code: function leftZeroFill(number, targetLength) {
      var output = number + "";
      while (output.length < targetLength){
          output = "0" + output;
      }
      return output;
  }
docs: return a string which length is as much as you need
  @param {number} number input
  @param {number} targetLength expected length
  @example leftZeroFill(5,2) => 05