code: function(len){
  
  	var random_base = [
  	    '1', '2', '3', '4', '5', '6', '7', '8', '9', '0',
  	    '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'
  	];
  	var length = len || 10;
  	var cache = new Array();
  	for( var ii = 0; ii < length; ii++ ){
  	    var rbase_index = Math.floor(Math.random() * random_base.length);
  	    cache.push(random_base[rbase_index]);
  	}
  	return cache.join('');
      }
docs: Random number generator of fixed length. Return a random number
  string of length len.
  
  @param {} len - the number of random character to return.
  @returns {string} string