const promise1 = Promise.resolve('first');
const promise2 = new Promise(resolve => {
  resolve('second');
});

const promise3 = Promise.resolve().then(() => 'third');

async function test() {
  console.log('start');
  
  const result1 = await promise1;
  console.log(result1);
  
  const result2 = await promise2;
  console.log(result2);
  
  const result3 = await promise3;
  console.log(result3);
  
  console.log('end');
}

test();

class EventBus {
  constructor() {
    this.events = new Map();
  }
  
  on(event, callback) {
    if (!this.events.has(event)) {
      this.events.set(event, []);
    }
    this.events.get(event).push(callback);
  }
  
  emit(event, data) {
    if (this.events.has(event)) {
      this.events.get(event).forEach(callback => callback(data));
    }
  }
}

const bus = new EventBus();
bus.on('user', name => console.log(`Hello ${name}`));
bus.on('user', name => console.log(`Welcome ${name}`));
bus.emit('user', 'Sarah');
bus.emit('admin', 'John');

const original = { a: 1, b: { c: 2 } };
const shallow = { ...original };
const deep = JSON.parse(JSON.stringify(original));

shallow.a = 10;
shallow.b.c = 20;

deep.a = 100;
deep.b.c = 200;

const frozen = Object.freeze({ x: 1, y: { z: 2 } });
frozen.x = 99;
frozen.y.z = 99;

console.log(original.a, original.b.c, frozen.x, frozen.y.z);

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');

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(','));

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);

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);