在文本框輸入固定數字時，選中多選框的某個元素，限制多選框的選中個數為什么沒有效果?

在你原來的基礎上改了一點點

<form>
  請選擇你愛好:
  <br>
  <input type="checkbox" name="hobby" id="hobby1"> 音樂
  <input type="checkbox" name="hobby" id="hobby2"> 登山
  <input type="checkbox" name="hobby" id="hobby3"> 游泳
  <input type="checkbox" name="hobby" id="hobby4"> 閱讀
  <input type="checkbox" name="hobby" id="hobby5"> 打球
  <input type="checkbox" name="hobby" id="hobby6"> 跑步
  <br>
  <input type="button" value="全選" onclick="checkall();">
  <input type="button" value="全不選" onclick="clearall();">
  <p>請輸入您要選擇愛好的序號，序號為1-6:</p>
  <input id="wb" name="wb" type="text">
  <input name="ok" type="button" value="確定" onclick="checkone();">
</form>
<script type="text/javascript">
  function checkall() {
    //全選
    var hobby = document.getElementsByTagName("input");
    for (var i = 0; i < hobby.length; i++) {
      if (hobby[i].type == "checkbox") {
        hobby[i].checked = true;
      }
    }
  }

  function clearall() {
    // 全不選
    var hobby = document.getElementsByName("hobby");
    for (var i = 0; i < hobby.length; i++) {
      if (hobby[i].checked == true) {
        hobby[i].checked = false;
      }
    }

  }

  function checkone() {
    //選一個   
    var j = document.getElementById("wb").value;
    var hobby = document.getElementsByName('hobby');
    if (j <= 0 || j > 6) {
      alert("請輸入正確的數字！");
    } else
    if (autoCheck("hobby", 3, 'checkone')) {

      hobby[j - 1].checked = true;
    }

  }
  // 輸入時 限制單個選中的最大個數
  function autoCheck(name, limit, type) {
    var hobby = document.getElementsByName(name);
    if (type == 'checkone') {
      var num = 1;
    } else {
      var num = 0;
    }

    for (var i = 0; i < hobby.length; i++) {
      if (hobby[i].checked) {
        num++
      }
    }
    console.log(num)
    if (num <= limit) {
      return true;
    } else {
      alert('超限了親')
      return false;
    }
  }
  //  直接選擇時限制單個選中的最大個數
  function sigleCheck() {
    var hobby = document.getElementsByName('hobby');
    if (autoCheck("hobby", 3)) {
      for (var i = 0; i < hobby.length; i++) {
        hobby[i].onclick = function (e) {

          if (autoCheck("hobby", 3, 'sigleCheck')) {

            e.target.checked = true;
          } else {
            e.target.checked = false;
          }


        }


      }

    }
  }
  sigleCheck()

</script>
</body>

把你script里面的代碼替換成下面代碼即可實現需求，為了方便書寫直接我使用es6語法，需確保你的瀏覽器支持

const limit = 3;

const hobby = [].map.call(document.querySelectorAll("input[name='hobby']"), el => el);

const checkedLength = () => hobby.filter(el => el.checked).length;

hobby.forEach(el => {
  el.addEventListener('change', e => {
    if (checkedLength() > limit) {
      e.target.checked = false;
    }
  });
});

function checkall() {
  hobby.forEach(el => el.checked = true)
}

function clearall() {
  hobby.forEach(el => el.checked = false)
}

function checkone() {
  const index = document.getElementById("wb").value;
  if (index > 0 && index < 6) {
    const el = hobby[index - 1];
    if (!el.checked) {
      if (checkedLength() < limit) {
        el.checked = true;
      } else {
        alert("只能選擇" + limit + "項哦!");
      }
    }
  } else {
    alert("請輸入正確的數字！");
  }
}