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ts2fable npm version

Fable parser for TypeScript declaration files.


Install it with npm and run the ts2fable command on a TypeScript declaration file.

npm install -g ts2fable
ts2fable react.d.ts

The parser just displays the results on screen, but you can redirect the output to a file:

ts2fable react.d.ts > Fable.Import.React.fs

You can find more information about how to interact with JavaScript from F# here. Please note the parser is not perfect and some tweaking by hand may be needed.


Some JavaScript/TypeScript features have no direct translation to F#. Here is a list of common workarounds adopted by the parser to solve these problems:

  • Module methods: Module functions in F# have several limitations (no overload, optional or rest parameters). To overcome this, the parser wraps module methods in a dummy static typed named Globals. For example:
module ReactDom =
    type [<Import("react-dom")>] Globals =
        static member render(element: DOMElement<'P>, container: Element, ?callback: Func<Element, obj>): Element = jsNative

Note the Import attribute is on the Globals value, not the module.

  • Erased unions: TypeScript union types work differently from F# and its only purpose is to specify the types allowed for a function argument. In F# they are translated as erased unions: they're checked at compiled time but they'll be removed from the generated JS code.
type CanvasRenderingContext2D =
    abstract fillStyle: U3<string, CanvasGradient, CanvasPattern> with get, set
let ctx: CanvasRenderingContext2D = failwith "dummy"
ctx.fillStyle <- U3.Case1 "#FF0000"
  • Constructor functions: In JS any function can become a constructor just by calling it with the new keyword. In the parsed files, interfaces with this capability will have a Create method attached:
type CanvasRenderingContext2DType =
    abstract prototype: CanvasRenderingContext2D with get, set
    [<Emit("new $0($1...)")>] abstract Create: unit -> CanvasRenderingContext2D
  • Callable interfaces: In the same way, JS functions are just objects which means applying arguments directly to any object is legal in JS. To convey, the parser attaches an Invoke method to callable interfaces:
type Express =
    inherit Application
    abstract version: string with get, set
    abstract application: obj with get, set
    [<Emit("$0($1...)")>] abstract Invoke: unit -> Application