Object pool

type Thumbnail = { name: string; flipX?: boolean };

class Session {
  id: number;
  flipX = false;
  constructor(id: number) {
    this.id = id;
  }
}

// The pool: every session made once, lent to one job at a time.
class SessionPool {
  private idle: Session[] = [];
  constructor(size: number) {
    for (let id = 1; id <= size; id++) this.idle.push(new Session(id));
  }
  acquire(): Session | null {
    return this.idle.shift() ?? null;
  }
  release(session: Session): void {
    session.flipX = false;
    this.idle.push(session);
  }
}

// One job on a borrowed session. An omitted flipX keeps the session's setting.
function job(session: Session | null, thumbnail: Thumbnail): string {
  if (!session) return `${thumbnail.name}: no session free`;
  if (thumbnail.flipX !== undefined) session.flipX = thumbnail.flipX;
  const look = session.flipX ? 'mirrored' : 'original';
  return `${thumbnail.name}: session ${session.id}, ${look}`;
}

const pool = new SessionPool(2);
const first = pool.acquire();
const second = pool.acquire();
const third = pool.acquire();
const a = job(first, { name: 'A', flipX: true });
const b = job(second, { name: 'B' });
const c = job(third, { name: 'C' });
if (first) pool.release(first);
if (second) pool.release(second);
const retry = pool.acquire();
const cRetry = job(retry, { name: 'C' });
console.log([a, b, c, cRetry].join('\n'));