Testing JavaScript using BrowserSync, Mocha and PhantomJS

Repository Code of this tutorial

It’s a good idea to test your JavaScript applications using a testing framework. It improves the integrity of your software and enables you to do Test Driven Development.

You can do your testing in Mocha, but you can also expand it a bit further with browser specific behaviour testing. This article is aimed to get you started with testing using BrowserSync, Mocha and PhantomJs.

What we’re going to make

We are going to make a fairly straightforward test that can be tested in the browser and on command-line. This way we can also integrate our tests with services like Travis CI to automatically perform tests on future releases.

At the end of the article you’ll have a project where you can write your tests in. You’ll have JavaScript code that can be tested on command-line and an HTML page where the script you want to test will live. It will allow us to actually see the tests.

All changes will be watched and checked automatically. You can keep your focus on writing code while the test results get updated elsewhere.

We’re going to use Gulp for this workflow. You can also do without, but it comes in handy later when we want to watch our files for changes and automatically reload the tests in BrowserSync.

For bundling our code and not breaking the Mocha way of testing Node.js code we’re going to use Browserify.

1. Getting started

First off, create a project folder which will hold the files and navigate to that folder in your terminal.

Init an npm project:

npm init

Install Gulp globally if you haven’t already:

npm install -g gulp

Install all the dependencies we’ll be using:

npm install --save browser-sync browserify gulp gulp-mocha-phantomjs mocha-phantomjs mocha vinyl-source-stream

The reason I install gulp-mocha-phantomjs, mocha-phantomjs and mocha is for easier access later on.

2. Creating something to test

Let’s say we want to create a JavaScript library that changes the text inside a given DOM element to something else.

In order to know what to do next is to determine the way our module will be used and what the outcome should be.

We’ll call the module TextChanger which holds a method called replaceText. The replaceText method will accept two parameters: a DOM element and a string with text which will be placed inside.

If we’re going to follow a TDD approach we need to write tests for this method before even implementing the code of TextChanger.

Create a new folder called test and place a file called tests.js in it. Place the following inside the file:

var assert = require("assert");

describe('TextChanger', function(){
    var element = document.createElement("section");
    element.appendChild(
        document.createElement("span")
            .appendChild(
                document.createTextNode("Replace me")
            )
    );

    describe('#changeText(element, text)', function() {
        it('should replace the content of the element with given text', function() {
            assert.equal(false, true);
        });

        it('should throw and error if element is not a DOM element', function() {
            assert.equal(false, true);
        });
    });
});

These tests do not do anything yet. They fail because false is not true. I do this on purpose because we have to write correct tests first.

Would you run this test using just mocha test/tests.js, Mocha would complain ReferenceError: document is not defined. Mocha is not run in a browser environment, just like Node.js. In order to enable testing in a browser environment we need to use PhantomJS.

3. An HTML document to run the tests

In order to run the tests in a browser environment we’re going to need an HTML document which will load the necessary scripts and styles. While Mocha can test .js files, Mocha PhantomJS takes .html files. In this document we’re going to load Mocha and our tests.

Create a new file in the test folder called tests.html and place the following content in it.

<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title>TextChanger tests</title>
    <link rel="stylesheet" href="../node_modules/mocha/mocha.css"/>
</head>
<body>
    <div id="mocha"></div>
    <script src="../node_modules/mocha/mocha.js"></script>
    <script>mocha.setup('bdd')</script>
    <script src="tests-browserify.js"></script>
    <script>
        if (window.mochaPhantomJS) {
            mochaPhantomJS.run();
        } else {
            mocha.run();
        }
    </script>
</body>
</html>

We are loading Mocha from the node_modules folder, our tests (which do not exist yet) and running the tests in the end.

The reason we need a different test file from the one we created earlier is because the browser doesn’t understand require(). We can solve this by using Browserify to bundle our script.

4. Watching and serving the tests

In this step we’re going to create our workflow. We’ll use Gulp as our runner, use BrowserSync as our web server which can automatically reload and Browserify to bundle our test script.

Create a gulpfile.js file in the root of your project. Add the following contents.

var gulp = require("gulp"),
    browserSync = require("browser-sync"),
    browserify = require("browserify"),
    source = require("vinyl-source-stream"),
    mochaPhantomJS = require("gulp-mocha-phantomjs");

gulp.task("browser-sync", function () {
    "use strict";
    browserSync({
        server: {
            //serve tests and the root as base dirs
            baseDir: ["./test/", "./"],
            //make tests.html the index file
            index: "tests.html"
        }
    });
});

gulp.task("browserify", function() {
    "use strict";
    return browserify("./test/tests.js")
        .bundle()
        .on("error", function (err) {
            console.log(err.toString());
            this.emit("end");
        })
        .pipe(source("tests-browserify.js"))
        .pipe(gulp.dest("test/"))
        .pipe(browserSync.reload({stream:true}));
});

gulp.task("test", function () {
    "use strict";
    return gulp.src("./test/tests.html")
        .pipe(mochaPhantomJS());
});

gulp.task("serve", ["browserify", "browser-sync"], function () {
    "use strict";
    //when tests.js changes, browserify code and execute tests
    gulp.watch(["test/tests.js", "src/text-changer.js"], ["browserify", "test"]);
});

The browser-sync task makes sure the test folder and the root of the project get served as a web server. It will also set tests.html as the index so it will open said file when browsing to http://localhost:3000. When this task is run it automatically opens the link in your browser.

The browserify task bundles the tests.js file and creates a browser runnable tests file.

The test tasks executes mocha-phantomjs and outputs the outcome in the terminal. This is the same output you get in your browser.

To start serving the tests and rerun them every time you make a change in the tests file you need to run the following command in your terminal.

gulp serve

5. Finishing the tests

If you have your web server running using the gulp serve command we just implemented you’ll see a web page with two errors complaining that true does not equal false. Which is good. Now we can make write some useful tests.

With the watcher in Gulp running it will run Browserify and reload the tests once you make a change. You don’t have to run the test command yourself.

Let’s create a file where the source of TextChanger will be defined. Create a folder called src and create a new file called text-changer.js. Leave it empty for now.

Open up the test/tests.js file, it’s time to fill in the tests.

First we need to require the TextChanger module. Do so after var assert = require("assert"); by adding:

var assert = require("assert"),
    textChanger = require("../src/text-changer.js");

Now that we’ve included our empty module, we can use it in our tests. The first test will check if the given element holds a textNode with a string value. The textChanger object will be a factory which returns an object which holds the method replaceText. Given this info the first test can be made.

it('should replace the content of the element with given text', function() {
    textChanger().replaceText(element, "test");

    assert.equal(element.childNodes[0].nodeValue, "test");
});

The second test checks if the method throws an error if no real DOM element is given. Which can be done by doing:

it('should throw and error if element is not a DOM element', function() {
    assert.throws(function () {
        textChanger().replaceText(null, "test");
    }, /DOM element/);
});

Now that the tests are ready to check our implementation we can continue to coding the module. The biggest advantage we have now is that we know what the implementation should look like.

6. Implementing your module

It is time to make our tests succeed. We know how the module has to work, now we have to make it work.

Open up the empty src/text-changer.js file.

At first we need a little base to fill in our implementation. Place the following content inside.

var textChanger = {
    replaceText: function (element, newText) {
        //check if element is a DOM element

        //remove all the children

        //add textNode to element
    }
};

module.exports = function () {
    return Object.create(textChanger);
};

Now that we have this basis, we see that we might need two extra methods. I’ll call them isDomElement and removeChildren. You could (and should) go back to the tests file and write some tests for these methods too. To keep this tutorial short, I’ll skip this part.

Add two extra methods in the textChanger object called isDomElement and removeChildren. Give them the following implementation:

isDomElement: function (element) {
    return (element && element.nodeType && element.nodeType === 1);
},

removeChildren: function (element) {
    while (element.firstChild) {
        element.removeChild(element.firstChild);
    }
}

So far we’re not doing anything in the replaceText method which gets called yet. It’s time to implement the first rule we defined in our tests: “it should replace the content of the element with given text”.

Place the following in the replaceText method and save the file:

this.removeChildren(element);

element.appendChild(
    document.createTextNode(newText)
);

Look at your browser and terminal. You’ll see that the first test is now succeeding. Great! Time to put a check for the given element in the method as well.

Put the following code at the beginning of the replaceText method:

if (!this.isDomElement(element)) {
    throw new Error("element is not a DOM element");
}

Save the file and… success! Both tests succeed and should be green in the future of your application. It’s okay to refactor your tests, as long the integrity stays the same.

The finished implementation should look like this:

var textChanger = {
    replaceText: function (element, newText) {
        if (!this.isDomElement(element)) {
            throw new Error("element is not a DOM element");
        }

        this.removeChildren(element);

        element.appendChild(
            document.createTextNode(newText)
        );
    },

    isDomElement: function (element) {
        return (element && element.nodeType && element.nodeType === 1);
    },

    removeChildren: function (element) {
        while (element.firstChild) {
            element.removeChild(element.firstChild);
        }
    }
};

module.exports = function () {
    return Object.create(textChanger);
};

Onwards

Now you can easily test your JavaScript modules and scripts in your terminal and browser at the same time. Without having to reload your browser or running a test task all the time.

Using the following command you can execute the test in your terminal: node ./node_modules/mocha-phantomjs/bin/mocha-phantomjs test/tests.html. Keep in mind that you still have to Browserify your code after you make changes to your tests and implementations.

I hope this gave you a quick intro into testing your JavaScript code. It takes some time to get used to write your tests before your code, but it’s worth it in the end.