JavaScript

const user = {
  profile: {
    settings: {
      theme: 'dark',
      notifications: null
    }
  }
};

const result1 = user?.profile?.settings?.theme;
const result2 = user?.profile?.settings?.notifications?.email;
const result3 = user?.profile?.preferences?.language ?? 'en';
const result4 = user?.profile?.settings?.notifications?.push?.('test');

console.log(result1, result2, result3, result4);

console.log(typeof myFunction);
console.log(typeof myVar);
console.log(typeof myLet);
console.log(typeof myConst);

var myVar = 'hello';
let myLet = 'world';
const myConst = 'test';

function myFunction() {
  return 'hoisted';
}

console.log(myFunction());
console.log(myVar);

const Flyable = {
  fly() { return 'flying'; }
};

const Swimmable = {
  swim() { return 'swimming'; }
};

function applyMixins(target, ...mixins) {
  mixins.forEach(mixin => {
    Object.assign(target.prototype, mixin);
  });
}

class Bird {}
class Fish {}

applyMixins(Bird, Flyable, Swimmable);
applyMixins(Fish, Swimmable);

const eagle = new Bird();
const shark = new Fish();

console.log(eagle.swim());
console.log(shark.fly?.() || 'undefined method');

const target = { name: 'Sarah', age: 25 };

const handler = {
  get(obj, prop) {
    if (prop in obj) {
      return obj[prop];
    }
    return `Property '${prop}' not found`;
  },
  set(obj, prop, value) {
    obj[prop] = value.toString().toUpperCase();
    return true;
  }
};

const proxy = new Proxy(target, handler);
proxy.city = 'boston';
console.log(proxy.name);
console.log(proxy.city);
console.log(proxy.country);