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Latest commit

Increase Version number to `0.9.0`

**Instructions to upgrade TypeScript**:

Note: When upgrading `TypeScript.fs`: don't update TS directly in ts2fable, but use `typescript.d.ts` from another project. 
Reason: unbundled ts2fable CLI tool uses TS from `node_modules` -- which might not work when already upgraded (Though probably only an issue for major TS upgrades, not minor ones)

./build.cmd BuildCli --typescript-special-rules

npm run ts2fable -- --noresizearray --convertpropfns --remove-obsolete "path/to/typescript.d.ts" "./src/TypeScript.fs"

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March 26, 2017 00:21

ts2fable npm version

Fable parser for TypeScript declaration files.


Install it with yarn or npm. With yarn it is:

yarn global add ts2fable

With npm it is:

npm install -g ts2fable

Run the ts2fable command on a TypeScript file and also specify the F# output file. The F# namespace in taken from the output filename. In this example, it is Yargs.

yarn add @types/yargs --dev
ts2fable node_modules/@types/yargs/index.d.ts src/Yargs.fs

You can also use --export(or -e) option to collect from multiple tsfiles

In below sample: All the related ts files in npm packages uifabric and office-ui-fabric-react will be compiled to OfficeReact.fs as a bundle

ts2fable node_modules/office-ui-fabric-react/lib/index.d.ts test-compile/OfficeReact.fs -e uifabric office-ui-fabric-react

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. Please submit bugs as issues on GitHub.

Online Version

You can also try an in-browser version here

The online version will be updated automatically when commits is merged


Succesfull builds on the master branch are uploaded and tagged as next. You can help us test these builds by installing them with:

yarn global add ts2fable@next

or build directly from source:

  • git clone
  • cd ts2fable
  • Compile:
    • Windows: ./fake.cmd build
    • Linux: ./ build
  • Run:
    • node -require esm ./build/cli/ts2fable.js Path/to/Declaration.d.ts Path/to/Output.fs
    • or: npm ts2fable Path/to/Declaration.d.ts Path/to/Output.fs

-require esm is needed, because fable outputs code with ES modules.
If you want to run ts2fable directly without esm (ECMAScript module loader):

  • build: ./fake.cmd build -t Cli.BuildRelease
  • call: node ./dist/ts2fable.js ...

Test Suites

  • ./test: Mocha tests
    • Run: ./fake.cmd build -t RunTest
    • Watch: ./fake.cmd build -t WatchAndRunTest
    • functionTests.fs: Test ts2fable functions
    • fsFileTests.fs: Test file translation using small snippets and compare output against expected results or content
  • ./test-compile: Translate actual TypeScript declaration files with ts2fable into F#. Then compile with F# compiler to ensure they are valid F#.
    • Run: ./fake.cmd build -t BuildTestCompile
    • Setup for translation of .ts files: ./build.fsx > Target RunCliOnTestCompile
    • Setup for .NET compilation: ./test-compile/test-compile.fsproj

Run both test suites: ./fake.cmd build -t CliTest

Debug Test:

  • In VS Code: Ctrl+Shift+P > Run Task > WatchTest
  • Add your test to ./test/fsFileTests.fs and prefix with mocha only (See below sample)

Sample Test:

only "duplicated variable exports" <| fun _ ->
    let tsPaths = ["node_modules/reactxp/dist/web/ReactXP.d.ts"]
    let fsPath = "test-compile/ReactXP.fs"
    testFsFiles tsPaths fsPath  <| fun fsFiles ->
        |> getTopVariables
        |> List.countBy(fun vb -> vb.Name)
        |> List.forall(fun (_,l) -> l = 1)
        |> equal true
  • Press F5 (or launch Run Mocha Tests) to debug this test


  • Start with: ./fake.cmd run -t WebApp.Watch
  • Launch in Browser: localhost:8080


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:

  • 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