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WinJS and ES6

It’s time for a new episode about WinJS and WinJS Contrib ! this time we will talk about using ECMAScript 6 with WinJS and more specifically about using JavaScript classes.

With WinJS pages, you use object literal to define your pages, something like :

WinJS.UI.Pages.define('/pages/home/home.html', {    
    processed: function(element, option){ 
    ready : function(element, options){

WinJS will build a constructor for the page and add the content of the object literal as a mixin on that constructor. When the page is invoked, the constructor is used to produce a new object that hold those methods on its prototype. What is great is that you could call WinJS.UI.Pages.define multiple time on the same page, using some external mixin. This model is very interesting because you could work in the best possible way for a developper : composition.

This programming model has no limit in terms of what you can do, however it has some drawbacks. First, a lot of developpers are not JavaScript gurus. Using OO languages along with JavaScript, they are not familiar or comfortable with the concept of mixins and this syntax looks like some esotheric thing. It’s also a different programming model than the business logic where you may enforce something more « object oriented ». I know « object oriented » is not in the « pure » JavaScript paradigm, but that’s where JavaScript is going with things like ES6 classes.

The other problem with this syntax is about the tooling. More often than not, intellisense goes nuts and does not provide you any help about a variable you created in another function, 5 lines of code above.

Last but not least, the object literal makes things like ECMAScript 6 classes, or Typescript a hell to work with WinJS. This is really odd because : 1 – Typescript is from Microsoft and it does not fit well with their own UI/front-end framework, and 2 – if you play with Windows 10 preview and dev tools, you may have noticed that, if you create an universal app in JavaScript, you can use ES6 in your application. It has not been officially announced yet but it looks like Windows 10 UAP in HTML and JavaScript uses some form of EdgeHTML (the Spartan engine). It means that you can use all the ES6/HTML/CSS goodness that comes along with it, as well as the performance benefits.

So, how could you use JavaScript / ES6 / Typescript classes with WinJS ? in fact it is quite simple. Last time we looked at the basic features of WinJS Contrib, and more specifically the fact that WinJS Contrib override WinJS page control. Once you put WinJS Contrib, you could call WinJS.UI.Pages.define with a constructor instead of an object literal.

class HomePage{
    ready(element, options){
WinJS.UI.Pages.define('/pages/home/home.html', HomePage);

If you start playing with ES6 or Typescript, you may also use some of the other ES6 features. One that could help you improve readability and reduce code is arrow functions. Arrow functions are anonymous function with a syntax identical to lambdas in C#. In addition to syntactic sugar, the arrow functions does not change scope. If you use arrow function instead of functions, the « this » scope is preserved, and you avoid writing tons of ugly code like « that = this ».

To demonstrate those features, we made an ES6 version of the demo application using JavaScript classes. This version is still a Windows 8 version transpiled using Babel, but if you want to have a lot a fun, you could bring it to a Windows 10 UAP (it’s just copying files) and drop Babel, it will work just fine and it is a wonderful playground to experiment with new features of HTML/Javascript in the new rendering engine in Windows 10.

If you want a more direct look, you could check the « searchbar » control that we used to illustrate WinJS Contrib basics on the previous post.

Une réponse à “WinJS and ES6

  1. Pingback: Using WinJS controls as webcomponents | Le Post de MCNEXT

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