JavaScript

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

try {
  throw new CustomError('Something went wrong');
} catch (e) {
  console.log(e instanceof Error);
  console.log(e instanceof CustomError);
  console.log(e.constructor.name);
  console.log(e.name);
}

const wm = new WeakMap();
const obj1 = { name: 'first' };
const obj2 = { name: 'second' };
const obj3 = obj1;

wm.set(obj1, 'value1');
wm.set(obj2, 'value2');
wm.set(obj3, 'value3');

console.log(wm.get(obj1));
console.log(wm.get(obj2));
console.log(wm.get(obj3));
console.log(wm.has(obj1));
console.log(wm.size);

function createCounter() {
  let count = 0;
  return {
    increment: () => ++count,
    decrement: () => --count,
    getValue: () => count
  };
}

const counter1 = createCounter();
const counter2 = createCounter();
counter1.increment();
counter1.increment();
counter2.increment();
console.log(counter1.getValue(), counter2.getValue());
counter1.decrement();
console.log(counter1.getValue(), counter2.getValue());

const moduleMap = new Map();

async function loadModule(name) {
  if (moduleMap.has(name)) {
    return moduleMap.get(name);
  }
  
  const module = await Promise.resolve({
    default: () => `Module ${name} loaded`,
    version: '1.0.0'
  });
  
  moduleMap.set(name, module);
  return module;
}

loadModule('auth').then(m => console.log(m.default()));
loadModule('auth').then(m => console.log(m.version));
loadModule('db').then(m => console.log(m.default()));

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);
}