create unique id with javascript

Question

I have a form where a user can add multiple select boxes for multiple cities. The problem is that each newly generated select box needs to have a unique id. Can this be done is JavaScript?

UPDATE: here is the part of the form for selecting cities. Also note that i'm using some php to fill in the cities when a specific state is selected.

<form id="form" name="form" method="post" action="citySelect.php">
<select id="state" name="state" onchange="getCity()">
    <option></option>
    <option value="1">cali</option>
    <option value="2">arizona</option>
    <option value="3">texas</option>
</select>
<select id="city" name="city" style="width:100px">

</select>

    <br/>
</form>

Here is the javascript:

$("#bt").click(function() {

$("#form").append("<select id='state' name='state' onchange='getCity()'><option></option><option value='1'>cali</option><option value='2'>arizona</option><option value='3'>texas</option></select><select id='city' name='city' style='width:100px'></select><br/>");});

Answer 1

could you not just keep a running index?

var _selectIndex = 0;

...code...
var newSelectBox = document.createElement("select");
newSelectBox.setAttribute("id","select-"+_selectIndex++);

EDIT

Upon further consideration, you may actually prefer to use array-style names for your selects...

e.g.

<select name="city[]"><option ..../></select>
<select name="city[]"><option ..../></select>
<select name="city[]"><option ..../></select>

then, on the server side in php for example:

$cities = $_POST['city']; //array of option values from selects

EDIT 2 In response to OP comment

Dynamically creating options using DOM methods can be done as follows:

var newSelectBox = document.createElement("select");
newSelectBox.setAttribute("id","select-"+_selectIndex++);

var city = null,city_opt=null;
for (var i=0, len=cities.length; i< len; i++) {
    city = cities[i];
    var city_opt = document.createElement("option");
    city_opt.setAttribute("value",city);
    city_opt.appendChild(document.createTextNode(city));
    newSelectBox.appendChild(city_opt);
}
document.getElementById("example_element").appendChild(newSelectBox);

assuming that the cities array already exists

Alternatively you could use the innerHTML method.....

var newSelectBox = document.createElement("select");
newSelectBox.setAttribute("id","select-"+_selectIndex++);
document.getElementById("example_element").appendChild(newSelectBox);

var city = null,htmlStr="";
for (var i=0, len=cities.length; i< len; i++) {
    city = cities[i];
    htmlStr += "<option value='" + city + "'>" + city + "</option>";
}
newSelectBox.innerHTML = htmlStr;

Answer 2

another way it to use the millisecond timer:

var uniq = 'id' + (new Date()).getTime();

Answer 3

Like others said you can use a running index, or if you don't like the idea of using a variable just pull the id of the last city in the list and add 1 to its id.