Microsoft has announced WinJS 3.0:

The Windows Library for JavaScript (WinJS) project is pleased to announce the general availability of its first release – WinJS 3.0 – since the open source project began at //BUILD 2014.

Much of WinJS will run on any modern browser but the browser support matrix has a number of gaps:

You can also see what runs where from this status table.

But what is WinJS? Note that it comes from the Windows apps team, not the web development team at Microsoft. WinJS was designed to enable app development for Windows 8 “Metro” (also known as the Windows Runtime) using JavaScript, CSS and HTML. Back in 2010, when Microsoft signalled the end of Silverlight and the rise of HTML 5 for browser-based applications, early versions of WinJS would already have been in preparation. Using WinJS you can share code across a Windows 8 app, web apps, and via an app packager like Apache Cordova, in apps for Android and iOS as well.

Note that Cordova is now integrated into Visual Studio, using the catchy name Multi-Device Hybrid App:

If you want to know what kind of controls and components are on offer in WinJS, you can find out using the excellent demo site here. This is Firefox:

Quick summary then: WinJS lets you build apps that look like Windows 8 Store apps, but which run cross-browser and cross-platform. But who wants to do that?

Maybe Microsoft does. The messaging from the company, especially since CEO Satya Nadella took over from Windows guy Steve Ballmer, is “any device”, provided of course that they hook up to Microsoft’s services. That messaging is intended for developers outside the company too. Check out the current campaign for Microsoft Azure, which says “consume on any device”.

This could be a web application, or it could be a client app using Azure Mobile Services or an ASP.NET Web API application to connect to cloud data.

You do not have to use WinJS to consume Microsoft’s services of course. Why would developers want to use the look and feel of a rather unloved app platform, rather than the native look and feel of Android or iOS? That is an excellent question, and in most cases they will not. There could be cases though, for example for internal business apps where users care most about functionality. What is the current stock? What is the lead time? Show me this customer’s order history. A WinJS app might not look right for the platform, but the UI will be touch-friendly, and ease of rollout across the major mobile platforms could trump Apple’s design guidelines.

If you are writing a pure web application, users expectations concerning native look and feel are not so high. The touch-oriented design of WinJS is its main appeal, though other web frameworks like JQuery Mobile also offer this. The “Metro” design language is distinctive, and Microsoft will be making a renewed push for Windows Store apps, or Universal Apps, as part of the new wave of Windows called Windows 9 or “Threshold”. WinJS is the way to build apps for that platform using JavaScript and HTML, with the added bonus of easy porting to a broad range of devices.

This is a hard sell though. I am impressed by the effort Microsoft has put into making WinJS work cross-platform, but will be surprised to see much usage outside Windows Store apps (including Windows Phone). On the other hand, it does help to keep the code honest: this really is HTML and JavaScript, not just a wrapper for Windows Runtime APIs.