text: var srcString = "";
  
  function md5(src) {
      if (typeof(src) !== 'string') {
          return null;
      }
      var step1Res = addTail(src);
      function addTail(src) {
          var len = src.length;
          var mod = len %
      }
  }
  
  var destString = md5(srcString);
  console.log("the result is ", destString);