Developing Bindings to JavaScript Libraries

WebSharper is designed to enable the programmer to easily interoperate with existing JavaScript code. This section documents the techniques and guidelines for creating F# bindings to existing JavaScript libraries.

The development of WebSharper bindings to JavaScript involves:

  • Writing class and function stubs.
  • Providing static types for the stubs, possibly writing wrapper types or using inlining where appropriate.
  • Defining CSS, JavaScript and other resources and packaging the bindings library.
  • Providing configuration sections for the library users.


A stub is an F# class or a function that has no F# implementation, but instead represents existing JavaScript code. The stubs provide F# identity, type safety and code completion to JavaScript-implemented functionality.

For example, assume a JavaScript file with the following declarations:

function counter() {
    var value = 0;
    return function () { return value++; };

function Counter() {
    this.value = 0; = function () {
        this.value ++;
        return this.value;

The corresponding F# module with stubs might look like this:

module Counter =

    [<Name "counter">]
    let makeCounter () : (unit -> int) = X<_>

    type Counter() =

        val mutable value: int

        [<Name "next">]
        member this.Next() : int = 0

Given the stub definitions, the following F# code:

open Counter
let c = new Counter()

Will translate to the equivalent of:

var c = new Counter()

Things to note:

  • Marking a class declaration with StubAttribute enables all its members for use from F#.

  • StubAttribute-annotated members do not have to have meaningful bodies if they are not intended to be called on the server side. Unchecked.defaultof<_> or its shorthand X<_> is acceptable as a body of any such member.

  • NameAttribute can be provided when the inferred names (F# names) do not match the desired compiled names.

Static Typing

Stubs must provide types for all represented methods. The bindings author is expected to pick or construct types that reflect the expectations of the JavaScript code. To do this effectively requires an awareness of how WebSharper represents F# data in JavaScript.

For cases when static typing is difficult or impossible to determine, it is always legal to give all argument and return parameters the obj type. This approach gives the least information to the user, but is viable if the user is to properly cast the parameters. Note that box, unbox, upcast and downcast are all implemented as the identity function in WebSharper, and are safe when the source and target types of the cast have the same data representation, as all type information is erased during compilation.

Common scenarios for providing static typing to stubs include:

  • JavaScript code expects a scalar, such as a string, a number, or a boolean). Use the matching F# type string, int, double, bool.
  • JavaScript code expects an array. If the array has a fixed known length, use an F# tuple. If the array varies in length and is monomorphic, use an F# array. If the array varies in length and is polymorphic, use obj [].
  • JavaScript code expects a first-class function}. If at all possible, determine the parameter and return types and use p1 * p2 * ... * pn -> r.
  • JavaScript code expects a string from a fixed set of possible values. Use ConstantAttribute to create a new union type to represent this fixed set of values.
  • JavaScript code expects an object with a certain field structure. Express the structure as a new record type and use it as the type of the parameter.
  • JavaScript code expects either a value of type A, or a value of type B. Use member overloads where appropriate.
  • JavaScript code expects an object with optional fields. This is a common idiom for configuration objects in graphical user interface frameworks. The recommended way to expose the assumptions about these objects to F# is to create a class with DefaultValue fields, which will be left undefined if not explicitly set:
type ButtonConfiguration [<Inline "{}">] () =
    val mutable label: string

    val mutable onclick: unit -> unit


let cfg = new ButtonConfiguration(label = "Click me!")

Packaging and Configuration

The F# code written against stub classes can only work if the implementing JavaScript is available in the runtime environment. The recommended way to do it is to annotate all stub modules and classes with the RequireAttribute and define resource classes to include the necessary files. If that is properly done, WebSharper automatically includes the relevant JavaScript and CSS links on the page when any of the stubbed functionality is used, freeing the library user from manually keeping track of these assumptions.