JavaScript

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');

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);

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 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();

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

const counter1 = createCounter();
const counter2 = createCounter();
counter1.increment();
counter1.increment();
counter2.increment();
console.log(counter1.get(), counter2.get());
const { increment, get } = counter1;
increment();
console.log(get());

let obj1 = { name: 'Sarah' };
let obj2 = { name: 'Mike' };
let obj3 = { ref: obj1 };

obj1.circular = obj1;
obj2.partner = obj3;
obj3.partner = obj2;

let weakMap = new WeakMap();
weakMap.set(obj1, 'data1');
weakMap.set(obj2, 'data2');

obj1 = null;
obj2 = null;

console.log(weakMap.has(obj3.ref));
console.log(obj3.partner.name);