获取选定的下拉菜单选项的值

Question

So i know there have been many posts about this and ive read them, but the solutions recommended in this previous posts, dont work.

I have a function that dynamically creates me a drop down menu. Here it is:

function date(start, end) {

    output ="";
    for(i=start;i<=end;i++)
    {
    output += "<option value='"+i+"'>"+i+"</option>";
    }

    document.write(output);

}

And i call it like this:

Month:
        <select name="mm" id="mm" disabled onchange="activateDate();">
            <script type="text/javascript">
                date(1,12);
            </script>
        </select>

Note: activateDay just changes the attribute disabled in the next dropdown menu, because you should first select a year, then a month, then a day.

and when i try to get the selected value of the month so that i can give a month a number of days (31 for january, 28 for february)like this :

var a = document.getElementById("mm");
var month = a.options[a.selectedIndex].value;

And if i use this vars for a switch which looks for the month and returns the number of the days, then i give the number of days to my date() i get an empty drop down menu for my dates. The Javascript console in chrome says this:

Uncaught TypeError: Cannot read property 'options' of null

And points me to this line:

var month = a.options[a.selectedIndex].value;

So what am i doing wrong?

Answer 1

The error

Cannot read property 'options' of null

means that a is null in a.options[a.selectedIndex].value .

Since you have an element with id="mm" , check if:

• Some script has hijacked window.document
• Some script has hijacked document.getElementById
• Some script has deleted your select, or changed its id
• You use var a = document.getElementById("mm") before loading the select to the DOM

Probably its the last one, try running your code at onload event, or place your script just before closing </body> .

Answer 2

尝试使用此代码

var month = a.options[a.selectedIndex-1].value;