자바스크립트 랜덤(문자+숫자 )

function randNum(){
 var ALPHA = ['a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z','0','1','2','3','4','5','6','7','8','9'];
 var rN='';
 for(var i=0; i<8; i++){
  var randTnum = Math.floor(Math.random()*ALPHA.length);
  rN += ALPHA[randTnum];
 }
 return rN;
}