create unique id with javascript

Solution 1:

var id = "id" + Math.random().toString(16).slice(2)

Solution 2:

another way it to use the millisecond timer:

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

Solution 3:

const uid = function(){
    return Date.now().toString(36) + Math.random().toString(36).substr(2);
}

This Function generates very unique IDs that are sorted by its generated Date. Also useable for IDs in Databases.

Solution 4:

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;