Wednesday, 2 March, 2016 UTC


Polymer is a library (developped by Google) to ease the creation of components or more precisely of Web Components. I really like the idea of being able to build my own element in a standard way and to use it in a web page like any other HTML element. Unfortunately, at the moment, Chrome is the only browser to really implement the 4 specifications behind the term Web Components. In others browsers, the support varies from inexistant to experimental and then Web Components polyfills are the only way to have a better support in current browsers. Polymer version 1.0 was released in May last year and is said to be production ready. The version 1.3 was released lately.
While it's possible to build a single page application with Polymer, it's probably easier to start by writing a single component. As anybody, I started with the Build your first Polymer element tutorial which is a simple way to start but maybe a bit too simple. In any case, the best way to learn a technology is to really experiment it. That's why I decided to build a component called github-user-events which displays the public events of a user on Github after fetching them with the Gitub Events API. The idea behind this exercice is also to do it the right way™ with the tools, the documentation, the unit tests and almost everything I would do in a real project. As the demo shows, the component is working :-) The code is on Github but if I don't really plan to maintain nor further develop that project.
In Practice
So in practice, the documentation to follow is Create a reusable element. The requirements to start working on your Web Component are quite low and that's a damn good thing. bower, polyserve, your favorite text editor and basically you are good to start coding.
In the JavaScript code, one of the most important task is to define the properties of your component. The properties are a big part (if not 100%) of your component public API. The properties starting with an underscore are considered private and the same goes with the methods.
Polymer offers a catalog of ready to use elements which can serve as a basis for your element. For instance, in github-user-events I use several Paper elements (paper-card, paper-item and paper-spinner) which allow to easily follow Google's Material Design recommendations. In my component, I also use another element called iron-ajax. As its name suggests, it allows to do an AJAX request. At first, in an HTML document, writing something like:
<dom-module id="github-user-events">
  <!-- ... -->
  <iron-ajax auto handle-as="json" loading="{{ loading }}"
    on-response="_buildEventList" last-error="{{ error }}"></iron-ajax>
    // some JS using Polymer
was a bit odd. I guess it's because we are used to see a direct result of a tag either in the document structure or even the rendered version. Still, this is very handy and somehow I now see that as the injection of a dependency to do an AJAX request. After thinking about it, I've also come to the conclusion that I should have divided my component into two different components: one to deal with the Github API and an other one to display the event list.
One last handy detail, the seed-element (the recommended starting point for a reusable element) provides an index page that directly generates an API documentation for the element thanks to the iron-component-page element. That's a good incentive to write some API documentation from day one!
Unit tests
Unit testing is of course important. By definition, writing small and focused component should make this step easier but the efficiency in this area is also a matter of tooling.
The recommended way to unit test a Polymer component is to use Web Component Tester. It provides a command to run unit tests in an environment where several libraries like Sinon.JS, Chai, Lodash or Mocha are already loaded. Again, that's very handy as this reduces the work overload to write unit tests to a minimum. When running wct, the tests are executed in real browsers available on the system. It's at the same time a good and a not so good thing. Testing in an environment close to the real execution environment is interesting but on the other hand, that's quite slow, at least slower than using a headless browser like phantomjs. Also, while writing this blog post, I've figured out that Web Component Tester needs a network access to be executed.
The Test your elements page describes well how to actually write the unit tests. The only thing I struggled with was that most of my tests have to be written in the asycnrhonous way because of some internal Polymer optimizations.
Travis-CI can easily be set up to automate those tests at least in Chrome and/or Firefox. For others browsers, it seems like SauceLabs is a solution, but I did not test that yet.
The developer experience is very positive. I managed to build my component quite quickly with unit tests, a Travis-CI integration and an online demo. I still have tons of questions on various details and tasks about the component creation with Polymer but that will be for an other blog post. I think I'll also try to build a simple but complete Single Page Application because I'm quite curious on how such application can be architectured but that will also be for an other blog post.