JavaScript

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

const getTheme = (obj) => obj?.profile?.settings?.theme ?? 'light';
const getLanguage = (obj) => obj?.profile?.settings?.language ?? 'en';
const getNotifications = (obj) => obj?.profile?.notifications?.enabled ?? true;

console.log(getTheme(user));
console.log(getLanguage(user));
console.log(getNotifications(user));
console.log(getTheme(null));

const target = { name: 'John', age: 30 };
const handler = {
  get(obj, prop) {
    if (prop in obj) {
      return `[${obj[prop]}]`;
    }
    return `missing: ${prop}`;
  },
  set(obj, prop, value) {
    obj[prop] = value.toUpperCase();
    return true;
  }
};
const proxy = new Proxy(target, handler);
proxy.city = 'paris';
console.log(proxy.name);
console.log(proxy.city);
console.log(proxy.country);

const source = {
  value: 1,
  subscribers: new Set(),
  subscribe(fn) {
    this.subscribers.add(fn);
    return () => this.subscribers.delete(fn);
  },
  next(newValue) {
    this.value = newValue;
    this.subscribers.forEach(fn => fn(this.value));
  }
};

const mapped = {
  value: undefined,
  source,
  transform: x => x * 2,
  init() {
    this.source.subscribe(val => {
      this.value = this.transform(val);
      console.log(`Mapped: ${this.value}`);
    });
  }
};

mapped.init();
source.next(3);
source.next(5);
console.log(`Final: ${mapped.value}`);
source.next(2);

try {
  const obj = null;
  obj.property = 'value';
} catch (e) {
  console.log(e.name);
}

try {
  undeclaredVariable;
} catch (e) {
  console.log(e.name);
}

try {
  JSON.parse('invalid json');
} catch (e) {
  console.log(e.name);
}

const numbers = [1, 2, 3, 4, 5];
const result = numbers
  .filter(n => n % 2 === 0)
  .map(n => n * 2)
  .reduce((acc, n) => acc + n, 0);

const original = numbers.slice();
original.reverse();

const flattened = [[1, 2], [3], [4, 5]].flat();
const found = flattened.find(n => n > 3);

console.log(result);
console.log(original.length);
console.log(found);

const user = {
  name: 'Sarah',
  age: 28,
  city: 'Boston'
};

const keys = Object.keys(user);
const values = Object.values(user);
const entries = Object.entries(user);

const result = entries.map(([key, value]) => {
  return typeof value === 'string' ? key.toUpperCase() : value * 2;
});

console.log(result);