添加需要选择，但它已经有一个值

Question

我想要为＃34;品牌汽车＆＃34;提供一个必需的选择，添加属性但问题是我已经有一个值＆＃34;选择品牌名称＆＃34 ; 我该怎么做才能做到这一点？ （用于HTML5检查）

＆＃13;

＆＃13;

$("#category").change(function() {
	var category = parseInt($("#category").val(), 10);
	
	switch (category){
		case 2: //car
			$("#car_part").show("slow"); 
			$("#brand_car").prop('required',true);
		}
		....
	}

＆＃13;

<select name="category" id="category">
	<option value="0">Choose the catecory</option>
	<option value='2' id='cat2' > Cars </option>
	<option value='88'  id='cat88' > Trucks </option>
</select>

<select id="brand_car" name="brand_car">
	<option id="0">Choose the brand name</option>
	<option id="10" title="Audi">AUDI</option>
	<option id="16" title="BMW">BMW</option>
</select>

＆＃13;

＆＃13;

＆＃13;

Answer 1

这个怎么样？

  $("#category").change(function() {
    	var category = parseInt($("#category").val(), 10);
    	
    	switch (category){
    		case 2: //car
    			$("#car_part").show("slow"); 
    			$("#brand_car").prop('required',true);
                break;
    		}
    		
    	});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.0/jquery.min.js"></script>
<select name="category" id="category">
    	<option value="0">Choose the catecory</option>
    	<option value='2' id='cat2' > Cars </option>
    	<option value='88'  id='cat88' > Trucks </option>
    </select>

    <select id="brand_car" name="brand_car">
        <option selected="true" style="display:none;">Choose the brand name</option>
    	<option id="10" title="Audi">AUDI</option>
    	<option id="16" title="BMW">BMW</option>
    </select>


<!--If you want to display default text as option but want to disable it below will work-->

<select id="brand_car" name="brand_car">
        <option selected disabled>Choose the brand name</option>name</option>
	<option id="10" title="Audi">AUDI</option>
	<option id="16" title="BMW">BMW</option>
</select>

Answer 2

您可以为选择添加空值，以便在选择“选择品牌名称”

时需要

代码将是这样的：

<select name="category" id="category">
	<option value="0">Choose the catecory</option>
	<option value='2' id='cat2' > Cars </option>
	<option value='88'  id='cat88' > Trucks </option>
</select>

<select id="brand_car" name="brand_car">
	
    <option value="" >Choose the brand name</option>
	<option id="10" title="Audi">AUDI</option>
	<option id="16" title="BMW">BMW</option>
</select>