function trickyQuestion() {
    var a = 1;
    var b = 2;

    return (function() {
        delete a;
        delete b;
        return a + b;
    })();
}

console.log(trickyQuestion());

let a = 5; // binary: 0101
let b = 3; // binary: 0011
let c = a & b; // binary: 0001
let d = a | b; // binary: 0111
let e = a ^ b; // binary: 0110

console.log(c, d, e);

function trickyFunction(a) {
    a = a || 42;
    let result = (function(a) {
        return a * 2;
    })(a);
    return result;
}

console.log(trickyFunction(0));

let x = 1;
function outer() {
  let x = 2;
  function inner() {
    console.log(x);
    let x = 3;
  }
  inner();
}
outer();

const user = {
  name: 'John',
  greet() {
    const getName = () => {
      console.log(this.name);
    };
    getName();
  },
  farewell: function() {
    function getName() {
      console.log(this.name);
    }
    getName();
  }
};

user.greet();
user.farewell();

const arr = [1, 2, 3, 4, 5];
const result = arr
  .map(x => x * 2)
  .filter(x => x > 5)
  .reduce((acc, val) => {
    return acc + (val % 3 === 0 ? val : 0);
  }, 0);

console.log(result);

const items = new WeakSet();
const obj1 = { id: 1 };
const obj2 = { id: 2 };

items.add(obj1);
items.add(obj2);

obj2.value = 'test';
console.log(items.has(obj2));

obj2 = null;
setTimeout(() => {
  console.log(items.has(obj2));
}, 0);