Unit Testing Emscripten Library in Browser Using CMake and Nightwatch.JS

logo-nightwatchIn a previous blog post, I described how I took Emscripten-created JS and turned it into an UMD module.  One of the reasons I did this was because I wanted more control over the generated JavaScript and for it to be usable in more contexts, such as with the RequireJS module loader.

As I am a responsible developer, I desired to create a number of automated unit tests to ensure that the client-visible API for my Emscripten module works as I intended.  I began by searching for a browser automated test framework and settled upon Nightwatch.js.  Now I just had to figure out how to get Nightwatch.js tests running in my existing, CMake-based build system.  Here’s how I did it.

Configurig Nightwatch.JS

In order to use Nightwatch.JS, you must first configure it by creating a file called nightwatch.json. The first major decision you need to make is which WebDriver-implementing system you wish to use. Most users use Selenium, but you can also run an individual browser driver directly.

As I was not concerned with cross-browser compatibility — I assume that if the test works on one browser it will work on all major browsers — and I was looking for a system with a minimum number of build-time dependencies, I decided to use ChromeDriver automatically as my WebDriver implementation.

To make everything work, I did the following:

1. To automatically download chromedriver, add the following to CMakeLists.txt:

# Install chromedriver
  OUTPUT node_modules/chromedriver/package.json
  COMMAND npm install chromedriver
  chromedriver ALL
  DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/node_modules/chromedriver/package.json

2. To configure Nightwatch.JS to use chromedriver, create a nightwatch.json which looks like this (the purpose of nightwatch-globals.js will become clear shortly):

    "globals_path": "nightwatch-globals.js",
    "selenium" : {
        "start_process" : false
    "test_settings" : {
        "default" : {
            "selenium_host": "localhost",
            "selenium_port": 9515,
            "default_path_prefix": "",
            "desiredCapabilities": {
                "browserName": "chrome",
                "chromeOptions" : {
                    "args" : ["--no-sandbox"]
                "acceptSslCerts": true

3. To start and stop chromedriver when running tests, create a nightwatch-globals.js which looks like this:

var chromedriver = require('chromedriver');

module.exports = {
    before: function(done) {
    after: function(done) {

4. CMake will run the unit tests from ${CMAKE_CURRENT_BINARY_DIR}, so we’ll need to copy the above config files to ${CMAKE_CURRENT_BINARY_DIR}. Here’s how to do that:

# Copy nightwatch config files to target directory
  nightwatch.json ALL

  OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/nightwatch-globals.js
  COMMAND ${CMAKE_COMMAND} -E copy ${CMAKE_CURRENT_SOURCE_DIR}/nightwatch-globals.js ${CMAKE_CURRENT_BINARY_DIR}/nightwatch-globals.js
  DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/nightwatch-globals.js
  nightwatch-globals.js ALL
  DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/nightwatch-globals.js

Automatically Download Nightwatch.JS and Run Unit Tests

1. First, we need to create a Nightwatch.JS unit test. Here’s a sample test case from the Nightwatch.JS home page:

// google.js
module.exports = {
  'Demo test Google' : function (browser) {
      .waitForElementVisible('body', 1000)
      .setValue('input[type=text]', 'nightwatch')
      .waitForElementVisible('button[name=btnG]', 1000)
      .assert.containsText('#main', 'Night Watch')

2. To automatically download the Nightwatch.JS library, add the following lines to CMakeLists.txt:

# Install nightwatch
  OUTPUT node_modules/nightwatch/package.json
  COMMAND npm install nightwatch
  nightwatch ALL
  DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/node_modules/nightwatch/package.json

3. To run the above unit test as a CMake unit test, add the following lines to CMakeLists.txt:

    NAME nightwatch_test
    COMMAND ./node_modules/nightwatch/bin/nightwatch -t ${CMAKE_CURRENT_SOURCE_DIR}/google.js

You may want to separate your tests into multiple JavaScript files and execute them independently. Here’s one way to do that from CMake:

file(GLOB TESTCASE_SRC tests/*.js)
foreach (testPath ${TESTCASE_SRC})
  get_filename_component(testName ${testPath} NAME_WE)

  # Test all unit tests
    NAME browser_${testName}
    COMMAND ./node_modules/nightwatch/bin/nightwatch -t ${testPath}

Using Local Web Server when Running Test Cases

In certain cases, your unit tests be able to refer to local file: URLs, but things tend to be a lot easier if your unit tests reference URLs from a local web server. It’s really easy to get one up and running:

1. Download Node’s http-server module by adding the following to your CMakeLists.txt

# Install http-server
  OUTPUT node_modules/http-server/package.json
  COMMAND npm install http-server
  http-server ALL
  DEPENDS ${CMAKE_CURRENT_BINARY_DIR}/node_modules/http-server/package.json

2. Modify nightwatch-globals.js to start and stop the web server as part of the tests:

var chromedriver = require('chromedriver');
var http = require('http-server');

module.exports = {
    before: function(done) {
        this.server = http.createServer();
    after: function(done) {

Once this is done, your tests can refer to http://localhost:8080.

Note that http-server reads files from the current working directory, and CMake runs unit tests from ${CMAKE_CURRENT_BINARY_DIR}, so you may need to copy your test HTML and JavaScript to ${CMAKE_CURRENT_BINARY_DIR}. Here’s some CMake code which you might find helpful:

# Copy all .HTML files to binary directory
file(GLOB HTML_SRC *.html)
foreach (htmlPath ${HTML_SRC})
  get_filename_component(htmlFileName ${htmlPath} NAME)

  # Copy HTML to binary folder so they can be referred to by the test
    browser_copy_${htmlFileName} ALL

For a real-life, working example of all this in action, see the source code for my streaming percentiles library.

Creating UMD Module from Emscripten using CMake

By default, Emscripten creates a module which can be used from both Node.JS and the browser, but it has the following issues:

  1. The module pollutes the global namespace
  2. The module is created with the name Module (in my case, I require streamingPercentiles)
  3. The module cannot be loaded by some module loaders such as require.js

While the above issues can (mostly) be corrected by using –s MODULARIZE=1, it changes the semantics of the resulting JavaScript file, as the module now returns a function rather than an object. For example, code which previously read var x = new Module.Klass() would become var x = new Module().Klass(). I found this semantic change unacceptable, so I decided to abandon Emscripten’s -s MODULARIZE=1 option in favor of hand-crafting a UMD module.

I determined that the most appropriate pattern for my use case was the no dependencies pattern from UMD’s templates/returnExports.js. Applied to an Emscripten module, and using the default module name streamingPercentiles, the stanzas look like the following:


(function (root, factory) {
    if (typeof define === 'function' && define.amd) {
        // AMD.  Register as an anonymous module.
        define([], factory);
    } else if (typeof module === 'object' && module.exports) {
        module.exports = factory();
    } else {
        // streamingPercentiles is the 'default' name of the module
        root.streamingPercentiles = factory();
}(typeof self !== 'undefined' ? self : this, function () {


    return Module;

While I might be able to use Emscripten’s --pre-js and --post-js‘s options to prepend and append the above JavaScript files, these options do not guarantee in all cases that the above JavaScript files will be first and last. Therefore, I decided to prepend and append the JavaScript manually.

As my build system is CMake based, I needed to change change the compilation process to generate an intermediate file streamingPercentiles-unwrapped.v1.js, and then use some CMake magic to prepend and append the above JavaScript files:

add_executable(streamingPercentiles-unwrapped.v1.js ${STMPCT_JS_SRC})

file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/concat.cmake "
file(WRITE \${DST} \"\")

file(READ \${SRC1} S1)
file(APPEND \${DST} \"\${S1}\")

file(READ \${SRC2} S2)
file(APPEND \${DST} \"\${S2}\")

file(READ \${SRC3} S3)
file(APPEND \${DST} \"\${S3}\")
add_custom_command(OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/streamingPercentiles.v1.js
                   COMMAND ${CMAKE_COMMAND} -D SRC1=${CMAKE_CURRENT_SOURCE_DIR}/umdprefix.js
                                            -D SRC2=${CMAKE_CURRENT_BINARY_DIR}/streamingPercentiles-unwrapped.v1.js
                                            -D SRC3=${CMAKE_CURRENT_SOURCE_DIR}/umdsuffix.js
                                            -D DST=${CMAKE_CURRENT_BINARY_DIR}/streamingPercentiles.v1.js
                                            -P ${CMAKE_CURRENT_BINARY_DIR}/concat.cmake
                   DEPENDS ${CMAKE_CURRENT_SOURCE_DIR}/umdprefix.js ${CMAKE_CURRENT_BINARY_DIR}/streamingPercentiles-unwrapped.v1.js ${CMAKE_CURRENT_SOURCE_DIR}/umdsuffix.js)

With the above code, all of the original three issues are fixed without any semantic changes for users.

Board Games for Family Night

Playing board games is a major hobby of mine.  Every year I look forward to attending Gen Con, where I aggressively shop the dings and dents section of CoolStuffInc and play as many new games as I can.

We enjoy playing board games as a family.  I’ve been playing board games with my daughter since she was four; she’s eight now, and quite the gamer.  One of the things we’ve learned is that the recommended age range on a board game is merely a recommendation: the only real challenge for my daughter in playing advanced games was how well she was able to read.

I thought I’d share some of the games that we like to play together as a family.

Strategy Game

Winner: Tzolk’in: The Mayan Calendar
Runners-Up: The Castles of Burgundy, 7 Wonders, Kingsburg, Tikal
Want to try soon: Hansa Teutonica

In most strategy games, the theme is far less important than the mechanics, and Tzolk’in has my favorite game mechanic of all time:  Workers placed on six interconnected gears rotate to take the workers to different action spots.  Winning the game is all about placing and removing your workers at optimal times.

The game is beautiful, and there are many different ways to win the game.  My daughter, in her first time out and without any hints, soundly beat my wife and I with the Chichen Itza strategy.

Cooperative Game

Winner: Ghost Stories
Runners-Up: Pandemic, Shadows Over Camelot
Want to try soon: Pandemic Legacy: Season 1, Robinson Crusoe: Adventures on the Cursed Island

This is my wife’s favorite category.  We recently played Ghost Stories for the first time as a family, and while we lost (of course), we had a great time and we can’t wait to play again … once my daughter gets over her frustration of losing.  Now that I think about it, it might take a while before we can bring this game back to the table.

One vs. Everyone Else Game

Winner: Fury of Dracula (third edition)
Runners-Up: Betrayal at House on the Hill, Mansions of Madness

In any one-vs-everyone game, I tend to be the “one”.  When we played Fury of Dracula, it was no different – I was the famous vampire, trying to hide across Europe.  It was fascinating watching my daughter trying to track me down and seeing through my feints.  I thought I had her for a while, but she caught on eventually.  Once she did, I became cornered without any chance for escape.

I find it curious that all three examples here are from the horror genre…

Kid’s Choice

Winner: Steam Park
Runners-Up: Mysterium, Concept, Tokaido, The Downfall of Pompeii

My least-favorite category!

Every time my daughter gets free choice as to which game we’ll play, she invariably chooses Steam Park.  She absolutely loves the theme and the idea of building rollercoasters for people to ride.  Also, she tends to win – by a lot!

I don’t personally enjoy Steam Park or Mysterium all that much, so when it’s kid’s choice night, I cross my fingers and hope my daughter ends up picking Concept, Tokaido, or The Downfall of Pompeii.  Especially the latter: I enjoy watching my daughter getting excited about throwing my meeples into the volcano.


Winner: All racing games (e.g. Formula D)
Runners-Up: Galaxy Trucker, Blokus

These games, for whatever reason, just didn’t work with us.  We tend to find racing games like Formula D boring; there’s just not enough going on there to keep our interest.  My daughter was completely turned off by the theme of Galaxy Trucker, and Blokus had absolutely no replayability whatsoever.