Swizec Teller - a geek with a hatswizec.com

Senior Mindset Book

Get promoted, earn a bigger salary, work for top companies

Senior Engineer Mindset cover
Learn more

    Testing socket.io apps

    ![Nyan cat testing](https://swizec.com/wp-content/uploads/2012/11/nyan-cat-testing2.png

    Socket.io is probably one of the coolest things to come out of the JavaScript world in recent years. Finally! Something that lets web developers create real-time apps without the fuss of thinking about websockets and long polling and all the other hacks that need to be used.

    The idea is pretty simple, the server can emit an event and the client will pick it up. The converse also happens. Callbacks through the server-client barrier works as well. Socket.io takes care of deciding which of the real-time hacks should be used to make the magic happen.

    Thing is, the interplay of client and server makes socket.io apps a bit difficult to test.

    A good way I've found is the combination of Mocha, Chai and socket.io-client.

    First, something to test

    Let's take for example a very simple echo server. I used Express to make things easier to play with in the Chrome console. Here's the relevant part of app.js.

    var server = (exports.server = http
      .createServer(app)
      .listen(app.get("port"), function () {
        console.log("Express server listening on port " + app.get("port"))
      }))
    
    var io = require("socket.io").listen(server)
    io.set("log level", 0)
    
    // the important parts of echo server
    io.sockets.on("connection", function (socket) {
      socket.on("echo", function (msg, callback) {
        callback = callback || function () {}
    
        socket.emit("echo", msg)
    
        callback(null, "Done.")
      })
    })
    

    After not forgetting to load /socket.io/socket.io.js into the index page, I can now run the server, point my browser to http://localhost:3000 and play around in the console like this:

    > var socket = io.connect("http://localhost:3000")
    undefined
    > socket.on("echo", function (msg) { console.log(msg); })
    SocketNamespace
    > socket.emit("echo", "Hello World")
    SocketNamespace
    Hello World
    

    Automating the test

    Typing commands into a console, even clicking around a webpage is a rather arduous and boring process. The easiest way I've found to automate this is using Mocha and socket.io-client.

    First thing we're going to need is requiring everything and making sure the socket.io server is running.

    var chai = require('chai'),
        mocha = require('mocha'),
        should = chai.should();
    
    var io = require('socket.io-client');
    
    describe("echo", function () {
    
        var server,
            options ={
                transports: ['websocket'],
                'force new connection': true
            };
    
        beforeEach(function (done) {
            // start the server
            server = require('../app').server;
    
            done();
        });
    

    See, simple :)

    Now comes the interesting part, the actual test making sure our server does in fact echo what we ask it to.

    it("echos message", function (done) {
      var client = io.connect("http://localhost:3000", options)
    
      client.once("connect", function () {
        client.once("echo", function (message) {
          message.should.equal("Hello World")
    
          client.disconnect()
          done()
        })
    
        client.emit("echo", "Hello World")
      })
    })
    

    The idea behind this test is simple:

    1. Connect client to server
    2. Once there's a connection, listen for echo event from the server
    3. Emit echo event to the server
    4. Server responds and triggers our listener
    5. Listener checks correctness of response
    6. Disconnects client

    Disconnecting clients after tests is very important. As I've discovered, not disconnecting can lead to the socket accumulating event listeners, which in turn can fire completely different tests than what you expect. It also leads to tests that pass 70% of the time, but fail in random ways.

    In the end, our efforts are rewarded by a happy nyan cat.

    Nyan cat testing

    PS: you can see all the code on github.

    Maria Ramos from Webhostinghub.com/support/edu has translated this post into Spanish.

    Published on November 30th, 2012 in Client, JavaScript, Programming, Server, Uncategorized, WebSocket

    Did you enjoy this article?

    Continue reading about Testing socket.io apps

    Semantically similar articles hand-picked by GPT-4

    Senior Mindset Book

    Get promoted, earn a bigger salary, work for top companies

    Learn more

    Have a burning question that you think I can answer? Hit me up on twitter and I'll do my best.

    Who am I and who do I help? I'm Swizec Teller and I turn coders into engineers with "Raw and honest from the heart!" writing. No bullshit. Real insights into the career and skills of a modern software engineer.

    Want to become a true senior engineer? Take ownership, have autonomy, and be a force multiplier on your team. The Senior Engineer Mindset ebook can help 👉 swizec.com/senior-mindset. These are the shifts in mindset that unlocked my career.

    Curious about Serverless and the modern backend? Check out Serverless Handbook, for frontend engineers 👉 ServerlessHandbook.dev

    Want to Stop copy pasting D3 examples and create data visualizations of your own? Learn how to build scalable dataviz React components your whole team can understand with React for Data Visualization

    Want to get my best emails on JavaScript, React, Serverless, Fullstack Web, or Indie Hacking? Check out swizec.com/collections

    Did someone amazing share this letter with you? Wonderful! You can sign up for my weekly letters for software engineers on their path to greatness, here: swizec.com/blog

    Want to brush up on your modern JavaScript syntax? Check out my interactive cheatsheet: es6cheatsheet.com

    By the way, just in case no one has told you it yet today: I love and appreciate you for who you are ❤️

    Created by Swizec with ❤️