JavaScript

function processData() {
  try {
    console.log('start');
    throw new Error('oops');
    console.log('after throw');
  } catch (e) {
    console.log('catch');
    return 'caught';
  } finally {
    console.log('finally');
  }
  console.log('end');
}

const result = processData();
console.log(result);

const obj = {
  name: 'Sarah',
  getName() { return this.name; },
  getNameArrow: () => this.name
};

const { getName, getNameArrow } = obj;
const boundGetName = obj.getName.bind(obj);

console.log(getName());
console.log(getNameArrow());
console.log(boundGetName());
console.log(obj.getName());
console.log(obj.getNameArrow());

const userInput = "<script>alert('xss')</script>";
const sanitized = userInput.replace(/<script[^>]*>.*?<\/script>/gi, '');

const users = new Map();
users.set('admin', { password: 'secret123', role: 'admin' });
users.set('guest', { password: 'guest', role: 'user' });

function authenticate(username, password) {
  const user = users.get(username);
  return user && user.password === password ? user.role : null;
}

const role1 = authenticate('admin', 'secret123');
const role2 = authenticate('guest', 'wrong');
const role3 = authenticate('hacker', 'secret123');

console.log(sanitized);
console.log(role1, role2, role3);

class StateMachine {
  constructor() {
    this.state = 'idle';
    this.transitions = {
      idle: { start: 'running' },
      running: { pause: 'paused', stop: 'idle' },
      paused: { resume: 'running', stop: 'idle' }
    };
  }
  
  transition(action) {
    const validTransitions = this.transitions[this.state];
    if (validTransitions && validTransitions[action]) {
      this.state = validTransitions[action];
      return true;
    }
    return false;
  }
}

const machine = new StateMachine();
console.log(machine.transition('pause'));
console.log(machine.state);
console.log(machine.transition('start'));
console.log(machine.state);

const obj = { a: 1, b: { c: 2 } };
const frozen = Object.freeze(obj);
frozen.a = 99;
frozen.b.c = 88;
frozen.d = 77;

const sealed = Object.seal({ x: 10, y: 20 });
sealed.x = 30;
sealed.z = 40;
delete sealed.y;

console.log(obj.a, obj.b.c, obj.d);
console.log(sealed.x, sealed.y, sealed.z);

const arr = new Array(1000000).fill(0).map((_, i) => i);
const results = [];

function method1() {
  return arr.filter(x => x % 2 === 0).map(x => x * 2).slice(0, 3);
}

function method2() {
  const result = [];
  for (let i = 0; i < arr.length && result.length < 3; i++) {
    if (arr[i] % 2 === 0) {
      result.push(arr[i] * 2);
    }
  }
  return result;
}

console.log(method1().join(','));
console.log(method2().join(','));

async function fetchData() {
  console.log('1');
  return Promise.resolve('data');
}

async function processData() {
  console.log('2');
  const result = await fetchData();
  console.log('3');
  return result;
}

console.log('4');
processData().then(() => console.log('5'));
console.log('6');