> For the complete documentation index, see [llms.txt](https://docs.hackle.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.hackle.io/en/development-guide/sdk/user-identifier/client-user-identifier.md).
# Client User Identifier
On the client, you can use a User Identifier in two ways.
* Using the device identifier managed internally by the SDK
* Using a custom identifier
## Using the Device Identifier Managed Internally by the SDK
{% hint style="warning" %}
Available only for Client-side SDKs.
{% endhint %}
Client-side SDKs include functionality for managing the device identifier.\
Therefore, users can be automatically identified without passing a separate User Identifier.
For JavaScript, Android, and iOS, you can obtain the device identifier managed by the SDK — refer to the example code below.
```javascript
// Variation distribution
const experimentKey = 42
const variation = hackleClient.variation(experimentKey)
// Event tracking
hackleClient.track("purchase")
// Get the internally managed device identifier
const userId = Hackle.getUserId()
```
```java
// Variation distribution
int experimentKey = 42;
Variation variation = hackleApp.variation(experimentKey);
// Event tracking
hackleApp.track("purchase");
// Get the internally managed device identifier
String deviceId = hackleApp.getDeviceId();
```
```swift
// Variation distribution
let variation = hackleApp.variation(experimentKey: 42)
// Event tracking
hackleApp.track(eventKey: "purchase")
// Get the internally managed device identifier
let deviceId = hackleApp.deviceId
```
```javascript
```
## Using a Custom Identifier
The SDK identifies users via the identifier passed as a parameter.\
The identifier you pass can be a Primary Key you manage yourself, a device identifier, a member ID, an email address, a hash value, etc.
```javascript
// Variation distribution
const experimentKey = 42;
const user = { userId: "ae2182e0" };
const variation = hackleClient.variation(experimentKey, user);
// Event tracking
hackleClient.track("purchase", user);
```
```java
// Variation distribution
long experimentKey = 42L;
String userId = "ae2182e0";
Variation variation = hackleApp.variation(experimentKey, userId);
// Event tracking
hackleApp.track("purchase", userId);
```
```swift
// Variation distribution
let variation = hackleApp.variation(experimentKey: 42, userId: "ae2182e0")
// Event tracking
hackleApp.track(eventKey: "purchase", userId: "ae2182e0")
```
```javascript
const user = {
userId: "ae2182e0"
}
```
### Example: Creating a Custom Identifier
If you only need data for logged-in users, you can use the member ID or email address used at login as the identifier.
However, if you need to include non-logged-in users, it is recommended to use a value that can distinguish users based on the app, device, or browser.\
(For mobile apps, use a UUID or ADID value; for PC/Mobile web, use a cookie value.)
Below is an example of generating a User Identifier based on cookies.
```javascript
import Cookies from "js-cookie"
import uuid4 from "uuid4"
function getUserId() {
const key = "PCID" // Enter your preferred name
const id = Cookies.get(key)
if (id) {
return id
} else {
const id = uuid4()
const [top, second] = window.location.hostname.split(".").reverse()
const domain = `.${second}.${top}`
Cookies.set(key, id, { expires: 99999, domain: domain, path: "/" })
return id
}
}
```
```javascript
const app = require("express")()
const bodyParser = require("body-parser")
const cookieParser = require("cookie-parser")
const {v4: uuidv4} = require("uuid")
app.use(bodyParser.urlencoded({extended: false}))
app.use(cookieParser())
app.set("view engine", "ejs")
app.set("views", "views")
function extractDomain(hostname) {
const DOMAIN_MATCH_REGEX = /[a-z0-9][a-z0-9-]+\.[a-z.]{2,6}$/i;
const SIMPLE_DOMAIN_MATCH_REGEX = /[a-z0-9][a-z0-9-]*\.[a-z]+$/i;
let domain_regex = DOMAIN_MATCH_REGEX;
const parts = hostname.split(".");
const tld = parts[parts.length - 1];
if (tld.length > 4 || tld === "com" || tld === "org") {
domain_regex = SIMPLE_DOMAIN_MATCH_REGEX;
}
const matches = hostname.match(domain_regex);
return matches ? matches[0] : "";
}
app.use((req, res, next) => {
const domain = extractDomain(req.headers.host)
if (!req.cookies.deviceId) {
const deviceId = uuidv4()
res.cookie("deviceId", deviceId, {
maxAge: 365 * 10 * 365 * 24 * 60 * 60,
domain: domain,
path: "/"
})
req.cookies.deviceId = deviceId
}
next()
});
app.get("/", (req, res) => {
console.log(req.cookies.deviceId)
res.render("index")
});
app.listen(3000, () => {
console.log("App Start")
});
```
---
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