JavaScript - 检查是否选择了 <select> 标签的选项的方法
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JavaScript - a way check if an option of the <select> tag is selected
提问by Narek
I have a select tag and I want to check if a moth is selected.
我有一个选择标签，我想检查是否选择了飞蛾。
<select name="Birth_Month">
<option value="" SELECTED>- Month -</option>
<option value="January">January</option>
<option value="Fabruary">Fabruary</option>
<option value="March">March</option>
<option value="April">April</option>
<option value="May">May</option>
<option value="June">June</option>
<option value="July">July</option>
<option value="August">August</option>
<option value="September">September</option>
<option value="October">October</option>
<option value="November">November</option>
<option value="December">December</option>
</select>
So do this:
所以这样做：
if (document.registration_form.Birth_Month.value === '')
{
alert('Please fill select Month!');
}
But this JavaScript for some select-s work and for some of them, does not. Obviously, when the "- Month -" is selected the it returnes "- Month -" insted of "" (empty string). What I have done wrong? What is the best way of checking the tag's selection?
但是这个 JavaScript 对某些 select-s 有效，而对其中一些则无效。Obviously, when the "- Month -" is selected the it returnes "- Month -" insted of "" (empty string). 我做错了什么？检查标签选择的最佳方法是什么？
回答by Webveloper
Browsers didn't always have a .value property for <select> - we used to have to get the value of the <option>:
浏览器并不总是有 <select> 的 .value 属性 - 我们过去必须获取 <option> 的值：
var birthMonth=document.registration_form.Birth_Month;
if(birthMonth.options[birthMonth.selectedIndex].value===''){
I use jQuery .val() now. I don't remember which browsers lack the select.value property, and maybe those browsers are so old that we don't need to worry about them anymore. But jQuery doesn't use select.value - it loops through each of the options to find the selected option's value.
我现在使用 jQuery .val()。我不记得哪些浏览器缺少 select.value 属性，也许这些浏览器太旧了，我们不再需要担心它们了。但是 jQuery 不使用 select.value - 它遍历每个选项以查找所选选项的值。
Of course, if you know you'll always have a single blank option as the first option, just check for selectedIndex==0.
当然，如果你知道你总是有一个空白选项作为第一个选项，只需检查 selectedIndex==0。
回答by hex494D49
I believe integers are a better practice for option values. Anyhow, the snippet below doesn't care if your default option value is an empty string or a zero value.
我相信整数是选项值的更好实践。无论如何，下面的代码片段并不关心您的默认选项值是空字符串还是零值。
var birthMonth = document.registration_form.Birth_Month;
if(birthMonth.options[birthMonth.selectedIndex].value === false){
alert('Please fill select Month!');
}
