javascript - 加载具有数字1-20的asp下拉列表

我有一个ASP下拉列表，我需要加载数字1到20，默认情况下选择1。如何使用javascript做到这一点？我有一个示例代码，但下拉列表未加载。我想念什么吗？

<script>
    function quantitydropdown() {
        var ddl = document.getElementById('quantitydropdownid').getElementsByTagName("select")[0];

        for (var i = 1; i <= 100; i++) {
            var theOption = new Option;
            theOption.text = i;
            theOption.value = i;
            ddl.options[i] = theOption;
        }
    }
</script>

<select id="quantitydropdownid" onchange="javascript:quantitydropdown();" runat="server" style="width: 200px;"></select>

最佳答案

因此，当文档准备就绪时，我们将填充下拉列表：

// Set up event handler for when document is ready
window.addEventListener("DOMContentLoaded", function(){

  // Get reference to drop down
  var ddl = document.getElementById('quantitydropdownid');

  for (var i = 1; i < 21; i++) {
    var theOption = document.createElement("option");
    theOption.text = i;
    theOption.value = i;
    // If it is the first option, make it be selected
    i === 1 ? theOption.selected = "selected" :  "";
    ddl.options[i] = theOption;
  }
});

#quantitydropdownid { width:200px; }

<select id="quantitydropdownid" runat="server"></select>