js获取select的长度

// HTML
<select id="mySelect">
  <option value="1">Option 1</option>
  <option value="2">Option 2</option>
  <option value="3">Option 3</option>
</select>

// JavaScript
var selectElement = document.getElementById('mySelect');
var length = selectElement.options.length;
console.log(length); // 输出: 3

// JavaScript
var selectElement = document.getElementById('mySelect');
var options = selectElement.querySelectorAll('option');
var length = options.length;
console.log(length); // 输出: 3

window.onload = function() {
  var selectElement = document.getElementById('mySelect');
  var length = selectElement.options.length;
  console.log(length);
};

document.addEventListener('DOMContentLoaded', function() {
  var selectElement = document.getElementById('mySelect');
  var length = selectElement.options.length;
  console.log(length);
});