JavaScript

const arr = [1, 2, 3, 4, 5];
arr[Symbol.iterator] = function* () {
  for (let i = 0; i < 3; i++) yield arr[i] * 2;
};
console.log([...arr], JSON.stringify(arr));

class Money {
  #amount;
  constructor(amount) { this.#amount = amount; }
  [Symbol.toPrimitive](hint) {
    if (hint === 'number') return this.#amount;
    if (hint === 'string') return `$${this.#amount}`;
    return `Money(${this.#amount})`;
  }
}
const m = new Money(42);
console.log(`${m}`, m + 8, m == 42);

const str = "2024-01-15";
const result = str.replace(/(\d+)-(\d+)-(\d+)/, "$3/$2/$1");
const s2 = "abc".padStart(6, "12");
const s3 = [..."hello"].reverse().join("");
console.log(result, s2, s3);

function memoize(fn) {
  const cache = new Map();
  return (...args) => {
    const key = JSON.stringify(args);
    if (cache.has(key)) return cache.get(key);
    const result = fn(...args);
    cache.set(key, result);
    return result;
  };
}

let calls = 0;
const add = memoize((a, b) => { calls++; return a + b; });

const r1 = add(1, 2);
const r2 = add(2, 1);
const r3 = add(1, 2);
console.log(r1, r2, r3, calls);

const Loggable = {
  log() { return `[${this.constructor.name}] ${this.toString()}`; }
};

class Base {
  toString() { return 'Base instance'; }
}

Object.setPrototypeOf(Base.prototype, Loggable);

class Derived extends Base {
  toString() { return 'Derived instance'; }
}

const b = new Base();
const d = new Derived();

console.log(b.log(), d.log(), Object.getPrototypeOf(Derived.prototype) === Base.prototype);

class Config {
  constructor(options = {}) {
    this.retries = options.retries ?? 3;
    this.timeout = options.timeout ?? 1000;
  }
}
const cfg1 = new Config({ retries: 0, timeout: null });
const cfg2 = new Config({ retries: undefined, timeout: false });
cfg1.retries ??= 99;
cfg2.timeout ??= 99;
console.log(cfg1.retries, cfg1.timeout, cfg2.retries, cfg2.timeout);

let arr = [];
for (let i = 0; i < 3; i++) {
  if (i === 1) {
    let i = 10;
    arr.push(i);
    continue;
  }
  arr.push(i);
}
console.log(arr.join('-'));

const a = [] + {};
const b = {} + [];
const c = 1 + '1' - 1;
const d = '5' + 3 - 2;
const e = true + true;
console.log(a, b, c, d, e);