I was running some code like this:

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 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:

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

Lessons in this unit:

0:	Introduction
1:	How to handle errors in React
2:	Fix the “Objects are not valid as a React child” error
3:	Fix Uncaught Error Objects are not valid as a React child
4:	▶︎ How to use useEffect callback with event callbacks
5:	How to debug a React application
6:	How to fix the dangerouslySetInnerHTML did not match error in React
7:	React, how to fix the TypeError: resolver is not a function error
8:	How to fix the "cannot update a component while rendering a different component" error in React
9:	Testing React components
10:	How to configure HTTPS in a React app on localhost