Javascript/認証フォーム

var smallAlphabet = "abcdefghijklmnopqrstuvwxyz"
var largeAlphabet = smallAlphabet.toUpperCase();
var number = "0123456789";
var seed = smallAlphabet + largeAlphabet + number;
var code = document.getElementById("code");
var randomCode = "";
//4~6文字
var numArray = [3, 4, 5];
var randomNumber = numArray[Math.floor(Math.random()*numArray.length)];
//background
var back = ["green", "blue", "red"];
var randomBack = back[Math.floor(Math.random()*back.length)];
//3or4or5のrandom文字
for( var i=0; i