带计算器的十进制输入

Question

for (var i = 0; i < operator.length; i++) {
  operator[i].addEventListener('click', function() {
    if (this.id == "clear") {
      printHistory("");
      printOutput("");
    } else if (this.id == "backspace") {
      var output = reverseNumberFormat(getOutput()).toString();
      if (output) {
        //if output has a value
        output = output.substr(0, output.length-1);
        printOutput(output);
      }
    } else {
      var output = getOutput();
      var history = getHistory();
      if (output == "" && history != "") {
        if (isNaN(history[history.length-1])) {
          history = history.substr(0, history.length-1);
        }
      }
      if (output != "" || history != "") {
        output = output == ""?output: reverseNumberFormat(output);
        history = history+output;
        if (this.id == "=") {
          var result = eval(history);
          printOutput(result);
          printHistory("");
        } else {
          history = history+this.id;
          printHistory(history);
          printOutput("");
        }
      }
    }


  });
}

我试图让该十进制按钮弹出我的计算器，尝试添加一个if语句，该语句检查输出是否为十进制，这是行不通的。单击按钮后，我一直得到NaN作为输出，