Introduction to Socket.IO

Socket.IO is a library that makes us possible to implement a 2-way communication channel between client and server with low latency based on events. We can use Socket.IO to implement a WebSocket server. Originally Socket.IO was written in JavaScript but some other languages are also supported like Java and Python. In this tutorial, I will discuss with you more about Socket.IO, how to implement a WebSocket server using Socket.IO and how to connect and send messages to the Websocket server!

Socket Server

First, I will create a new NodeJS project to implement a WebSocket server as an example:

Next, we will add the dependency to the project:

Now, I will add the index.js file to start working with the NodeJS project:

We will initialize Socket.IO’s Socket Server object from NodeJS’s HTTP module as follows:

In the above example, I just initialized the Socket Server object from the HTTP Server object with default configurations. You can use another constructor with a server option parameter.

You can refer to the server options that Socket.IO supports here.

Once we have the Socket Server object, we can use the on() method of this object to add code to handle events from the client to the server.

There are 3 default events supported by Socket.IO Server: connection, connect, and new_namespace. Event connection and connect are the same!

The event connection is called when there is a connection from the client to the server and as you can see in the example above, I have code to print the words “Connected to client!”.

To start this server, add the following line to your project’s package.json file:

The output when running npm start will be as follows:

Now, I will create a client application to connect to the Websocket server that we just started.

Socket Client

The NodeJS project is as follows:

I also created a new file index.js and added dependency as follows:


We will create a new Socket Client object that connects to the Socket Server using the io() method as follows:

Here, any event that is emitted from the Socket server, we will print to the console of that client application!

The default events that Socket.IO supports for Socket Client are connect, disconnect, and connect_error.

Similar to when starting the server, we also add the following line:

into the package.json file of the socketio-client-example project to start the client application.

At this point, you will see the following output in the console log of the Socket server when starting the client application:

Send a message

If you modify the Socket Server code so that every time a client connects, the Socket Server will send a message to this client:

then when starting the client application, you will see an event sent from the server as follows:

As you can see, we use the emit() method of the Socket object with 2 parameters, event name, and data to send a message from the Socket Server to the client.

You can also send messages from the client to the Socket Server as follows:

On the Socket Server side, we also need to add code to handle this “welcome” event, for example as follows:

Run both the Socket Server and the client again, you will see that the Socket Server receives a message from the client as follows:

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