как сделать ограничение на ввод операций? Если операция уже есть в строке при нажатии на кнопку калькулятора она не вводилась

function add(value) {
  let inputValue = document.getElementById("textview");
  if (isDontHaveOperation(inputValue.value, value)) {
    inputValue.value += value;
  }
}

function che() {
  var res = [];
  var answer;

  if (document.getElementById("textview").value.includes("+")) {
    res = document.getElementById("textview").value.split('+');
    return answer = parseFloat(res[0]) + parseFloat(res[1]);
  }


  if (document.getElementById("textview").value.includes("-")) {
    res = document.getElementById("textview").value.split('-');
    return answer = parseFloat(res[0]) - parseFloat(res[1]);
  }

  if (document.getElementById("textview").value.includes("/")) {
    res = document.getElementById("textview").value.split('/');
    return answer = parseFloat(res[0]) / parseFloat(res[1]);
  }

  if (document.getElementById("textview").value.includes("*")) {
    res = document.getElementById("textview").value.split('*');
    return answer = parseFloat(res[0]) * parseFloat(res[1]);
  }
  if (document.getElementById("textview").value.includes("%")) {
    res = document.getElementById("textview").value.split('%');
    return answer = parseFloat(res[0]) % parseFloat(res[1]);
  } else {
    return answer = document.getElementById("textview").value;
  }
}

function equals() {
  var a = che()
  document.getElementById('textview').value = a;
}


function clean() {
  document.getElementById('textview').value = " ";
}


function backspace() {
  let exp = document.getElementById('textview').value;
  return document.getElementById('textview').value = exp.slice(0, -1);
}

const onlyNumber = e => {
  const value = e.value;
  e.value = value.replace(/\D/g, '')
}

function degree() {

  return document.getElementById('textview').value =
    parseFloat(document.getElementById("textview").value) ** 2;

}

function isDontHaveOperation(inputValue, newValue){
const operation = ["+","-","*","/","%"];
return operation.includes(newValue) ? inputValue.split("")
   .filter(c => operation.includes(c)).length < 1 : true;
 
}

<form name="form" action="">
  <input class="textview" id="textview" value="" oninput="onlyNumber(this)">
</form>
<table>
  <tr>
    <td><input class="button-calc button-calc__active" type="button" value="С" onclick="clean()"></td>
    <td><input class="button-calc button-calc__active" type="button" value="x^2" onclick="degree()"></td>
    <td><input class="button-calc button-calc__active" type="button" value="<-" onclick="backspace()"></td>
    <td><input class="button-calc button-calc__active" type="button" value="%" onclick="add('%')"></td>
  </tr>
  <tr>
    <td><input class="button-calc button-calc__active" type="button" value="1" onclick="add(value)"></td>
    <td><input class="button-calc button-calc__active" type="button" value="2" onclick="add(value)"></td>
    <td><input class="button-calc button-calc__active" type="button" value="3" onclick="add(value)"></td>
    <td><input class="button-calc button-calc__active" type="button" value="/" onclick="add(value)"></td>
  </tr>
  <tr>
    <td><input class="button-calc button-calc__active" type="button" value="4" onclick="add(value)"></td>
    <td><input class="button-calc button-calc__active" type="button" value="5" onclick="add(value)"></td>
    <td><input class="button-calc button-calc__active" type="button" value="6" onclick="add(value)"></td>
    <td><input class="button-calc button-calc__active" type="button" value="*" onclick="add(value)"></td>
  </tr>
  <tr>
    <td><input class="button-calc button-calc__active" type="button" value="7" onclick="add(value)"></td>
    <td><input class="button-calc button-calc__active" type="button" value="8" onclick="add(value)"></td>
    <td><input class="button-calc button-calc__active" type="button" value="9" onclick="add(value)"></td>
    <td><input class="button-calc button-calc__active" type="button" value="-" onclick="add(value)"></td>
  </tr>
  <tr>
    <td><input class="button-calc button-calc__active" type="button" value="." onclick="add(value)"></td>
    <td><input class="button-calc button-calc__active" type="button" value="0" onclick="add(value)"></td>
    <td><input class="button-calc button-calc__active" type="button" value="=" onclick="equals()"></td>
    <td><input class="button-calc button-calc__active" type="button" value="+" onclick="add(value)"></td>
  </tr>

</table>