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

let arr = [1, 2, 3, 4, 5];
console.log(arr.map(x => x * 2));

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);
});
promise.then(alert);

class Rectangle {
  constructor(height, width) {
    this.height = height;
    this.width = width;
  }
  get area() {
    return this.height * this.width;
  }
}

let arr = [1, 2, 3, 4, 5];
let [first, ...rest] = arr;
console.log(first);
console.log(rest);