И вот такие задачи были на собесе 👀

1️⃣ EventEmitter

Задача: реализовать EventEmitter с подпиской, отпиской и вызовом событий.

class EventEmitter {
  constructor() {}

  subscribe(event, callback) {
    this.events[event].push(callback);
  }

  unsubscribe(event, callback) {
    this.events[event] =
      this.events[event].filter(cb => cb !== callback);
  }

  fire(event, payload) {
    this.events[event].forEach(callback => callback(payload));
  }
}

const eventEmitter = new EventEmitter();

const callback = (payload) =>
  console.log(`Callback catch event "test" with payload: ${JSON.stringify(payload)}`);

2️⃣ JavaScript Event Loop

Задача: определить порядок вывода в консоль + объяснить Event Loop.

console.log('start');

setTimeout(() => console.log('timeout'), 0);

new Promise((resolve, reject) => {
  console.log('promise');
  reject();
})
  .then(() => console.log('promise'))
  .catch(() => console.log('promise1'))
  .catch(() => console.log('promise2'))
  .then(() => console.log('promise3'))
  .then(() => console.log('promise4'));

console.log('final');

3️⃣ TypeScript / Runtime

Задача: объяснить поведение instanceof и различия runtime vs compile-time.

interface User {}

class AdminUser implements User {
  name: string;
  role: string;

  constructor(name: string) {
    this.name = name;
    this.role = "admin";
  }
}

function checkGrants(user: User): boolean {
  return user instanceof AdminUser;
}

const adminUser2: AdminUser = {
  name: "admin",
  role: "admin",
};

console.log(checkGrants(new AdminUser("admin"))); // true
console.log(checkGrants(adminUser2)); // false

4️⃣ React Refactoring

Задача: провести рефакторинг компонента интернет-магазина.

export default function App() {
  const [query, setQuery] = useState("");

  const [products, setProducts] = useState([]);
  const [filtered, setFiltered] = useState<Product[]>([]);

  const [cartCount, setCartCount] = useState(0);
  const [cartSum, setSumCount] = useState(0);

  const onAdd = useCallback((p: Product) => {
    setCartCount(cartCount + 1);
    setSumCount(cartSum + p.price);
  }, []);

  const clear = () => {
    setCartCount(0);
    setSumCount(0);
  };

  useEffect(() => {
    fetchProducts(query).then((res) => {
      setProducts(res.items);
    });
  }, [query]);

  useEffect(() => {
    setFiltered(products.filter((p) => p.price >= 0));
  }, [products]);

  return (
    <div>
      <div style={{ fontFamily: "sans-serif", maxWidth: 720, margin: "20px auto" }}>
        <h2>Shop</h2>

        <div style={{ display: "flex", gap: 8, alignItems: "center" }}>
          <input
            value={query}
            onChange={(e) => setQuery(e.target.value)}
            placeholder="Search..."
          />

          <span>Cart: {cartCount}</span>
          <span>Sum: ${cartSum}</span>

          <div
            style={{ backgroundColor: "#808080", padding: "8px" }}
            onClick={() => clear()}
          >
            Clear
          </div>
        </div>

        <div style={{ marginTop: 12, border: "1px solid #eee", borderRadius: 8 }}>
          {filtered.map((product) => (
            <>
              <div style={{ display: "flex", justifyContent: "space-between" }}>
                <div>
                  <b>{product.name}</b>
                  <div style={{ fontSize: 12, opacity: 0.7 }}>
                    ${product.price}
                  </div>
                </div>

                <div style={{ display: "flex" }}>
                  <div
                    style={{ backgroundColor: "#808080", padding: "8px" }}
                    onClick={() => onAdd(product)}
                  >
                    Add
                  </div>
                </div>
              </div>

              <hr />
            </>
          ))}
        </div>

        <h6>Copyrite and footer</h6>
      </div>
    </div>
  );
}