> ## Documentation Index
> Fetch the complete documentation index at: https://docs.trysignal.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Alias

> Link an alias to a user (client and server)

# Next.js — Alias

Link an alias (e.g. anonymous ID) to a user. Use this to merge anonymous events (from before login) with the identified user. Available on both client and server.

## Client: useSignal

Get `alias` from `useSignal()`:

```tsx theme={null}
'use client';

import { useSignal } from '@signal-js/nextjs/client';
import { useEffect, useState } from 'react';

function LoginHandler({ userId, userTraits }) {
  const { identify, alias } = useSignal();
  const [anonymousId, setAnonymousId] = useState<string | null>(null);

  useEffect(() => {
    // Store anonymous ID before login
    const stored = localStorage.getItem('anonymous_id');
    if (stored) {
      setAnonymousId(stored);
    }
  }, []);

  const handleLogin = () => {
    // Identify the user
    identify(userId, userTraits);

    // Link anonymous ID if it exists
    if (anonymousId) {
      alias(anonymousId);
      localStorage.removeItem('anonymous_id');
    }
  };

  return <button onClick={handleLogin}>Log in</button>;
}
```

## Client: alias(alias)

Links an alias to the current user. Sends a `$create_alias` event.

| Parameter | Type     | Required | Description                                |
| --------- | -------- | -------- | ------------------------------------------ |
| `alias`   | `string` | yes      | Alias to link (e.g. previous anonymous ID) |

**When to use:** After login to merge anonymous events (from before login) with the identified user.

## Server: alias(distinctId, alias, sessionId?, windowId?)

Link an alias to a user from the server. Pass `sessionId` and `windowId` for client correlation.

| Parameter    | Type     | Required | Description                                              |
| ------------ | -------- | -------- | -------------------------------------------------------- |
| `distinctId` | `string` | yes      | Current user identifier (e.g. after login)               |
| `alias`      | `string` | yes      | Alias to link (e.g. previous anonymous ID)               |
| `sessionId`  | `string` | no       | From `x-signal-session-id` header for client correlation |
| `windowId`   | `string` | no       | From `x-signal-window-id` header for client correlation  |

```typescript theme={null}
// app/api/auth/login/route.ts
import { signal, getSessionInfo } from '@/lib/signal-server';
import { NextRequest } from 'next/server';

export async function POST(request: NextRequest) {
  const { sessionId, windowId, distinctId } = getSessionInfo(request);
  const body = await request.json();

  // Identify the user
  signal.identify({
    distinctId: body.userId,
    properties: { email: body.email, name: body.name },
    sessionId,
    windowId,
  });

  // Link anonymous ID if provided
  if (body.anonymousId) {
    signal.alias(body.userId, body.anonymousId, sessionId, windowId);
  }

  return Response.json({ success: true });
}
```

## How it works

1. **Before login:** User browses anonymously. Events are tracked with an anonymous ID.
2. **After login:** Call `identify(userId, traits)` to set the user's identity.
3. **Link alias:** Call `alias(anonymousId)` to link the anonymous ID to the identified user.
4. **Result:** All events (both anonymous and identified) are now associated with the same user profile.

## See also

* [Identify](/docs/integrations/nextjs/identify) — identify() and reset()
* [Capture](/docs/integrations/nextjs/capture) — Track events
* [Server tracking](/docs/integrations/nextjs/server-tracking) — Server-side patterns