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JavaScript file watching with Reason & Rescript in Dune

While banging my head against the wall trying to understand the Dune build system used with OCaml and the Melange project to compile ReasonML and Rescript to js, I came across these great simple videos by @TheEduardoRFS.

Most of below is from his hello intro. Thanks, Eduardo! Checkout his great videos at

You don't actually need this for Rescript but this gets you all the extra latest OCaml juice in the editor plus a whole lot more.


The following is a brief introduction on how to start coding in the mess that is the OCaml ecosystem.

You can call me Eduardo and I will be helping you to setup an OCaml environment ... TODO: joke with trying


What will you need?

To do that, you will need a couple of tools, they're. The OCaml compiler. A build system. And a language server for VSCode, because no one deserves to code without autocomplete. Everything can be installed through a package manager, so you just get the package manager and that should be it.

For this video we will be using esy, an "easy" package manager for OCaml, mostly because it simplifies the life of Windows users a lot.

Windows Warning

Warning, if you're using windows, I'm sorry for you. Ok, seriously, if you're using windows everything here must be executed as admin, because "windows bad", to run VSCode as admin you need to close all VSCode windows and execute it as admin.

Installing esy

To install esy, you're going to need the node package manager and git installed. If you have both, you can just put

npm install --global esy
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on your terminal.t should take a couple of seconds. But that's it.

Install project

After that you can just clone my example project on github,

git clone
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Enter the folder /hello

cd hello
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And call esy

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The first time running esy it will install everything that we need, so it will take a couple of minutes, but on subsequent executions everything is cached, so it should be quite fast. On windows it may take a long time and you need to run your VSCode and terminal as an admin.

Add a rule with an alias to the dune file to watch the JavaScript output, note the call to node. I just took a shot in the dark and it worked. I don't know if there are docs on this:

 (alias run_hello_js)
  (run node ./
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Add a script to esy to watch the js output

    "watch-js": "esy b dune build @run_hello_js -w",
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To check if everything is working, you can call

esy watch-js
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If you see Tu tu ru~ Mayushii desu!, that means everything is working and we can now setup VSCode.

Works for ReasonML, Rescript and OCaml

Run esy hello-reason and esy hello-rescript

> esy hello-reason

Hello, Reason!

> esy hello-rescript

Hello, Rescript!

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You should go to your VSCode and install the extension "OCaml Platform". That's it, now you can open the example project on VSCode, open the file and you should have everything working, autocomplete, types when you hover some identifier and in-editor typechecking.


Yeah, now you can play with OCaml, like a prefessional. If you have any question send a comment below, something something like and subscribe.

Redemon File Watcher

This example use the redemon file watcher.

Add redemon

opam pin add redemon
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Try the example from the docs.
Run the following them change index.js or create a file and you should see Hello World! in the terminal.

redemon --path=./foo echo "Hello World!"
Hello World!
# after a change
Hello World!
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Or pass node index.js as a command and see the output

redemon --path=./foo node foo/index.js
# after editing text
hello again
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Hope this helps!

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