function rastgele(kacAdet, hangisi) {
			  var text = "";
			  var harfler = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
			  var sayilar = "0123456789";

			  for (var i = 0; i < kacAdet; i++)
				if(hangisi == "harf"){
					text += harfler.charAt(Math.floor(Math.random() * harfler.length));
				}else{
					text += sayilar.charAt(Math.floor(Math.random() * sayilar.length));
				}

			  return text;
			}

//KULLANIMI
console.log(rastgele(5, 'sayi'));
console.log(rastgele(7, 'harf'));

kolay gele.