Integrating bucklescript with typescript

Recently I’ve started using Bucklescript (and Reason) over Elm for various reasons (Something I might blog about later). In this post I’m exploring integrating Bucklescript with Typescript.

Let’s start with a blank typescript project with webpack

You can find the inital commit of a Typescript project here.

There is nothing special in this commit, some npm dependencies: source-map-loader, ts-loader, typescript, webpack, webpack-dev-server. The ts-config.json file is generated with tsc --init, nothing is changed. In the webpack.config.js I have one entry file ./src/index.ts that get’s compiled to ./dist.

The index.html is just a bare html file that loads that compiled javascript. So if you run npm start and open your browser on the url that webpack-dev-server hosts, you should see Hello from Typescript in your console.

The first step was really easy, let’s see if the next step is any harder?

Integrating Bucklescript and Reason

Let’s first install bs-loader and bs-platform locally. bs-platform contains the bucklescript compiler.

To work with bucklescript we need a bsconfig.json file:

  "name": "typescript-with-bucklescript",
  "version": "0.1.0",
  "sources": [
  "package-specs": ["es6"],
  "bsc-flags": [ "-bs-super-errors -no-alias-deps", "-color", "always"]

Nothing to special in here, we look for our source files in src, compile to es6 and enable some compiler flags.

In webpack.config.js we add the bs-loader and resolve .ml and .re files:

--- a/webpack.config.js
+++ b/webpack.config.js
@@ -14,7 +14,7 @@ module.exports = {
         publicPath: '/dist/',
     resolve: {
-        extensions: ['.js', '.ts'],
+        extensions: ['.js', '.ts', '.ml', '.re'],
     module: {
         rules: [
@@ -30,6 +30,15 @@ module.exports = {
                 loaders: ['ts-loader'],
                 exclude: /node_modules/
+            {
+                test: /\.(re|ml)$/,
+                use: {
+                    loader: 'bs-loader',
+                    options: {
+                        module: 'es6',
+                    }
+                }
+            },
\ No newline at end of file

Now we are able to create 2 files: src/

let hello name =
  Js.log {j|Hi $(name), from Bucklescript.|j}

and src/

let hello name => Js.log {j|Hi $(name), from Reason.|j};

both of which can be called from index.ts:

const helloBucklescript = require('./Bucklescript').hello;
const helloReason = require('./Reason').hello;


When you run npm start you will get an error from typescript Cannot find name 'require', to solve this we need to install @types/node

Now you can run npm start and everything should work. The full commit of this step can be found here.

You might wonder why I’m using require to import the modules. That’s because I didn’t get import {x} from module syntax working. Maybe I need typescript type definitions for this?

A strange bug

While I was playing with this setup I noticed something strange. It looks like the compilation phase is always one step late. If I change something in the Bucklescript file, it only get’s picked up after a next webpack build. I’m not sure why this is the case.

The easiest way to show the problem is like this:

"scripts": {
+    "clean": "bsb -clean",
+    "build": "webpack",
     "start": "webpack-dev-server --inline --hot"

If we first run npm run clean and then npm run build we get the error that .../lib/es6/src/Reason.js does not exist. If you would run npm run build again the build would succeed because the file will be there.

Well, after some searching, it looks like this is a bug in bs-loader so I submitted a PR. The bug only manifests itself because we load 2 files and bs-loader won’t wait for the compiler for the second file.

After this fix everything works as intended, and you can start using Bucklescript/Reason together with Typescript.


Apart from 2 things:

  • Why doesn’t the import syntax work? (I’m sure I’m missing something here…)
  • The bug in bs-loader

integrating Bucklescript/Reason with Typescript was trivial and I’m planning on using it in some projects to have some real integrations.

Thomas Coopman

Software Consultant