let x = 10;
let y = '5';
let z = x + y;

function test() {
  var x = 1;
  if (true) {
    var x = 2;
    console.log(x);
  }
  console.log(x);
}
test();

let arr = [1, 2, 3, 4, 5];
let sum = arr.reduce((a, b) => a + b, 0);

let promise = new Promise((resolve, reject) => {
  setTimeout(() => resolve('done!'), 1000);
});

class Person {
  constructor(name) {
    this.name = name;
  }
  sayHello() {
    return `Hello, ${this.name}`;
  }
}
let john = new Person('John');
john.sayHello();

let x = function() {
  console.log(this);
};
let y = {
  x: x
};
y.x();