Includes在函数参数上使用时不是函数

function calcTotalCook(prep, rest, cook) {
  //console.log(rest);

  if (prep.includes('hours')) {
    prep = prep.replace('hours', '');
    prep = Number(prep) * 60;
  }

  if (rest.includes('hours')) {
    rest = rest.replace('hours', '');
    rest = Number(rest) * 60;
  }

  if (cook.includes('hours')) {
    cook = cook.replace('hours', '');
    cook = Number(cook) * 60;
    console.log(cook);
  }

  document.getElementById('totalTime').innerText = Number(prep) + Number(rest) + Number(cook);
}

<div class="fourth clearfix">
  <h2>Prep Time:</h2>
  <input type="text" id="prepTime" name="prepTime" class="inputField" value="">
</div>
<div class="fourth">
  <h2>Rest Time:</h2>
  <input type="text" id="restTime" name="restTime" class="inputField" value="">
</div>
<div class="fourth">
  <h2>Cook Time:</h2>
  <input type="text" id="cookTime" name="cookTime" class="inputField" value="">
</div>
<div class="fourth">
  <h2>Total Time:</h2>
  <label id="totalTime" name="totalTime"></label>
</div>
<input type="button" class="addButton" value="Calc" onclick="calcTotalCook(document.getElementById('prepTime').value, document.getElementById('restTime').value, document.getElementById('cookTime').value);">