The place to find a quick answer to your questions


This is the section for quick questions or to miss the holes in the documentation. Check the first question below to learn how you can help make the FAQ a great resource for Fable users.


How can I add a new question?

Make sure the question is not answered already either here or in the documentation. Then just click here to edit this page in Github, add your question to the bottom and create a PR. Maintainers will write an answer and ask you if that solves the issue. If it does, the PR will be merged and the question will be added to the page so it can be referenced by other users.


Can I use Type Providers with Fable?

Erasure Type Providers compatible with netstandard should be compatible with Fable, however they must generate code that doesn't call .NET APIs unsupported by Fable. At the time of writing there are no Type Providers designed for Fable available.

How can I run a certain piece of code only in my production build?

Using a Compiler Directive.

#if !DEBUG
printfn "only printing in production bundle"

You can define compilation directives using the define option in your Fable client. Latest fable-loader will automatically define DEBUG when running Webpack in development mode.

How are numbers compiled to JS?

All numeric types including decimal become JS number (64-bit floating type), except for longs and big integers. Tom Clarke has documented in much more detail the differences in numeric types between .NET and JS, check it out.


Which operators are used in Fable.Core?

The Fable.Core.JsInterop module provides three main operators to interact with JS (all of them erased in generated JS code): ? to dynamically access a property of an object, !! for unsafe casting (equivalent to unbox) and !^ to pass arguments to a method accepting Erased Unions.

open Fable.Core.JsInterop

let jqElement = Browser.window?jQuery
let result: float = !!myObj?bar(5)
myObject?aProperty <- 30

let myMethod (arg: U3<string, int, Test>) = ...
myMethod !^5 // Same as: myMethod (U3.Case2 5)

How can I create a JS object?

The Fable.Core.JsInterop module provides three helpers to create JS object. The first one, createObj, is used to dynamically create a JS object with string keys:

open Fable.Core.JsInterop

// { a: 5, b: aValue }
createObj [
    "a" ==> 5
    "b" ==> aValue

If you have an interface definition for the JS object you can just use an F# Object Expression. However, in many cases you only want to set a few fields, as when you pass options to a JS library, in those cases you can use jsOptions and a lambda to declare your object and have the compiler type-check the fields:

type IFoo =
    abstract foo: string with get, set
    abstract bar: int with get, set
    abstract optionalField: obj with get, set

let myJsApi (options: IFoo) = importMember "foo-module"

// JS: { foo: "foo", bar: 5 }
myJsApi (jsOptions(fun (o: IFoo) ->
    o.foo <- "foo"
    o.bar <- 5

If you use these helpers multiple times, you may want to define custom prefix operators for a terser syntax. For example:

let inline (~%) x = createObj x
let inline (~%%) x = jsOptions x

anotherJsApi %["bar" ==> 10]
myJsApi %%(fun o -> o.foo <- "foo")

Finally, some libraries (like React bindings) use F# unions to represent fields of JS objects. In those cases you can use keyValueList to transform a list of union cases to a JS object.

type MyOptions =
    | Foo of string
    | Bar of int

// JS: { foo: "foo", bar: 5 }
// Note we use the caseRules argument to change the first letter
keyValueList CaseRules.LowerFirst [
    Foo "foo"
    Bar 5

Library authors

Which files need to be included in the nuget package?

The F# source code and the F# project file have to be included in the Nuget Package in a folder named fable within the package. This may sound complicated but it's only a matter of adding a couple of lines to your project file and let the dotnet pack command do all the rest.

<!-- Add source files to "fable" folder in Nuget package -->
    <Content Include="*.fsproj; **\*.fs" PackagePath="fable\" />

Possible cryptic error message when you don't include the project file and/or source files is Cannot find root module.