const original = Object.freeze({ a: [1, 2, 3] });
const copy = { ...original };

copy.a.push(4);

console.log(original.a);
console.log(copy);

const user = {
  profile: {
    name: 'Alice',
    settings: {
      notifications: {
        email: true,
        sms: false
      }
    }
  },
  getPreference(type) {
    return this.profile?.settings?.notifications?.[type] ?? 'not configured';
  }
};

const admin = {
  profile: {
    name: 'Admin',
    settings: null
  },
  getPreference: user.getPreference
};

console.log(admin.getPreference('email'));

let arr = [1, 2, 3];
for (i = 0; i < arr.length; arr[i++] = i++);
console.log(arr);

const arr = [1,2,3];
arr[Symbol.iterator] = function*() { yield *[3,2,1, arr[0]]; };

for (const n of arr) {
   console.log(n);
}

const companies = [
  { name: 'TechCorp', founded: 2010 },
  { name: 'DataSystems', founded: 2015 },
  { name: 'WebSolutions', founded: 2008 }
];

const activeClients = new WeakSet();

activeClients.add(companies[0]);
activeClients.add(companies[2]);

companies.pop();

const result = [
  activeClients.has(companies[0]),
  activeClients.has(companies[1]),
  typeof activeClients.size
];

console.log(result);

const obj = {
  value: 64,
  increment() {
    this.value--;
  }
};

const increment = obj.increment;
increment();

console.log(obj.value);

function* generator() {
  yield 'a';
  yield* 'bcd';
  yield 'e';
}
console.log([...generator()]);

function processConfig(config) {
  const defaults = {
    timeout: 1000,
    retries: 3,
    enabled: false,
    count: 0
  };
  
  const settings = {
    ...defaults,
    ...config
  };
  
  const effectiveTimeout = settings.timeout ?? 500;
  const effectiveRetries = settings.retries ?? 1;
  const effectiveEnabled = settings.enabled ?? true;
  const effectiveCount = settings.count ?? 5;
  
  console.log([effectiveTimeout, effectiveRetries, effectiveEnabled, effectiveCount]);
}

processConfig({ timeout: null, retries: 0, enabled: undefined });

let funcs = [];

for (var i = 0; i <= 3; i++) {
  funcs.push(() => i);
}

console.log(funcs[0]());
console.log(funcs[1]());
console.log(funcs[2]());

const numbers = [33, 2, 8];
numbers.sort();
console.log(numbers);

❓Что будет на выходе?

const x = "5";
const y = 3;
const result = x - y;
console.log(result);

console.log('Start');

setTimeout(() => {
  console.log('Timeout 1');
}, 0);

Promise.resolve().then(() => {
  console.log('Promise 1');
}).then(() => {
  console.log('Promise 2');
});

setTimeout(() => {
  console.log('Timeout 2');
}, 0);

console.log('End');

let proto = { a: 1 };
let obj = Object.create(proto);

Object.defineProperty(obj, 'a', {
  value: 3,
  writable: true,
  enumerable: true,
  configurable: true
});

console.log(obj.a);
proto.a = 3;
console.log(obj.a);

const a = 9007199254740991n;
const b = 2n;

function performCalculation() {
  const c = a + 1n;
  const d = c / b;
  const e = d * 2n - 1n;
  
  const result = Number(e) === Number(a);
  console.log(result);
}

performCalculation();

const obj = {
  [Symbol('a')]: 'hidden',
  a: 'visible',
  [Symbol.for('b')]: 'registered',
  b: 123
};

const symbol1 = Symbol.for('b');
const symbol2 = Symbol.for('b');

console.log(obj.a + ', ' + 
  obj[Symbol('a')] + ', ' + 
  obj[symbol1] + ', ' + 
  (symbol1 === symbol2));