Generate unique random alphanumeric strings #learnjs

gistfile1.js

/**
 * #learnjs
 * Generate unique random alphanumeric strings
 * @return {int} length 
 */
function passGen(length) {
  var str = '',
  randomChar = function () {
     var num = Math.floor(Math.random() * 62),
     charCodeNum = (num < 36) ? num + 55 : num + 61;
     return (num < 10 ? num : String.fromCharCode(charCodeNum));
   };
   while (str.length < length) {
     str += randomChar();
   }
   return str;
}