Tag Archives: ES6

ES6+ code coverage with Babel plugin


Running unit tests against an ES6+ source code base has now become an almost trivial task, thanks to Babel and all the ecosystem around it. There are a lot of good resources explaining how to do that, with different tools and frameworks.

On the other hand, code coverage on this kind of tests is a rather less covered subject …

If you apply regular code coverage solutions to that case, you end up with reports based on the transpiled code, not your original source code in ES6+, which isn’t really relevant …

Here comes isparta

This problem was solved by isparta, a code coverage tool for ES6+, using Babel, which provides code coverage reports using istanbul (which is also a code coverage tool … 😉 more infos here).

Using isparta, you can generate code coverage from unit tests on ES6+ source code base, directly against your original source code (not the transpiled one). This works great, combined with tools like karma-coverage, you can output coverage reports under any format (html, lcov … that can be processed by various tools/services like Jenkins or coveralls.io …).

isparta not maintained anymore

No Maintenance Intended

About two weeks ago, @duglasduteil added the “No Maintenance Intended” badge to his module. This should remind us that behind every open source project there are developers maintaining them (most of the time on there free time).

So, now may be the time to find a replacement for isparta … I was looking for an alternative for a few days when I eventually ended up on twitter with @kentcdodds who mentionned dtinth/babel-plugin-__coverage__.

ES6+ code coverage using a Babel plugin

Since Babel v6, you can make your very own plugins that can do much more than simply transpile ES6+ to ES5.

Babel is a generic multi-purpose compiler for JavaScript. More than that it is a collection of modules that can be used for many different forms of static analysis.

Babel Plugin Handbook

That’s what @dtinth has done with babel-plugin-__coverage__. He made a babel plugin that instrument your code, injecting metadata that will be processed by istambul (which is used under the hood by karma-coverage and other coverage report tools).

The great thing is that, since you go through Babel anyway to transpile your source code, the only thing you have to do to get those infos to feed tools like karma-coverage (or any istambul-based coverage reporter) is to activate this plugin in your .babelrc file …

Using a babel plugin for coverage is a no-brainer.


You won’t have to add complex configuration or tools anymore. @dtinth‘s plugin is 300 LOC, it’s his first babel plugin that he made over two nights. As far as I have tested it, it works very well.

This is clear that using a babel plugin for that kind of purpose is the right way to proceed. Since it’s becoming easier to setup, we might see more projects using ES6+ including code coverage reports in a near futur.

You can see an example of setup on my project topheman/react-es6-redux.

Checkout it out

RxJS – first steps


I’ve been hearing about RxJS for a little more than 2 years in meetups and conferences, but never took the time to test it in a project (lots of other things came before on my list 😉 ).

At the end of last month, I decided to try it after watching the CycleJS Courses by André Staltz. Even if you’re more into React/Redux (like I am), I recommend it.

Before starting, I created a boilerplate for Webpack / Babel projects, gathering configurations, development & build workflow, good practices I used to setup each time I was creating one of my projects using Webpack. That way, everything would be in one place (and could be shared).

One thing RxJS is very good at is managing events (and multiple events) that have intermediate states (such as drag’n drop). So you will find the following features on topheman/rxjs-experiments (at least for the moment):

I could have coded those features in VanillaJS, but when you’ll take a look at the code, you’ll see how much simplier it is in RxJS.

Test the Demo!

I’m still a beginner at reactive programming. I can understand how it simplifies async event management. The number of operators is overwhelming though … It can be hard to find the right ones (or the right way) to use them – I’ve been said that at the end, you only use a handful of them – a little like when you switch from imperative to functional programming.

I’ll continue to add examples (for me to train) – if you’re an RxJS expert and find a better way to write my code, please consider to send me a PR.


Learn functional programming with Redux

A few weeks ago, I implemented redux on my react project now renamed topheman/react-es6-redux.

What is Redux ?

Redux is a library made by Dan Abramov that evolves the ideas of Flux, avoiding its complexity (and lots of boilerplate). You can use it anywhere (client/server) with any library. Its goal is to solve the problem of state management in applications.

To do that, you have a single store that holds the state of your whole app as an object.

This store dispatches actions (make this kind of call from anywhere inside your app).

Those actions will pass through “reducers” which will process them and return a new state: (previousState, action) => newState (they are called “pure function” because no matter what, given the same arguments, they should always return the same result – no side effects).

Since all the app state exists in one place, you can combineReducers (split them so that they’ll each handle their part of the state).

This was a very short description of what is redux – more infos on redux.js.org.

How about functional programming ?

This paradigm has been around for a long time (it was there before Object Oriented programming) and if you’ve never heard of it, you’ve been using it for sure. I have been using both but I must say that I did learned design patterns in OOP but I never took any real interest in functional programming until recently, mostly because I was used to OOP.

What’s great with Redux is that if you digg just a little, you’ll learn a lot about:

  • immutability
  • functional programming
  • ES6+

If you’re doing UI in JavaScript, you should embrace those. Don’t be afraid, we’re already developping in ES6, you might find very interesting the approach of functional programming and immutability …


Optimize your bundle’s weight with webpack


The usage of a package manager for the dependencies in front-end JavaScript development has become mainstream for some time now. One of the downsides is that since it’s now very easy to install and require external modules in your application, if you’re not careful, you’ll end up with a big main.js file in production …

To avoid that, you can use multiple strategies of optimization:

  • uglify
  • deduplicate code
  • split your bundle in multiple ones and only load them on demand

But the very first thing to do is to drop the unnecessary code. For that, webpack provides the Webpack.DefinePlugin which lets you inject your own variables at build time, so that if they are set to false in a conditional, the minification step will drop the dead code.

But if you are using ES6 modules import statement, there is a catch …


module.exports = {
    new webpack.DefinePlugin({
      '__DEVTOOLS__': false //set it to true in dev mode
    new webpack.optimize.UglifyJsPlugin({
        warnings: true


import { DevTools, logger } from 'some-devtools';


With this configuration, the code inside the if statement will be dropped at minification since it will be inlined as if(false). But you’ll still be importing those packages that you won’t be using (aka dead code).

In this case you shouldn’t be using ES6 static imports but CommonJS require syntax:

  const { DevTools, logger } = require('some-devtools');

That way, if __DEVTOOLS__ === false, not only the calls inside this if statements will be dropped but the modules DevTools and logger wont even be part of the bundle, which will make it lighter.

Note: I made this example with webpack, but this kind of feature is available on other module bundlers such as browserify. An other way to resolve that kind of problem would be to specify a list of modules to exclude at build time in production mode …

Update: If you’re doing universal JavaScript (running your code on both client and server side), a good practice would be to use process.env.DEVTOOLS instead of __DEVTOOLS__. A good example is the React setup where you make your own production version of React.


Upgraded to react v0.14


The genesis of the project:

On october 7th, 2015, Facebook released the v0.14 of React. I took it as an opportunity to get back at React and upgrade those projects to the latest version, to check it out.

The easy part

Upgrading React was in fact the easy part … Since my app was react-warnings free, as said on the upgrade-guide, I only had to:

  • switch to react-dom (and react-dom/server for the server-side). As of now the render engine is a different module (React already did the render on browser and server, but now that there is react-native, it really makes sense to split)
  • switch to react-addons-* modules and drop React.addons which were deprecated (more modularity)

The tricky part

I was using react-router@0.13.2 and by upgrading to react@0.14.0, I ran into invalid peerDependencies problem (react-router@0.13.x couldn’t work with a version of React over 0.13).

So I also upgraded react-router to the v1.0.0-rc3, which has a new API … In fact, the refactor went pretty smoothly for the client-side part (the upgrade guide is well documented).

But for the server-side, I hit an issue: on previous versions, you could attach pretty much any attribute to the object passed from the router to your components. Now if you do that, it won’t get all the way down to your components. The workaround I finally found is to attach those data to the params attribute of the object passed by the router and retrieve them in the props.params of the component … There might be a better way (the createElement API …)

UPDATE: Since then, react-router has released the v0.13.4 which is compatible with react@0.13.x, but I’m glad I did the upgrade, so now I have a client & a server-side project that work with the latest versions of React & react-router!


I refactored some components with the syntax introduced in react@0.14 for stateless functional components using ES6 fat arrow (no this, class or anything alike).

As I was at it, I enhanced the build routines:

  • I added banners on html/js/css containing description/version/git revision (something I’m doing by default now on my projects) – for those who are building their project on heroku, check this commit 😉
  • I fixed react-hot-reload for the client-side project (this is a great workflow)
  • I fixed livereloading for the server-side project (when you make changes on a component in development, not only your browser has to reload but your also has your node server, since there is server-side rendering and they both need to have the same version)


This sprint was a great exercise to get back into React – both client and server-side. I also got to play with webpack and gulp (yeah, I enjoy setting up build routines 🙂 ). But moreover, coding in ES6 is great … I enjoy it, I did it on my previous project and please, don’t get left behind, don’t fear webpack/Babel, those are great tools that you could easily setup via a yeoman generator or a boilerplate from git …

I’ll keep using React, I do like this library. My next steps will be to add some animation/transition and setting up redux (or a flux-like implementation).


All the code is available on github, each tag has its own release providing a changelog with a list of items pointing to the commits of the version, so if you want to take a peak, please do.