Add option options to the select control dynamically in Javascript

When writing a client script, you may need to add the option to the select control. Code Is easier to achieve:
VaR opt = New Option ('mytext', 'myvalue ');
Here, parameter 1 is the value displayed to the user, and parameter 2 is the actual parameter value.
To add an option, you can append it to the options array and delete it. You can set the option value to null. Such:
<SCRIPT type = "text/JavaScript" Language = "JavaScript">
....
Add:
Function btnadd_onclick ()"
{
VaR opt = New Option ('New York ', '1 ');
Document. frmcity. Citys. Options [0] = OPT;
}
Delete:
Function btndel_onclick ()
{
Document. frmcity. Citys. Options [0] = NULL;
}
...
<Form ID = "frmcity" name = "frmcity">
...
<Select name = "citys" id = "citys">
<Option value = "1"> New York </option>
</SELECT>
<Input type = "button" value = "add City" onclick = "btnadd_onclick ()"/>
<Input type = "button" value = "Del City" onclick = "btndel_onclick ()"/>
</Form>
The above Code simply completes the corresponding functions