使用html，JS

Question

您将如何验证某人只会检查不超过2个选项？这是一个非常奇怪的问题，谷歌搜索，找不到文章。我更喜欢JS方法。

 <select multiple>
      <option value="cheese">Cheese</option>
      <option value="pepperoni">Pepperoni</option>
      <option value="sausage">Sausage</option>
      <option value="mushrooms">Mushrooms</option>
    </select>

Answer 1

您将查看选择的click事件中的selectedOptions.length，然后在该计数大于2时不允许进一步选择。

$(function() {
  // Get references to the different select elements and convert
  // them into Arrays:
  var pizzaSelect =
    Array.prototype.slice.call(
      document.querySelectorAll("#pizzaOptions > option"));
  var sodaSelect =
    Array.prototype.slice.call(
      document.querySelectorAll("#soda > option"));
  
  // Loop over each list's options, executing the supplied 
  // callback function for each one.
  pizzaSelect.forEach(checkSelectionCount);
  sodaSelect.forEach(checkSelectionCount);
  
  function checkSelectionCount(opt, idx, ary){
    // Add a click event listener to each option:
    opt.addEventListener("click", function(event){
      // Is a 3rd option clicked and being selected?
      if(this.parentNode.selectedOptions.length === 3 && this.selected) { 
        this.selected = false; // De-select it.
      }
    });
  }
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<select multiple id="pizzaOptions">
      <option value="cheese">Cheese</option>
      <option value="pepperoni">Pepperoni</option>
      <option value="sausage">Sausage</option>
      <option value="mushrooms">Mushrooms</option>
    </select>
<br><br>
<select multiple id="soda">
      <option value="coke">Coke</option>
      <option value="sprite">Sprite</option>
      <option value="dr_pepper">Dr. Pepper</option>
</select>