Run package.json scripts upon any file changes in a folder

This article explains how to make a package.json script re-run whenever a file in a folder changes.

My practical problem: I want to automatically regenerate the CSS, going through a PostCSS pipeline, upon file changes.

The approach I describe will work for any kind of automatic file and folder watching, not just for this specific case.

I have this script in action, which I run using yarn build:css:

"scripts": {
  "build:css": "postcss src/tailwind.css -o static/dist/tailwind.css",
}

and I want to re-run it whenever something changes in the layouts folder, which contains all the HTML files that build up my site.

If you’re familiar with Tailwind, it creates a slightly big CSS files with all the things you might need, and you can optimize it by removing any class you don’t use.

Every time I change something in there, I want to regenerate the CSS, and trigger the purge and minification I set up in the PostCSS setup.

How to do this?

Install the watch npm package:

npm install watch

and add the watch script to your package.json file. You already had build:css from before, we just add a script that watches the layouts folder and runs build:css upon every change:

"scripts": {
  "build:css": "postcss src/tailwind.css -o static/dist/tailwind.css",
  "watch": "watch 'npm run build:css' ./layouts"
}

Now run npm run watch or yarn watch.