masautt

let obj = { key: undefined }

// Check if the key is defined (warning: key could exist but set to undefined)
obj["key"] !== undefined;

//Check if the key is defined in the object (includes if set to undefined)
"key" in obj;

//Check if the key doesn't exist?
!("key" in obj);

masautt

let obj = { key: undefined }

// Check if the key is defined (warning: key could exist but set to undefined)
obj["key"] !== undefined; //false, but key still exists

//Check if the key is defined in the object (includes if set to undefined)
"key" in obj; // true, regardless of actual value

//Check if the key doesn't exist?
!("key" in obj); // false

masautt

var obj = { first: "John", last: "Doe" };

Object.keys(obj).forEach(function(key) {
    console.log(key, obj[key]); // first, John
                                // last, Doe
});

masautt

// Change the class altogether?
document.getElementById("MyElement").className = "MyClass";

// Add to the class?
document.getElementById("MyElement").className += "MyClass";

// Add or Remove a new class?
document.getElementById("MyElement").classList.add('MyClass');
document.getElementById("MyElement").classList.remove('MyClass');

masautt

//ES 7+
Object.entries(obj).length === 0 && obj.constructor === Object

//ES 5+
Object.keys(obj).length === 0 && obj.constructor === Object

//ES 5-
function isEmpty(obj) {
  for(var prop in obj) {
    if(obj.hasOwnProperty(prop)) {

masautt

var arr = ["7", "2", "5", "9"];

// Sort in ascending order?
arr.sort(); // --> 2,5,7,9

// Descending order?
arr.reverse(); // --> 9,7,5,2

masautt

let arr = [
  { prop: "7" },
  { prop: "3" },
  { prop: "6" },
  { prop: "1" }
]

//Sort in ascending 
arr.sort(function(a, b) {
    return (a.prop - b.prop);

masautt

let idLength = 6;
Math.random().toString(36).slice(idLength); // y80wde7

masautt

<a href="https://google.com" target="_blank">GO TO GOOGLE.COM IN SEPARATE WINDOW</a>

masautt

let str = "Masautt";
let subString = "sau";

console.log(str.includes(subString)); // true