You can use all tools of the JS ecosystem.

Several js libs already have a Fable binding :

Example with jest


You should install js test runner :

  npm install jest --save-dev

And Fable binding :

  # nuget
  dotnet add package Fable.Jester
  # paket
  paket add Fable.Jester --project ./project/path

Write tests

Now, you can write your first test :

open Fable.Jester

Jest.describe("can run basic tests", fun () ->
    Jest.test("running a test", fun () ->

See Jester documentation to more informations :


Like all transpiled languages to JS, you have to convert your project to JS. For this, you can use Webpack, but it isn't the ideal solution, because test runners generally prefer to have small files rather than a single big file. You can use the compiler directly via the fable-splitter tool.

  npm install fable-splitter --save-dev

You should create a config file splitter.config.js:

const path = require("path");

module.exports = {
  allFiles: true, // convert all files and not only entrypoint
  entry: path.join(__dirname, "./project/path.fsproj"),
  outDir: path.join(__dirname, "./output"),
  babel: { sourceMaps: "inline" } // enable sourceMaps to see F# code and not generated js in test reports

You should config Jest with another config file jest.config.js :

module.exports = {
  moduleFileExtensions: ['js'],
  roots: ['./output'],
  testMatch: ['<rootDir>/**/*.Test.js'],
  coveragePathIgnorePatterns: ['/\.fable/', '/[fF]able.*/', '/node_modules/'],
  testEnvironment: 'node',
  transform: {}

roots should be equals to outDir of compilator. testMatch indicate file pattern name with test. coveragePathIgnorePatterns, testEnvironment, transform improve performance of runner. You can read Jest doc to see more :

Now, you can build:

  npx fable-splitter -c splitter.config.js

And run test:

  npx jest --config=jest.config.js

Youhou! You can see the test result :)

You can specify this command on npm in package.json :

  "scripts": {
    "test": "fable-splitter -c splitter.config.js && jest --config=jest.config.js",

And now run with a single command:

  npm test

Watch mode

Running each time is slow. You can use the watch feature to take advantage of the compiler and runner cache, and run tests whenever a file changes.

Currently, Fable doesn't have official plugins for the different runners. So you have to execute these two commands in parallel:

  npx fable-splitter -c splitter.config.js -w
  npx jest --config=jest.config.js --watchAll

You add a npm script in package.json :

  "scripts": {
    "test": "fable-splitter -c splitter.config.js && jest --config=jest.config.js",
    "watch-test:build": "fable-splitter -c splitter.config.js -w",
    "watch-test:run": "jest --config=jest.config.js --watchAll",
    "watch-test": "npm-run-all --parallel watch-test:*"

I use npm-run-all to run several commands in parallel. You should install with:

  npm install --save-dev npm-run-all

Now, run

  npm run-script watch-test

Enjoy :)