Fable

2.13.0 verified Fri May 01 auth: no javascript

Fable is an F# to JavaScript/TypeScript, Python, Dart, Rust, and Erlang/Elixir (BEAM) compiler. Current stable version is 2.13.0 (npm package for CLI integration) with compiler core at v5.0.0. The project releases frequently with alphas/RCs. Unlike F# Compiler Services directly, Fable translates F# into clean, readable code for multiple targets. Key differentiator: it covers F#-to-multiple-backends in one tool, with growing language support including quotations and inlining. Note: npm 'fable-compiler' package is a wrapper for JS-based tooling; the actual compiler is the dotnet tool 'fable'.

Common errors

error Cannot find module 'fable-library' ↓

cause The fable-library npm package is not installed or not in node_modules.

fix

Run: npm install fable-library

error error FS3033: The fable-compiler package is not compatible with the .NET SDK version ↓

cause Incompatible .NET SDK version (Fable 5 requires .NET 8+).

fix

Install .NET 8 SDK from dotnet.microsoft.com and update global.json if present.

error Error: 'emit' is not defined (in browser) ↓

cause Using 'emit' macro or Fable.Core attributes without appropriate import or polyfill.

fix

Ensure Fable.Core is imported: import { emitExpr } from 'fable-core'

Warnings

breaking Fable 5 requires .NET 8 SDK and has breaking changes in target output; references to Fable.Core libraries must be updated. ↓

fix Update .NET SDK to 8.0+, check migration guide for changes in generated code.

deprecated The 'fable-compiler' npm package is deprecated in favor of dotnet tool; direct npm usage for compilation is discouraged. ↓

fix Use 'dotnet fable' command instead of 'fable-compiler' CLI.

gotcha String interpolation format specifiers may be dropped or cause compilation errors in non-JS targets (Python, Dart, Rust, BEAM). ↓

fix Review interpolated strings in code; use format functions where specifiers are required.

gotcha Fable does not support all .NET APIs; you must use Fable-specific equivalents (e.g., 'FSharp.Collections.Array' instead of 'System.Array'). ↓

fix Refer to Fable-friendly library replacements; use 'Fable.Core' and 'fable-library'.

breaking From v5.0.0-rc.6: Python modulo behavior for bigint changed from truncated (.NET semantics) to floored (Python semantics), breaking existing code. ↓

fix If using modulo on bigint in Python target, adjust logic to expect Python-style floored division.

Install

npm install fable-compiler

yarn add fable-compiler

pnpm add fable-compiler

Imports

default and named exports (JS/TS)
wrong
```
import fableModule from './fsharp-module'
```
correct
```
import { add, multiply } from './fsharp-module'
```
F# functions are exported as named exports; avoid default import.

FSharpRef

wrong

import { FSharpRef } from 'fable-library'

correct

import { FSharpRef } from 'fable-library/Builtins'

When using fable-library directly, use subpath imports.

Fable.Core types
wrong
```
import { emitExpr } from 'fable-library'
```
correct
```
import { emitExpr } from 'fable-core'
```
Fable.Core is separate from fable-library; ensure correct package.

Quickstart

Compiles an F# project to JavaScript using the dotnet tool, then imports the generated module.

// 1. Install Fable dotnet tool:
// > dotnet tool install --global fable

// 2. Create an F# project with fable.config.json
// Example fable.config.json:
// {
//   "sourceMaps": true,
//   "target": "javascript"
// }

// 3. Compile F# to JavaScript:
// > dotnet fable fsharp-project.fsproj --outDir ./output --lang js

// 4. In your JavaScript:
// import { greet } from './output/MyModule.js';
// console.log(greet("World"));