const numbers = [1, 2, 3, 4, 5];

const result = numbers
  .filter(num => num % 2 === 0)
  .map(num => num * num)
  .reduce((acc, num) => acc + num, 0);

console.log(result);

const sym1 = Symbol('description');
const sym2 = Symbol('description');

const obj = {
  [sym1]: 'value1',
  [sym2]: 'value2'
};

console.log(obj[sym1]);

function trickyScope() {
  var x = 10;
  if (true) {
    let x = 5;
    console.log(x);
  }
  console.log(x);
}

trickyScope();

function trickyScope() {
  var a = 10;
  let b = 20;
  const c = 30;

  if (true) {
    var a = 40;
    let b = 50;
    const c = 60;
    console.log(a, b, c);
  }

  console.log(a, b, c);
}

trickyScope();

function trickyFunction(num) {  
  let result = 0;  
  for (let i = 0; i < num; i++) {  
    result += i;  
    if (i % 2 === 0) {  
      result *= 2;  
    } else {  
      result -= 1;  
    }  
  }  
  return result;  
}  
console.log(trickyFunction(4));

function trickyScope() {
    let x = 10;
    if (true) {
        let x = 20;
        console.log(x); // A
    }
    console.log(x); // B
    {
        let x = 30;
        console.log(x); // C
    }
    console.log(x); // D
}

trickyScope();