我的JS程式碼無法產生字母等級

Question

我在最後用 JS 寫了一個函數，一旦你得到 5 個科目的平均值，它就會給你一個字母等級，但它沒有向我顯示任何東西 我現在迷路了

我寫的第四個函數似乎沒有產生任何字母等級。我相信其他一切都是對的

function getHandleValue(idName) {
  const value = parseInt(document.getElementById(idName).value);
  console.log(value);
  return value;
}

function getTotal() {
  //console.log("app js starts loading")
  let english = getHandleValue('english');
  let math = getHandleValue('math');
  let physics = getHandleValue('physics');
  let computer = getHandleValue('computer');
  let science = getHandleValue('science');
  //console.log("app js ends loading")
  let total = english + math + physics + computer + science;
  document.getElementById('total').innerHTML = total;
  return total;

}

function getAverage() {
  // option  1
  // const total = parseInt(document.getElementById('total').innerHTML);
  // const average = total / 5;
  // document.getElementById('average').innerHTML = average;

  // option 2
  const average = getTotal() / 5;
  document.getElementById('average').innerHTML = average;
}

function letterGrade() {
  letterGrade;
  if (grade >= 90 && grade <= 100)
    letterGrade = 'A';

  else if (grade >= 80 && grade <= 89)
    letterGrade = 'B';

  else if (grade >= 70 && grade <= 79)
    letterGrade = 'C';

  else if (grade >= 60 && grade <= 69)
    letterGrade = 'D';

  else if (grade > 1 && grade <= 59)
    letterGrade = 'F';

  let average = letterGrade;
  document.getElementById('Grade').innerHTML = Grade;
}

Answer 1

letterGrade 未正確聲明，請執行以下操作：

let letterGrade = ' '

這宣告了 letterGrade 變數。