改了函数以后，一个数字都不显示了

let currentQuestionIndex = 0;
const questions = [];
 
function addOrSubtract(num1, num2, operator) {
  return operator === '+' ? num1 + num2 : Math.max(0, num1 - num2); // 使用Math.max确保结果不为负
}
 
function displayQuestion(question) {
  document.getElementById('question').textContent = question;
}
 
function generateQuestion() {
  const operators = ['+', '-'];
  let question = '';
  for (let i = 0; i < 3; i++) {
    const number = Math.floor(Math.random() * 4) + 1; // 1-4的随机数
    const operator = i < 2 ? operators[Math.floor(Math.random() * 2)] : '';
 
    // 立即更新问题显示
    displayQuestion(question += operator + number);
 
    if (operator !== '') { // 如果有运算符，添加空格
      displayQuestion(question += ' ');
    }
 
    setTimeout(() => {
      // 在这里计算答案，因为我们需要当前的问题字符串
      const answer = addOrSubtract(addOrSubtract(questions[questions.length - 1] ? questions[questions.length - 1].answer : 0, parseInt(number), operator), number, operator);
      
      questions.push({ question: question, answer: answer.toString() });
 
      // 只在最后一轮结束后清空问题显示
      if (i === 2) {
        displayQuestion('');
      }
    }, (i + 1) * 1000);
  }
}
 
function checkAnswer() {
  const userAnswer = parseInt(document.getElementById('answerInput').value);
  if (userAnswer === questions[currentQuestionIndex].answer) {
    alert('正确！');
  } else {
    alert('错误，正确答案是: ' + questions[currentQuestionIndex].answer);
  }
  document.getElementById('answerInput').value = '';
  if (currentQuestionIndex < 2) {
    currentQuestionIndex++;
    document.getElementById('question').textContent = '';
    generateQuestion();
  } else {
    alert('所有题目已完成！');
  }
}
 
// 初始化第一题
generateQuestion();

="100%" border="0" cellpadding="0" cellspacing="0">
            ="STYLE2">
              ="STYLE2">
              ="STYLE2">
              
="60" align="center" style="font-size: 50px; line-height: 60px; padding-top: 15px;">
="question">
            
            
="150" align="center" style="font-size: 20px; line-height: 30px;">="text" id="answerInput" placeholder="输入答案">
    ="checkAnswer()">下一题
            
            
="300" align="center">
              
            
          

下面是原来的generateQuestion()函数，结果会为负数，但还能显示数字，为了结果不为负数，改成上面generateQuestion()函数后，怎么一个数字都不显示了？

function generateQuestion() {
    const operators = ['+', '-'];
    let question = '';
    for (let i = 0; i < 3; i++) {
        const number = Math.floor(Math.random() * 4) + 1; // 1-4的随机数
        const operator = i < 2 ? operators[Math.floor(Math.random() * 2)] : '';
        setTimeout(() => {
            document.getElementById('question').textContent = number + (i < 2 ? operator : '');
        }, (i + 1) * 1000);
        question += number + (i < 2 ? operator : '');
    }
    questions.push({ question: question, answer: eval(question) });
}