on 12/7/2016 5:42 AM

We started work on WebSharper 4 more than a year ago, open-sourced it in May and published first beta packages in August. It is a project with a big scope and still some features are planned before we would call it stable. This article is an introduction and also a status report.

What is WebSharper 4?

WebSharper is a toolset consisting of a .NET-to-JavaScript compiler, web framework and libraries. WebSharper 4 adds support for tranlating C# and expands F# language and .NET main library coverage for client-side availability. Main features are:

  • Write client and server code in a single language, or now with a mix of C# and F#. Communication between the client and server is transparent and type-safe.
  • You can also specify exact translation of method with attributes, this JavaScript code is checked for validity at compile-time or in code service.
  • Lightweight JavaScript runtime and generated code. Simple .NET types translate to JavaScript built-in types. No reflection support and some types cannot be checked against in client-side code but these give compile-time warnings or errors.
  • Metaprogramming: compile-time type information is used to generate output. Generating or transforming output code with custom logic is also possible with just a type definition and an attribute.

What is currently available?

WebSharper 4 beta packages are available under the codename Zafir on NuGet. Go to try.websharper.com to explore, write, test and share code snippets and mini-applications easily in both C# and F#. New documentation is under work, hosted on GitHub and browsable at websharper.com/docs. First full tutorial for C# newcomers presents a small CRUD application.


You can find previous change log of all beta releases on GitHub: beta1, beta2, beta3, beta3-bugfix, beta4, beta5.

Current vsix installers are available under Downloads in the "Other versions" section and here: Zafir.FSharp.vsix, Zafir.CSharp.vsix

New features

  • C#-to-JavaScript compiler fully compatible both ways with F# libraries. Code analyzer for giving you WebSharper-specific warnings and errors as you type.
  • Many new .NET framework features are usable client-side, including delegates, Tasks (usable for remote calls too), Linq methods.
  • Code dependency exploration for smaller output for single-page applications, with optional source mapping.

Track new releases on GitHub.

F#-specific new features

  • Not relying on ReflectedDefinition produces smaller .dll files and have improved compilation running time. JavaScript attribute now can be set on assembly level too, [<JavaScript(false)>] can remove a member or type from the compilation scope.
  • All F# language features are now supported, including object expressions, byref and & operator, inner generic functions, pattern matching on arrays, statically resolved type parameters.
  • Correct object-oriented behavior in JavaScript translation. WebSharper now fully supports method overrides, interface implementations, static constructors, base calls, constructor chaining, having no implicit constructor, self identifier on constructors.
  • Module let values now work as in .NET, not all initialized in arbitrary order on page load, only on first access of a value from a single file.
  • Better error reporting, translation failures are reported at the exact location of the expression.

For upgrading your WebSharper 3 projects, check out the update guide.

The future

There are a couple major features planned for the final release sometime in 2017, and also general quality improvements like API cleanup, more documentation and tutorials.

Planned features

  • C# 7 and F# 4.1 support. These include better interoperability between the two languages (newly added implicit conversions) which would simplify using from C# even those libraries in the WebSharper ecosystem or older projects which were not updated by hand to have C#-friendly overloads.
  • Support for .NET Core by the WebSharper server runtime and libraries.
  • TypeScript interoperability, including a code generator to generate C# or F# code from .d.ts, and .d.ts output for WebSharper projects.

Planned optimizations

  • Generated code optimizations for performance. For example transforming curried and tupled F# function arguments into multi-argument functions. The proxies for standard .NET classes are implemented in F# in WebSharper, so less function object creation by them would benefit C# WebSharper projects too.
  • Compiler and server runtime performance. For example better metadata format that allows partial deserialization which would reduce compiler running time and server startup time.
  • More .NET coverage, including client-side support for collection interfaces like IDictionary and better translation of type checks and conversions of value types.

Feedback and questions are welcome at the WebSharper forums and issue reports on GitHub.

Happy coding!

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