JavaScript

class Vehicle {
  constructor(type) {
    this.type = type;
  }
  describe() {
    return `I am a ${this.type}`;
  }
}

class Car extends Vehicle {
  constructor(brand) {
    super("car");
    this.brand = brand;
  }
  describe() {
    return `${super.describe()} made by ${this.brand}`;
  }
}

const myCar = new Car("Toyota");

console.log(myCar.describe());
console.log(myCar instanceof Car);
console.log(myCar instanceof Vehicle);
console.log(Object.getPrototypeOf(Car) === Vehicle);

function testScope() {
  var x = 'outer';
  let y = 'outer';
  
  if (true) {
    var x = 'inner';
    let y = 'inner';
    console.log(x, y);
  }
  
  console.log(x, y);
}

testScope();

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

const config = {
  name: 'John',
  profile: null
};

console.log(user.profile?.settings?.theme);
console.log(config.profile?.settings?.theme);
console.log(user.profile?.preferences?.language);
console.log(config.profile?.settings?.theme ?? 'light');

const user = {
  name: 'Sarah',
  age: 25,
  greet() {
    return `Hello, I'm ${this.name}`;
  }
};

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

console.log(keys.length);
console.log(values.includes('Sarah'));
console.log(entries[2][0]);

function createUser(name = 'Guest', age = 0, active = true) {
  return { name, age, active };
}

const users = [
  createUser('Sarah', 25),
  createUser('Mike'),
  createUser('Emma', undefined, false),
  createUser(null, 30)
];

console.log(users.map(u => `${u.name}-${u.age}-${u.active}`).join('|'));

class CustomError extends Error {
  constructor(message) {
    super(message);
    this.name = this.constructor.name;
  }
}

try {
  throw new CustomError("Something went wrong");
} catch (error) {
  console.log(error.name);
  console.log(error instanceof Error);
  console.log(error instanceof CustomError);
  console.log(typeof error.stack);
}

const cache = new Map();

function memoize(fn) {
  return function(...args) {
    const key = JSON.stringify(args);
    if (cache.has(key)) {
      return cache.get(key);
    }
    const result = fn(...args);
    cache.set(key, result);
    return result;
  };
}

const fibonacci = memoize((n) => {
  if (n <= 1) return n;
  return fibonacci(n - 1) + fibonacci(n - 2);
});

console.log(fibonacci(5));
console.log(cache.size);