var arr = Array.from({ length: 5 }, (v, i) => i * 2);
console.log(arr);

function trickyCount(n) {
  if (n <= 1) return n;
  return trickyCount(n - 1) + trickyCount(n - 2);
}

function wrapCount(n) {
  return trickyCount(n) - trickyCount(n - 4);
}

console.log(wrapCount(6));

function mystery(x) {
  return (function(y) {
    return x + y;
  })(x * 2);
}

const result1 = mystery(2);
const result2 = mystery(5);
const result3 = mystery(-1);

console.log(result1, result2, result3);

function mysteriousFunction(a) {
  let result = 0;
  for (let i = 1; i <= a; i++) {
    if (i % 3 === 0 && i % 5 === 0) {
      result += i * 2;
    } else if (i % 3 === 0) {
      result += i;
    } else if (i % 5 === 0) {
      result -= i;
    }
  }
  return result;
}

console.log(mysteriousFunction(15));

function tricky() {
  let a = 1;
  let b = 2;
  const result = (function(a) {
    a = 3;
    return a + b;
  })(a);
  return result;
}

console.log(tricky());

let symbol1 = Symbol('description');
let symbol2 = Symbol('description');

const obj = {
  [symbol1]: 'value1',
  [symbol2]: 'value2'
};

console.log(obj[symbol1]);
console.log(symbol1 === symbol2);

const wm = new WeakMap();
const obj1 = {};
const obj2 = {};
wm.set(obj1, 'object 1');
wm.set(obj2, 'object 2');
wm.delete(obj1);
console.log(wm.has(obj1));