Question

如果会话中没有任何值，那么我想显示选择选项。

<select class="selectpicker show-tick" id="vehicle" data-live-search="true" required="required">
  <option data-tokens=""><%=session.getAttribute("operator1") %></option>
  <option data-tokens="Select option">Select option</option>
  <option data-tokens="car">Car</option>
  <option data-tokens="bike">Bike</option>
  <option data-tokens="truck">Truck</option>

当前，如果null中没有值，它将显示Session。我已经使用了此代码，但无法正常工作。

window.onload = function()
{
  var DropdownList = document.getElementById('vehicle');
  var SelectedIndex = DropdownList.selectedIndex;
  var SelectedValue = DropdownList.value;
  var SelectedText = DropdownList.options[DropdownList.selectedIndex].text;

        if (SelectedText == null)
        {
           DropDownList.SelectedValue == Select option;
        }
         else
        {
          // otherwise display session value
        }
}

Answer 1

这只能通过在JSP表达式标记中使用三元运算符来实现，并且避免编写JavaScript。

使用：

<option data-tokens=""><%= session.getAttribute("operator1") != null ? session.getAttribute("operator1") : "Select option" %></option>

删除：

<option data-tokens="Select option">Select option</option>

Answer 2

如果您想知道为什么代码无法正常工作，请在几种情况下关注

window.onload = function() {
      var DropdownList = document.getElementById('vehicle');
      var SelectedIndex = DropdownList.selectedIndex;
      var SelectedValue = DropdownList.value;
      var SelectedText = DropdownList.options[DropdownList.selectedIndex].text;

        if (SelectedText == null) { // this might actually not work as you think, in case you might need to use `"null"`
           // DropDownList.SelectedValue == Select option;
           DropDownList.value = "Select option";
        } else {
             // otherwise display session value
        }
 }

关于

<input type="tel" id="tel" value="<%=session.getAttribute("mob1") %>" placeholder="Enter number" required="required" />

您尝试过

<input type="tel" id="tel" value="<%=session.getAttribute("mob1") != null ? session.getAttribute("mob1") : "" %>" placeholder="Enter number" required="required" />

如果会话值为空，则更改JavaScript中的下拉值

2 个答案: