App.fs

module FableApp

open Fable.Core
open Fable.Core.JsInterop
open Fable.Import
open Fable.Import.Node
open Fable.Import.ts

let makeFactorialFunction() =
    let functionName = ts.createIdentifier "factorial"
    let paramName = ts.createIdentifier "n"
    
    let parameter = 
        ts.createParameter(
            None, // decorators
            None, // modifiers
            None, // dotDotDotToken
            paramName |> BindingName.Case1 |> U2.Case2);

    let condition =
        ts.createBinary(
            paramName,
            SyntaxKind.LessThanEqualsToken,
            ts.createLiteral(1))

    let ifBody =
        ts.createBlock(
            [| ts.createReturn(ts.createLiteral(1)) |],
            true) // multiline
    let decrementedArg = ts.createBinary(paramName, SyntaxKind.MinusToken, ts.createLiteral(1))

    let recurse =
        ts.createBinary(
            paramName,
            SyntaxKind.AsteriskToken,
            ts.createCall(functionName, None, [| decrementedArg |])); // typeArgs
    let statements: Statement array = 
        [|
            ts.createIf(condition, ifBody)
            ts.createReturn(recurse)
        |]

    ts.createFunctionDeclaration(
        None, // decorators
        Some [| ts.createToken(SyntaxKind.ExportKeyword) |], // modifiers
        None, // asteriskToken
        functionName |> U2.Case2 |> Some,
        None, // typeParameters
        [| parameter |],
        ts.createKeywordTypeNode(SyntaxKind.NumberKeyword) :> TypeNode |> Some, // returnType
        ts.createBlock(statements, true) |> Some // multiline
    )

let resultFile = 
    ts.createSourceFile("someFileName.ts", "", ScriptTarget.Latest, false, ScriptKind.TS); // setParentNodes

let printerOptions = createEmpty<PrinterOptions>
printerOptions.newLine <- Some NewLineKind.LineFeed

let printer = ts.createPrinter(printerOptions);

let result = printer.printNode(EmitHint.Unspecified, makeFactorialFunction(), resultFile);

printfn "%s" result

dotnet_fable_webpack.txt

C:\Users\camer\fs\ts2fable\src [(blog_2017-10-16)]> dotnet fable webpack
Fable (1.2.3) daemon started on port 61225
CWD: C:\Users\camer\fs\ts2fable
cmd /C node "C:\Users\camer\fs\ts2fable\node_modules\webpack\bin\webpack.js"
Bundling for development...
Parsing ./FableApp.fsproj...
fable: Compiled src\FableApp.fsproj
fable: Compiled src\App.fs
Hash: b5936d27100724ba5734
Version: webpack 3.6.0
Time: 11920ms
        Asset     Size  Chunks                    Chunk Names
    bundle.js  5.59 MB       0  [emitted]  [big]  main
bundle.js.map  6.24 MB       0  [emitted]         main
  [13] C:/Users/camer/.nuget/packages/fable.core/1.2.3/fable-core/Date.js 7.49 kB {0} [built]
  [18] C:/Users/camer/.nuget/packages/fable.core/1.2.3/fable-core/Util.js 20.4 kB {0} [built]
  [75] C:/Users/camer/.nuget/packages/fable.core/1.2.3/fable-core/Symbol.js 324 bytes {0} [built]
  [76] C:/Users/camer/.nuget/packages/fable.core/1.2.3/fable-core/Long.js 37.3 kB {0} [built]
  [77] ./src/FableApp.fsproj 25 bytes {0} [built]
  [78] ./src/App.fs 1.56 kB {0} [built]
 [145] C:/Users/camer/.nuget/packages/fable.core/1.2.3/fable-core/String.js 21.8 kB {0} [built]
 [146] C:/Users/camer/.nuget/packages/fable.core/1.2.3/fable-core/Int32.js 1.12 kB {0} [built]
 [150] C:/Users/camer/.nuget/packages/fable.core/1.2.3/fable-core/TimeSpan.js 2.27 kB {0} [built]
 [151] C:/Users/camer/.nuget/packages/fable.core/1.2.3/fable-core/RegExp.js 3.56 kB {0} [built]
    + 142 hidden modules
Closing Fable daemon...

node_bundle.txt

C:\Users\camer\fs\ts2fable\src [(blog_2017-10-16)]> node ..\public\bundle.js
export function factorial(n): number {
    if (n <= 1) {
        return 1;
    }
    return n * factorial(n - 1);
}