Hello World! in Rescript React

Talking a bit about Rescript, it is a new language that combines the best parts of JS and OCaml. It has a compiler that compiles to JS and leverages OCaml’s static and sound type system. Many people prefer this approach to TypeScript. The compiler was originally named BuckleScript but in 2020, it was rebranded to ReScript because of its Reason-like syntax.

Rescript docs are well written, organized, and maintained actively. Link to their documentation.

If you have a basic knowledge of Rescript syntax and React, then we are good to go.

Setting up the project

  • Start with the command npm init -y. If you simply do npm init it will ask you to fill in a couple of data, if you want them to be default values add the -y tag.
$ npm init -y
  • Now, let’s install the ReScript compiler locally. The reason we are installing it with --save-dev is because we need it as a dev dependency.The Rescript compiler will compile the *.res files to JS files. This is actually called source to source compilation.
$ npm install --save-dev bs-platform
  • Before, moving on to the configuration part, let’s install the rescript-react plugin.
$ npm install @rescript/react --save
  • Now, we have to set up a bsconfig.json file. Create this file and then copy and paste the following into it:


    "$schema": "https://raw.githubusercontent.com/rescript-lang/rescript-compiler/master/docs/docson/build-schema.json",
    "name": "project-name",
    "sources": [
        "dir": "src",
        "subdirs": true
    "package-specs": [
        "module": "es6",
        "in-source": true
    "suffix": ".bs.js",
    "namespace": true,
    "bs-dependencies": ["@rescript/react"],
    "ppx-flags": [],
    "reason": {"react-jsx": 3},
    "warnings": {
        "error": "+101"
    "refmt": 3

Let’s understand the config for a moment, going line by line:

  1. $schema: Editors like VSCode have a feature of schema autocompletion, and to get that we need to specify the schema.
  2. name: It is the name of the library or the main project name in simple words.

Note: the bsconfig.json name should be the same as the package.json name, to avoid confusing corner-cases. However, this means that you can’t use a camelCased names such as MyProject, since package.json and npm forbid you to do so (some file systems are case-insensitive).

  1. sources: We have to specify where the source files will reside. Usually, it is the /src directory.
  2. suffix: The file extensions after compilation by the Rescript compiler, can be .js or .bs.js. ( Although, the latter is always preferred )
  3. bs-dependencies: The list of Rescript dependencies, and since we are using the rescript-react we have to specify that here. There is also bs-dev-dependencies. These are very similar to how we mention node dependencies in package.json.
  4. reason: Since we will be using React-JSX, we have to specify {"react-jsx": 3}.

For normal Rescript projects, the bs-dependencies will have [] and reason will be {}. To learn more about the config refer here

  • To compile the source files we have to add two scripts in our package.json.


"scripts": {
    "clean": "bsb -clean-world",
    "start": "bsb -make-world -w",

start: will compile the res files and will run in watch mode i.e will continuously look for changes and compile it. It is always advised to run the start script and then code the Res files, cause the intelligent rescript compilation helps a lot and makes the coding easier for beginners to the language. clear: will clean/remove the previously compiled *.bs.jsfiles.

  • Now, install react and react-dom packages.
$ npm install react react-dom --save
  • Finally, we will use snowpack to build and bundle the project during development. In snowpack each file needs to be built only once and then is cached forever. When a file changes, Snowpack rebuilds that single file. There’s no time wasted re-bundling every change, just instant updates in the browser (made even faster via Hot-Module Replacement (HMR)). It is really fast. img Again we will install snowpack as a dev dependency:
$ npm install --save-dev snowpack
  • Make a snowpack.config.js file and copy-paste this. Since we are not using any kind of plugins there is nothing to specify over here.


// Example: snowpack.config.js
// The added "@type" comment will enable TypeScript type information via VSCode, etc.

/** @type {import("snowpack").SnowpackUserConfig } */
module.exports = {
    plugins: [
        /* ... */
  • Now, we have to add a couple of npm scripts for snowpack to start the server and build the project.


"scripts": {
    "dev": "snowpack dev",
    "build": "snowpack build"

At, this point we are done setting up dependencies, and your final package.json should look like this:


  "name": "react-res-test",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "clean": "bsb -clean-world",
    "start": "bsb -make-world -w",
    "dev": "snowpack dev",
    "build": "snowpack build"
  "keywords": [],
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "bs-platform": "^9.0.1",
    "snowpack": "^3.0.13"
  "dependencies": {
    "@rescript/react": "^0.10.1",
    "react": "^17.0.1",
    "react-dom": "^17.0.1"

Start writing your Hello World Code!!

  • First start off with a simple index.html in your root directory. Paste this basic template. One important thing is that there should be a <div> with id="root" inside which the React components will be rendered. This is how React works, if you already have some experience then you would know. Also, we have to include the compiled index.bs.js file. You can simply copy-paste this HTML. If you will use index.css then create a file and add its relative path.


<!DOCTYPE html>

<html lang="en">
    <meta charset="utf-8" />
    <title>ReScript React</title>
    <link rel="stylesheet" href="index.css" />
    <div id="root"></div>
    <script type="module" src="src/index.bs.js"></script>
  • Now create a folder src and create a new file src/index.res. Inside the index.res file just copy-paste this for the time being. We will dive into the code later on. Currently, let’s test if all the setup we did was a success.
switch ReactDOM.querySelector("#root") {
| Some(root) => ReactDOM.render(<div> {React.string("Hello World!")} </div>, root)
| None => Js.log("Error: could not find react element")

NOTE: Here we are querying for #root in the DOM(index.html) i.e looking for an element with id="root". If in the index.html you have created the div with some other id let’s say xyz then over here you have to replace #root with #xyz.

  • Now, in the terminal run the command to compile the index.res file.
$ npm run start

Once you run this, a new file index.bs.js will be created. ( This is why we included the index.bs.js file in <script> tag in index.html ).

To build the react app run this is another terminal window.

$ npm run dev

This will start a snowpack server at localhost:8080 with hot reload. Visit the URL in your browser and if you see Hello World! Congratulations!! You are now ready to create your rescript-react project.

NOTE: Don’t keep your index.css file empty if you have linked it to the index.html.

Final folder structure will be like this:

├── bsconfig.json
├── index.css
├── index.html
├── package.json
├── package-lock.json
├── snowpack.config.js
└── src
    ├── index.bs.js
    └── index.res

Was it a long process? Yeah, kind of. But every time you won’t have to go through all. Here I have already created a repo with all these steps. Simply clone it and get started:


Link: https://github.com/arnabsen1729/rescript-react-starter