const wm = new WeakMap();
class Counter {
  #id;
  static #nextId = 0;
  constructor() {
    this.#id = Counter.#nextId++;
    wm.set(this, { count: 0, id: this.#id });
  }
  inc() {
    const data = wm.get(this);
    data.count++;
    return data.count;
  }
}
const a = new Counter();
const b = new Counter();
a.inc(); a.inc();
b.inc();
console.log(`${a.inc()} ${b.inc()} ${wm.has(a)} ${wm.get(a).id} ${wm.get(b).id}`);

const createCounterModule = (() => {
  let instances = 0;
  return function() {
    instances++;
    let count = 0;
    return {
      increment() { count++; return count; },
      id: instances
    };
  };
})();

const a = createCounterModule();
const b = createCounterModule();
console.log(a.increment(), a.increment(), b.increment(), a.id, b.id);

export {};

const base = {
  greet() { return `Hello, ${this.name}`; }
};
const mid = Object.create(base);
mid.name = 'Mid';
const child = Object.create(mid);
child.name = 'Child';

console.log(
  child.greet(),
  mid.greet(),
  Object.getPrototypeOf(child) === mid,
  child.hasOwnProperty('greet')
);

class StringBuilder {
  #parts = [];
  append(s) {
    this.#parts.push(s);
    return this;
  }
  toString() {
    return this.#parts.join('');
  }
}

const sb = new StringBuilder().append('a').append('b').append('c');
console.log(`${sb}-${sb.append('d')}`);

function curry(fn) {
  return function curried(...args) {
    if (args.length >= fn.length) return fn.apply(this, args);
    return (...more) => curried.apply(this, [...args, ...more]);
  };
}

function multiply(a, b, c = 1, ...rest) {
  return a * b * c * (rest.reduce((s, n) => s + n, 0) || 1);
}

const curried = curry(multiply);
console.log(curried(2)(3), multiply.length);

const target = { a: 1, b: 2 };
const handler = {
  get(obj, prop, receiver) {
    if (prop in obj) {
      return Reflect.get(obj, prop, receiver);
    }
    return `missing:${String(prop)}`;
  },
  set(obj, prop, value, receiver) {
    if (typeof value === 'number') {
      return Reflect.set(obj, prop, value * 2, receiver);
    }
    return Reflect.set(obj, prop, value, receiver);
  }
};
const p = new Proxy(target, handler);
p.c = 5;
p.d = 'hello';
console.log(p.a, p.c, p.d, p.z);

const sym = Symbol('id');
const target = { a: 1, [sym]: 2 };
Reflect.defineProperty(target, 'b', { value: 3, enumerable: false });

const handler = {
  get(obj, prop, receiver) {
    if (prop === 'a') return Reflect.get(obj, prop, receiver) * 10;
    return Reflect.get(obj, prop, receiver);
  }
};

const proxy = new Proxy(target, handler);
console.log(proxy.a, proxy.b, Reflect.ownKeys(target).length, Object.keys(target).length);

let a = 0;
let b = null;
let c = '';
let d;

a ||= 10;
b ??= 20;
c &&= 'changed';
d ??= a + b;

console.log(a, b, c, d);

function build(a, b = a + 1, c = (() => a + b)()) {
  a = 100;
  return [a, b, c];
}
console.log(build(1), build(1, 2), build(5, 5, 5));

let outerType;
{
  function greet() {
    return 'inner';
  }
  outerType = typeof greet;
}
console.log(outerType, typeof greet);