number of property-key arguments — calling
  with one key behaves like the chapter's single-key hasProperty, and
  calling with several behaves like a proper multi-key check, without
  needing two separate functions.
- `keys.every((key) => key in obj)` only returns true if EVERY given key
  exists on the object — a single missing key correctly fails the whole
  check, same as a hand-written `"id" in obj && "email" in obj && ...`
  chain would, but without repeating the `in` check per field.
- The return type `obj is Record<K, unknown>` means every key passed in
  is known to exist on `obj` inside the narrowed branch, with each key's
  value typed as `unknown` (since hasProperties has no way to know the
  actual value types) — callers still need their own narrowing or
  validation for the values themselves.