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])