Monday, March 14, 2016

N4JS: Emphasising the Java in JavaScript

For the last three years, I have worked at NumberFour leading a team which creates a type-safe extension for ECMAScript (aka JavaScript) called N4JS. I'm happy to tell you that yesterday, NumberFour announced N4JS to go open source (press release, PDF). You can find the project home page at:

N4JS bridges the strengths of JavaScript and Java; the result is a typed JavaScript superset that is dynamic, flexible and type-safe. This first release mainly targets Node.js developers, enabling them to create large and maintainable projects.

The idea behind N4JS is to bring Java's type system to JavaScript and to make JavaScript really as type-safe as Java. This is probably also the main difference to TypeScript, which focuses on ease of transition from untyped to typed JavaScript. I don't want to go into the details here, we already created a longer description of the differences to TypeScript

As a result, N4JS brings many features known from Java to ECMAScript. It is based on ECMAScript 2015 (almost all new features are supported, missing things will be added soon). It therefore supports classes as in ECMAScript 2015 (or Java). It also introduces interfaces with default methods similar to Java 8. Also the generics are quite similar to Java, including support for generic types, generic methods, and wildcards (i.e. use-site variance). We have actually ported the type inference algorithm for type variables introduced with Java 8 to N4JS. Of course we had to adjust the type system to match the semantics of ECMAScript. N4JS therefore also supports union and intersection types or "this"-type (great for builder pattern).

The most notable thing about N4JS is that it not only supports Java's nominal typing but also TypeScript's structural typing. It even supports different variants of structural typing, e.g., only taking fields into account. We also have created a short introduction to this very special feature of N4JS.

There are many other bells and whistles in N4JS such as dependency injection (as in JSR330/Google Guice) or testing with annotations (as in JUnit). And this is only the beginning. With the foundation finished, we are looking forward to adding more sophisticated features such as more program analysis.

Although we call the current release a "public alpha", N4JS is pretty stable already. We are using N4JS internally for over a year now and we have a rather large code base (and more than 70.000 tests!). Of course, there are a lot of known issues (we will migrate them to GitHub issues soon)...

N4JS comes with an Eclipse-based IDE. Our goal is to provide the same IDE support for N4JS as JDT for Java. You can either download the N4JS IDE as a standalone product or use the update site to add N4JS to your Eclipse Mars installation. We also provide an Oomph script to set-up an Eclipse IDE ready for developing N4JS itself. See the GitHub pages for source code and details.

Well, this has become a rather long posting already. I will post more about N4JS, its features and new developments in the future either here or on the N4JS developer blog. Last but not least, I want to thank the people behind N4JS. In particular I want to thank the N4JS team at NumberFour -- you did a great job and I'm looking forward to working on N4JS and other things with you! In place of the many helpers I want to thank Sebastian Zarnekow -- you did an amazing job (readers may want to look at the grammar file to get an idea of what kind of magic he had to add to Xtext to enable that)!

Update (14.3.2016, 19:00): The N4JS Eclipse project proposal is now public!