***

id: bfb38459-dc3c-4f49-856d-c95a1f6de902
title: once
slug: /js/reference/pubsub/client/once
description: once method for the PubSub.Client class.
max-toc-depth: 3
----------------

### once

* **once**(`event`, `fn`)

Attaches an event handler to the specified event. The handler will fire only once.

#### Parameters

| Name    | Type     | Description                                                                                          |
| :------ | :------- | :--------------------------------------------------------------------------------------------------- |
| `event` | `string` | Name of the event. See the list of [events](/docs/browser-sdk/v3/js/reference/pubsub/client/events). |
| `fn`    | Function | An event handler.                                                                                    |