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;

another way it to use the millisecond timer:

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

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.

In reply to @scott : Sometime JS go very fast... so...

var uniqueId = null,
getUniqueName = function(prefix) {
if (!uniqueId) uniqueId = (new Date()).getTime();
return (prefix || 'id') + (uniqueId++);
};

function uniqueid(){
// always start with a letter (for DOM friendlyness)
var idstr=String.fromCharCode(Math.floor((Math.random()*25)+65));
do {
// between numbers and characters (48 is 0 and 90 is Z (42-48 = 90)
var ascicode=Math.floor((Math.random()*42)+48);
if (ascicode<58 || ascicode>64){
// exclude all chars between : (58) and @ (64)
idstr+=String.fromCharCode(ascicode);
}
} while (idstr.length<32);


return (idstr);
}

Very short function will give you unique ID:

var uid = (function(){var id=0;return function(){if(arguments[0]===0)id=0;return id++;}})();

alert ( uid() );

put in your namespace an instance similar to the following one

var myns = {/*.....*/};
myns.uid = new function () {
var u = 0;
this.toString = function () {
return 'myID_' + u++;
};
};
console.dir([myns.uid, myns.uid, myns.uid]);

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

I'm working on a similar problem to the OP, and found that elements of the solutions from @Guy and @Scott can be combined to create a solution that's more solid IMO. The resulting unique id here has three sections separated by underscores:

1. A leading letter;
2. A timestamp displayed in base 36;
3. And a final, random section.

This solution should work really well, even for very large sets:

function uniqueId () {
// desired length of Id
var idStrLen = 32;
// always start with a letter -- base 36 makes for a nice shortcut
var idStr = (Math.floor((Math.random() * 25)) + 10).toString(36) + "_";
// add a timestamp in milliseconds (base 36 again) as the base
idStr += (new Date()).getTime().toString(36) + "_";
// similar to above, complete the Id using random, alphanumeric characters
do {
idStr += (Math.floor((Math.random() * 35))).toString(36);
} while (idStr.length < idStrLen);


return (idStr);
}

Here's my own take at it based on the xpath of the element created :

/** Returns the XPATH of an element **/
var getPathTo = function(element) {
if (element===document.body)
return element.tagName;


var ix= 0;
var siblings= element.parentNode.childNodes;
for (var i= 0; i<siblings.length; i++) {
var sibling= siblings[i];
if (sibling===element)
// stripped xpath (parent xpath + tagname + index)
return getPathTo(element.parentNode)+ element.tagName + ix+1;
if (sibling.nodeType===1 && sibling.tagName===element.tagName)
ix++;
}
}


/** hashcode function (credit http://stackoverflow.com/questions/7616461/generate-a-hash-from-string-in-javascript-jquery **/
var hashCode = function(str) {
var hash = 0, i, chr, len;
if (str.length === 0) return hash;
for (i = 0, len = str.length; i < len; i++) {
chr   = str.charCodeAt(i);
hash  = ((hash << 5) - hash) + chr;
hash |= 0; // Convert to 32bit integer
}
return hash;
};


/** Genaretes according to xpath + timestamp **/
var generateUID = function(ele)
{
return hashCode(getPathTo(ele)) + new Date().getTime();
}

First the xpath of the element is fetched.

The hashcode of the xpath is then computed. We therefore have a unique id per xpath.

The problem here is that xpath are not necesseraly unique if unique elements are generated on the fly. Thus we add the timestamp at the end.

Maybe we could also garantee more unique elements by adding a final Math.Random().

Here is a function (function genID() below) that recursively checks the DOM for uniqueness based on whatever id prefex/ID you want.

In your case you'd might use it as such

var seedNum = 1;
newSelectBox.setAttribute("id",genID('state-',seedNum));


function genID(myKey, seedNum){
var key = myKey + seedNum;
if (document.getElementById(key) != null){
return genID(myKey, ++seedNum);
}
else{
return key;
}
}

You could take advantage of closure.

var i = 0;
function generateId() {
return i++;
}

If you want to enclose it:

function generator() {
var i = 0;
return function() {
return i++;
};
}


var generateId = generator();
generateId(); //1
generateId(); //2

generator could accept a default prefix; generateId coud accept an optional suffix:

function generator(prefix) {
var i = 0;
return function(suffix) {
return prefix + (i++) + (suffix || '');
};
}


var generateId = generator('_');
generateId('_'); //_1_
generateId('@'); //_2@

This comes in handy if you want your id to indicate a sequence, very much like new Date().getTime(), but easier to read.

To avoid creating any counters and be sure that the id is unique even if there are some other components that create elements with ids on the page, you can use a random number and than correct it if it's not good enough (but you also have to set the id immediately to avoid conflicts):

var id = "item"+(new Date()).getMilliseconds()+Math.floor(Math.random()*1000);
// or use any random number generator
// whatever prefix can be used instead of "item"
while(document.getElementById(id))
id += 1;
//# set id right here so that no element can get that id between the check and setting it

You could generate an ID using a timer and avoiding duplicates using performance.now():

id = 'id' + performance.now()
dup = 'id' + performance.now()


console.log(id)
console.log(id.replace('.','')) // sexy id
console.log(id === dup) // false!

.as-console-wrapper{border-top: none !important;overflow-y: auto !important;top: 0;}

Note that the High resolution time API is available in all recent browsers.

EDIT(per Ciprian's comment): That is unfortunately not enough, performance.now() is only precise to the millisecond. Consider using it in conjuction with Math.random():

const generateId = () => `${performance.now()}${Math.random().toString().slice(5)}`.replace('.','')


let id = generateId()
let dup = generateId()


console.log(id)
console.log(id === dup) // false!


let ary = [...Array(1000)].map(_ => generateId())
console.log((new Set(ary)).size === 1000) // no dups!

.as-console-wrapper{border-top: none !important;overflow-y: auto !important;top: 0;}

I use a function like the following:

function (baseId) {
return baseId + '-' + Math.random().toString(16).slice(2);
}

In parameter baseId I indicate a prefix for the id to be easier to identify the elements.

Warning: This answer may not be good for the general intent of this question, but I post it here nevertheless, because it solves a partial version of this issue.

You can use lodash's uniqueId (documentation here). This is not a good uniqueId generator for say, db records, or things that will persist a session in a browser or something like that. But the reason I came here looking for this was solved by using it. If you need a unique id for something transient enough, this will do.

I needed it because I was creating a reusable react component that features a label and a form control. The label needs to have a for="controlId" attribute, corresponding to the id="controlId" that the actual form control has (the input or select element). This id is not necessary out of this context, but I need to generate one id for both attributes to share, and make sure this id is unique in the context of the page being rendered. So lodash's function worked just fine. Just in case is useful for someone else.

Combining random & date in ms should do the trick with almost no change of collision :

function uniqid(){
return Math.random().toString(16).slice(2)+(new Date()).getTime()+Math.random().toString(16).slice(2);
}
alert(uniqid()+"\r"+uniqid());

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.

Random is not unique. Times values are not unique. The concepts are quite different and the difference rears its ugly head when your application scales and is distributed. Many of the answers above are potentially dangerous.

A safer approach to the poster's question is UUIDs: Create GUID / UUID in JavaScript?

    const generateUniqueId = () => 'id_' + Date.now() + String(Math.random()).substr(2);


// if u want to check for collision
const arr = [];
const checkForCollision = () => {
for (let i = 0; i < 10000; i++) {
const el = generateUniqueId();
if (arr.indexOf(el) > -1) {
alert('COLLISION FOUND');
}
arr.push(el);
}
};

I think if you really want to have a unique ID then the best approach is to use a library like:
uuid or uniqueid

Note: Unique ID is not the same as Random ID

To use only date time milliseconds approach is wrong.
Nowadays computers are fast enough and able to run more than one iteration of a loop in a single millisecond.

npm install uuid

Importing the library:

If you are using ES modules

import { v4 as uuidv4 } from 'uuid';

And for CommonJS:

const { v4: uuidv4 } = require('uuid');

Usage:

uuidv4();


// This will output something like: 9b1deb4d-3b7d-4bad-9bdd-2b0d7b3dcb6d

No external libraries needed. Uniqueness proved.

You could do something like this.

// Function to generate unique id


const uniqueId = (length=16) => {
return parseInt(Math.ceil(Math.random() * Date.now()).toPrecision(length).toString().replace(".", ""))
}


// ----------------------------


document.querySelector("#butt01").onclick = () => {
document.querySelector("#span01").innerHTML = uniqueId()
}


ids = []
count = 0
document.querySelector("#butt02").onclick = () => {
for (let i = 0; i< 1000; i++){
ids.push(uniqueId())
}
for (el of ids){
for (ell of ids){
if(el == ell && ids.indexOf(el) != ids.indexOf(ell)){
count += 1
}
}
}
document.querySelector("#span02").innerHTML = `Found ${count} duplicated random values.`
}

<button id="butt01">Generate</button>
<br>
<span id="span01"></span>
<br>
<hr>
<br>
<button id="butt02">Check collision potentiality in 1000 cases</button>
<br>
<span id="span02"></span>

Multiply time in milliseconds since epoch with a random value to fixed size.

Run this to see possible collisions.

You would see there are no collisions whether it is 1000, 10000 or 1000000000.

It would have a very small chance if two users generate ids at the same time and gets the rame random number.

To increase the uniqueness you could multiply date more Math.random()s.

function generateId() {
       

return Math.random().toString(36).substring(2) +
(new Date()).getTime().toString(36);
}


console.log(generateId())

Look at this functions, it will get ur job done.

If u want uppercase and lowercase chars in ur string:

function (length) {
var id = '';
while (id.length < length) {
var ch = Math.random()
.toString(36)
.substr(2, 1);
if (Math.random() < 0.5) {
ch = ch.toUpperCase();
}
id += ch;
}
return id;
}

Only lowercase chars:

function (length) {
var id = '';
while (id.length < length) {
id += Math.random()
.toString(36)
.substr(2, 1);
}
return id;
}

For generate unique id's:

const uid = () =>
String(
Date.now().toString(32) +
Math.random().toString(32) +
Math.random().toString(32)
).replace(/\./g, '')

For check that is works:

var size = 500000
var arr = new Array(size)
.fill(0)
.map(() => uid())


var b = new Set(arr)


console.log(
size === b.size ? 'all ids are unique' : `not unique records ${size - b.size}`
)

There are two packages available for this.

• For short unique id generation nanoid link

import { nanoid } from 'nanoid'
const id = nanoid()    // "Uakgb_J5m9g-0JDMbcJqLJ"
const id = nanoid(10)  // "jcNqc0UAWK"

• For universally unique id generation uuid link

import { v4 as uuidv4 } from 'uuid';
const id= uuidv4();    // quite big id

Simple Solution :)

const ID = (_length=13) => {
// Math.random to base 36 (numbers, letters),
// grab the first 9 characters
// after the decimal.
return '_' + Math.random().toString(36).substr(2, _length); // max _length should be less then 13
};
console.log("Example ID()::", ID())

let transactionId =${new Date().getDate()}${new Date().getHours()}${new Date().getSeconds()}${new Date().getMilliseconds()}

let transactionId =`${new Date().getDate()}${new Date().getHours()}${new Date().getSeconds()}${new Date().getMilliseconds()}`


console.log(transactionId)

You can use the Generator function, was introduced in ES6 (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*)

const idCreator = function* () {
let i = 0;
while (true) yield i++;
};


const idsGenerator = idCreator();
const generateId = () => idsGenerator.next().value;


console.log(generateId()) // 0
console.log(generateId()) // 1
console.log(generateId()) // 2
...

The shortest and without libraries, also works in nodejs

crypto.randomUUID();
// 'a63ae209-ec69-4867-af8a-6f4d1efe15c6'

https://developer.mozilla.org/en-US/docs/Web/API/Crypto/randomUUID

btn.onclick = () => myID.textContent = crypto.randomUUID()

<button id="btn">Generate ID</button>
<myID id="myID"></myID>

I managed to have a non digit unique id with this function, so i'll like to share :)

const id = btoa((Math.random(0,(new Date()).getTime())).toString()).slice(0, -10);

I sliced it for DB varchar limit reasons, but you're free to do without it.

Just two cents

function makeId(tokenLen) {
if (tokenLen == null) {
tokenLen = 16;
}
var text = "";
const possible = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789";
for (var i = 0; i < tokenLen; ++i)
text += possible.charAt(Math.floor(Math.random() * possible.length));


return text;
}