# Alias

The `alias` call lets you merge different identities of a known user.

{{< info >}}
Note that:

- You can use the `alias` event only for merging user identities. It does not update the user's `traits` or other common properties.
- RudderStack supports sending `alias` events only to select downstream destinations. See the [destination-specific documentation]({{< ref "destinations/streaming-destinations/_index.md" >}}) for more details.
{{< /info >}}

## Alias fields

In addition to the [Common Fields]({{< ref "event-spec/standard-events/common-fields.md" >}}), the `alias` call accepts the following fields:

| Field | Description | Data type | Presence |
| :-----| :-----------| :----------| :---------|
| `userId` | A unique identifier for the user in the database. Either `userId` or `anonymousId` should be present. | String | Optional, if `anonymousId` is already set. |
| `previousId` | The user's previous identifier. | String | Required |

{{< warning >}}
The field names can change slightly depending on the SDK. However, the functionality remains the same.

See the [SDK-specific documentation]({{< ref "sources/event-streams/sdks/_index.md" >}}) for the implementation specifics and details on the above fields.
{{< /warning >}}

See the [Warehouse Schema]({{< ref "destinations/warehouse-destinations/warehouse-schema.md#alias" >}}) guide for more information on how RudderStack records the information in the warehouse table for an `alias` event.

### User ID vs Previous ID

- The `previousId` attribute refers to the previous user identifier. It could be either:
  - An `anonymousId` assigned to the user (for more details see [Identify API documentation]({{< ref "event-spec/standard-events/identify.md" >}})).
  - A previously set `userId` to identify the user via the `identify` call - this could be an email address, a database ID, device ID, or any other unique user identifier.
- The `userId` is the user's new identity or an existing identity that you want to merge with `previousId`. As mentioned above, it could be a new email address, database ID, device ID, or any other unique user identifier.

## Sample payload

Here is a sample payload of an `alias` call:

```json
{
  "type": "alias",
  "previousId": "gbelson@example.com",
  "userId": "13bd56c84a562"
}
```

The corresponding event that generates the above payload via the [JavaScript SDK]({{< ref "sources/event-streams/sdks/rudderstack-javascript-sdk/_index.md" >}}) is:

```javascript
rudderanalytics.alias("13bd56c84a562")
```

{{< info >}}
The RudderStack SDK automatically passes the user's `anonymousId` as `previousId` in the payload.
{{< /info >}}

{{< warning >}}
When instrumenting your website with the JavaScript SDK, the `alias` call must be made from the client-side as the `anonymousId` is generated via the browser. Similarly, if you're using a server-side SDK, the `alias` call must be made from the server-side as the session ID is set as the `anonymousId`.
{{< /warning >}}