In December 2015 Apple open-sourced Swift, which has been a real success. Many developers are contributing, not only via pull requests directly into the source code, but also by helping to define the shape of the language in the swift-evolution repository. One of the things that came with Swift was server-side development. There's a new version of Swift developed for Linux with a toolset that includes a package manager, the LLDB debugger, and the REPL. This opens a whole set of new possibilities for a lot of companies, such as IBM who are currently making a huge investment in its framework Kitura.
There's a lot of small frameworks that take advantage of these new possibilities, so have a look here where Edward Jiang does a great job of introducing four of the more promising frameworks already in the market. I'm divided between Vapor and Kitura:
Kitura is based on Express.js, so if you are already using it you will find almost no differences at project level structure and so on.
If you want to have a look of all the stack that IBM has, you can go here.
You are a backend developer and you thinking: "Nah... I'm not going to learn a new whole language..." Well, don't forget that Swift, first of all, is a type-safe language. And it's a hybrid in that it supports Object-Oriented and Functional programming. A huge advantage of this is the ability to introduce yourself into functional programming, but still be "safe" in your object-oriented skills. Yes, you would have to learn a new language... But it's Swift! Not convinced yet? Take a look at these 2 articles that compare server side Swift (in this case Vapor) with a lot of other well-known languages:
Please note that these tests were made before Swift 3.0 came out.
If you are a iOS developer, and you need to have a Web service for your app, you should definitely consider one of these options, since you won't need to learn another language to deploy your Web service. If you are a backend developer, you can always learn a new, type-safe, language and then who knows? Maybe start creating your own iOS apps?
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