Challenge 3: Add a Third Required Field to the Builder — Possible Solution
====================================================================

type RequestBuilderState = {
  url?: string;
  method?: string;
  headers?: Record<string, string>;
};

class RequestBuilder<State extends RequestBuilderState = {}> {
  private state: State;
  constructor(state: State) {
    this.state = state;
  }

  url(value: string): RequestBuilder<State & { url: string }> {
    return new RequestBuilder({ ...this.state, url: value });
  }

  method(value: string): RequestBuilder<State & { method: string }> {
    return new RequestBuilder({ ...this.state, method: value });
  }

  headers(value: Record<string, string>): RequestBuilder<State & { headers: Record<string, string> }> {
    return new RequestBuilder({ ...this.state, headers: value });
  }

  build(
    this: RequestBuilder<Required<RequestBuilderState>>
  ): { url: string; method: string; headers: Record<string, string> } {
    return this.state as { url: string; method: string; headers: Record<string, string> };
  }
}

// --- Usage ---

const request = new RequestBuilder({})
  .url("/users")
  .method("POST")
  .headers({ "Content-Type": "application/json" })
  .build(); // ✅ all three required fields were set

console.log(request);
// { url: "/users", method: "POST", headers: { "Content-Type": "application/json" } }

// Each of these fails to compile — .build() itself is not a callable
// member on a RequestBuilder missing any one required field:

// new RequestBuilder({}).url("/users").method("POST").build();
// ❌ Property 'build' does not exist on type
//    'RequestBuilder<{ url: string; method: string }>' in a way that
//    satisfies `this: RequestBuilder<Required<RequestBuilderState>>` —
//    `headers` was never set.

// new RequestBuilder({}).url("/users").headers({}).build();
// ❌ missing `method`

// new RequestBuilder({}).method("POST").headers({}).build();
// ❌ missing `url`

WHY THIS WORKS
--------------
- Adding `headers` followed the exact same pattern as `url` and `method`:
  a setter method that returns `RequestBuilder<State & { headers: ... }>`,
  accumulating one more required key onto the tracked State type.
- `Required<RequestBuilderState>` expands to
  `{ url: string; method: string; headers: Record<string, string> }` — all
  three properties now non-optional — so `build()`'s `this` parameter type
  demands all three have been set, not just the original two.
- Because `this` parameter types are structurally checked, calling
  `.build()` on any RequestBuilder instantiation whose State is missing
  even one of the three required keys fails to satisfy that `this`
  constraint, and TypeScript reports `.build` as not callable there — the
  compile-time equivalent of a runtime "missing required field" error,
  just impossible to ship in the first place.