/* Closure */
const buttons = document.getElementsByClassName('three-buttons');
for (var i = 0; i < 3; i++) {
  buttons[i].addEventListener(function(){
    console.log(i)
  });
}
// Click 1st button -> 2
// Click 2nd button -> 2
// Click 3rd button -> 2

/* Type */
console.log(NaN === NaN);
// -> false
console.log([1,2,3] == [1,2,3]);
// -> false

/* Prototype */
Array.prototype.push('😜');
let arr = [];
console.log(arr.length)
// -> 0
console.log(arr[0])
// -> '😜'

/* Arguments is not an array*/
(function test() {
  arguments.map(arg => arg*2)
// -> arguments.map is not a function
  Array.from(arguments).map(arg => arg*2)
})(1, 2, 3, 4)

/* float vs int toString */
console.log(2.toString());
// -> SyntaxError
console.log(2..toString());
// -> '2'

/* Object use string as key */
var a={},
    b={foo: 1},
    c={bar: 2};

a[b]=123;
a[c]=456;

console.log(a[b]);
// -> 456 (a['object object'] = 456)