const source = {
  count: 0,
  get value() {
    this.count++;
    return this.count;
  }
};

const { value, ...rest } = source;
const spread = { ...source };

console.log(value, spread.value, source.count, JSON.stringify(rest));

const compose = (...fns) => x => fns.reduceRight((acc, fn) => fn(acc), x);
const pipe = (...fns) => x => fns.reduce((acc, fn) => fn(acc), x);
const addOne = x => x + 1;
const double = x => x * 2;
const square = x => x * x;
console.log(compose(square, double, addOne)(3), pipe(square, double, addOne)(3));

let i = 0;
const key = () => `k${i++}`;
const obj = { [key()]: 1, [key()]: 2, [key()]: 3 };
const sym = Symbol('x');
const keyObj = { toString() { return 'dyn'; } };
const obj2 = {
  [sym]: 'secret',
  [`a${1 + 1}`]: 'computed',
  ['a' + 2]: 'overwritten',
  [keyObj]: 'fromObj'
};
console.log(`${Object.keys(obj).join(',')} ${obj2.a2} ${obj2.dyn} ${obj2[sym]}`);

const 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 square = memoize((x) => {
  calls++;
  return x * x;
});

console.log(square(4), square(4), square(-4), calls);

const handler = {
  get(target, prop, receiver) {
    if (!(prop in target)) {
      return `no ${String(prop)}`;
    }
    return Reflect.get(target, prop, receiver);
  },
  set(target, prop, value) {
    if (typeof value === 'number' && value < 0) {
      throw new RangeError('negative');
    }
    return Reflect.set(target, prop, value);
  }
};

const obj = new Proxy({ a: 1, b: 2 }, handler);
obj.c = 10;
console.log(obj.a, obj.c, obj.missing, obj.hasOwnProperty('a'));

let counter = 0;
const state = {
  get value() {
    counter++;
    return counter % 3 === 0 ? null : counter;
  }
};
let result;
result ??= state.value;
result ??= state.value;
result ??= state.value;
result ??= state.value;
console.log(result, counter);

const s = "JavaScript";
const str = new String(s);
console.log(str === s, str == s, typeof str, str.at(-1), s.at(-1));

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