Variabel

var firstName = "Doyok";
console.log(firstName);

firstName = "Rzk";
console.log(firstName);

/* output: 
Doyok
Rzk
*/


x = 100;
var x;
console.log(x);

/* output: 100 */


y = 100;
let y;
console.log(y);

/* ReferenceError: z is not defined */

const z = 100;
console.log(z);

z = 200;
console.log(z)

/* TypeError: Assignment to constant variable. */