code: function getRandomStringNotIn(string) {
      const randomString = Math.random().toString(36).substr(2);
      if (string.indexOf(randomString) === -1) {
          return randomString;
      }
      return getRandomStringNotIn(string);
  }
docs: get a random string which is not in the string
  
  @param {string} string the parameter string
  @return {string} the random string