How to Use the useEffect Callback with Event Callbacks

In this tutorial, we will discuss how to utilize the useEffect callback with event callbacks, specifically focusing on socket.io connections in a React application. This will ensure that the client-side application displays the up-to-date value of a variable when the corresponding event is called on the server.

Let’s consider the following code snippet:

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

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

}, [socket]);

Initially, we might assume that upon executing this code, the client-side application would log the current value of the connectedusers variable whenever the newuserconnected event is triggered on the server. However, we could face an issue where the value of the connectedusers variable remains “stuck in time” at the moment we define the event.

To resolve this problem, we need to include the connectedusers variable in the dependency array of the useEffect call:

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

Including the connectedusers variable ensures that the useEffect callback is triggered whenever the value of connectedusers changes, allowing the client-side application to display the most up-to-date value.

By following this approach, you can effectively use the useEffect callback with event callbacks in your React application.