불변 빌더

const base = makeMutableBuilder().url("/x");
// If base is shared, later .method() calls mutate everyone's copy.

class IB<S> {
  constructor(private readonly d: Record<string, unknown> = {}) {}
  with<K extends string, V>(k: K, v: V): IB<S & Record<K, V>> {
    return new IB<S & Record<K, V>>({ ...this.d, [k]: v });
  }
}

const a = new IB<{}>().with("url", "/x");
const c = a.with("method", "GET");
// a still only has url; c has url and method.

const base = new IB<{}>().with("url", "/api");
const get = base.with("method", "GET");
const post = base.with("method", "POST");
console.log("base untouched; get and post diverge");

class IB<S> {
  constructor(private readonly d: Readonly<Record<string, unknown>> = {}) {}
}

interface Out { url: string; method: string }
class IB<S> {
  constructor(private readonly d: Record<string, unknown> = {}) {}
  with<K extends string, V>(k: K, v: V): IB<S & Record<K, V>> {
    return new IB<S & Record<K, V>>({ ...this.d, [k]: v });
  }
  build(this: IB<{ url: string; method: string }>): Out { return this.d as Out; }
}
const out = new IB<{}>().with("url", "/u").with("method", "GET").build();
console.log(out.url);

const jsonBase = new IB<{}>().with("header", "application/json");
const usersReq = jsonBase.with("url", "/users");
const ordersReq = jsonBase.with("url", "/orders");
console.log("two independent requests from one base");