# How to use useEffect callback with event callbacks

> How to fix a useEffect event callback reading a stale variable value, by adding that variable to the hook dependency array so it stays up to date.

Author: [Flavio Copes](https://flaviocopes.com/about/) | Published: 2022-05-30 | Topics: [React](https://flaviocopes.com/tags/react/) | Canonical: https://flaviocopes.com/fix-useeffect-event-callback/

I was running some code like this:

```js
useEffect(() => {
  if (!socket) return

  socket.on('newuserconnected', (username) => {
    console.log(connectedusers)
  })
}, [socket])
```

to initialize a callback for an event `newuserconnected` on a socket.io connection.

I assumed that after doing so, any time I called that event on the server, the client-side ([React](https://flaviocopes.com/react/) app) would print the **current value** at runtime of the variable `connectedusers` (imagine I was updating it somewhere else in the app).

But no, the value of that variable was "stuck in time" at the moment I defined that event.

What I had to do to fix the problem was to add that variable value to the list of variables that the `useEffect` call depends on:

```js
useEffect(() => {
  //...
}, [socket, connectedusers])
```