Javascript - innerHTML not working with HTML select menus

I would suggest simply not to use innerHTML on a select - it just seems wrong. select elements have easy to use methods to add new options:
`document.getElementById('day').options.add(new Option("1", "1"))`

the parameters in the above object creation are:

new Option("optionText", "optionValue")

Just wanted to add to this answer, because it might clarify to someone who get to this post.

This is a known issue for IE.

KB article with workaround: http://support.microsoft.com/kb/276228

Also: dupe of: innerHTML replace does not reflect

EDIT: Here is my working sample based on your code:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
 <title>Selects</title>
 <meta http-equiv="content-type" content="application/xhtml+xml; charset=UTF-8" />
 <style rel="stylesheet" type="text/css">
 </style>
 <script>
function showOutboundDays(month) 
{ 
    if(month==4 || month==6 || month==9 || month==11) 
        document.getElementById('day').innerHTML='<option value="1">1</option><option value="2">2</option>';
    else if(month==2) 
        document.getElementById('day').innerHTML='<option value="1">3</option><option value="1">4</option>';
    else  
        document.getElementById('day').innerHTML='<option value="1">5</option><option value="1">6</option>';
}
 </script>
 </head>
<body>
    <select onchange="showOutboundDays(this.value);">
        <option value="1">January</option>
        <option value="2">February</option>
        <option value="3">March</option>
        <option value="4">April</option>
        <option value="5">May</option>
        <option value="6">June</option>
        <option value="7">July</option>
        <option value="8">August</option>
        <option value="9">September</option>
        <option value="10">October</option>
        <option value="11">November</option>
        <option value="12">December</option>
    </select>
    <br />
    <select id="day">
    </select>
</body>
</html>

You should not be using innerHTML to modify tags. You should be using removeChild(element); and appendChild(element);

First you set your select box in a variable for legibility and editing purposes;

var select = document.getElementById('days');

Then you clear the select box

while ( select.childNodes.length >= 1 )
{
    select.removeChild(select.firstChild);       
}

Finally you fill it again with the appropriate values

for(var i=1;i<=days;i++)
{
    newOption = document.createElement('option');
    newOption.value=i;
    newOption.text=i;
    select.appendChild(newOption);
}

So at the end with your code and my code here you get the following:

function showOutboundDays(month, year)
{
    var days=null;

    if(month==4 || month==6 || month==9 || month==11)
        days=30;
    else if(month==2)
    {
        //Do not forget leap years!!!
        if(year % 400 == 0 || (year % 4 == 0 && year % 100 != 0)) //Provided by Justin Gregoire
        {
            days=29;
        }
        else
        {
            days=28;
        }
    }
    else 
        days=31;


    var select = document.getElementById('days');

    while ( select.childNodes.length >= 1 )
    {
        select.removeChild(select.firstChild);       
    }

    for(var i=1;i<=days;i++)
    {
        newOption = document.createElement('option');
        newOption.value=i;
        newOption.text=i;
        select.appendChild(newOption);
    }
}

Leap years are now included!

This is a bit of a hack but it's tiny and works in both FF and IE as a workaround to IE's inability to change innerHTML on select elements.

function swapInnerHTML(objID,newHTML) {
  var el=document.getElementById(objID);
  el.outerHTML=el.outerHTML.replace(el.innerHTML+'</select>',newHTML+'</select>');
}

another solution is to use Jquery

$('#day').html('<option value="1">1</option><option value="2">2</option>');

Javascript - innerHTML not working with HTML select menus

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