Using importmap in Ruby on Rails



I'm a big fan of software packaging

But sometimes you are building a site that really doesn't use code, specially javascript, that is that complicated, so why lose the ability to have all those libraries already available on the internet?

What's cool about having them published already? First, you don't have to compile just another version that looks the same as the original, so no wasting resources, second, you don't force your user to download that custom-but-really-not-custom file with the code, maybe the use already has the file in its cache, so no need to load it, even less waste.

So, I built a site called which gets the exchange rate between the Chilean Peso and the US Dollar every day and publishes it. It uses a Rails App and for the first time it uses an importmap. In my own words, importmap is a technique to declare external JS dependencies on the internet so there is no need to have them compiled locally.

You can read a lot about it in the Rails site, but I will just write down the shortcut version.

If you have a library you need to import, say ChartJS into your code, just type down in your rails directory bin/importmap pin chartjs. That command will resolve the URL of the latest version (by default it looks on and will write down into your importmap.rb file a command to pin the library to an URL.

My importmap.rb looks like this at this moment:

pin "application", preload: true
pin "@hotwired/turbo-rails", to: "turbo.min.js", preload: true
pin "@hotwired/stimulus", to: "stimulus.min.js", preload: true
pin "@hotwired/stimulus-loading", to: "stimulus-loading.js", preload: true
pin_all_from "app/javascript/controllers", under: "controllers"
pin "bootstrap", to: "[email protected]/dist/js/bootstrap.esm.js"
pin "@popperjs/core", to: "[email protected]/lib/index.js"
pin "chart.js", to: "[email protected]/dist/chart.esm.js"
pin "lazysizes", to: "[email protected]/lazysizes.js"

Cool, huh? Now we have our dependencies declared, so how do we use it?

Well.. we just use it. For example, where I use ChartJS (it is an Stimulus Controller) I can use it like this:

import { Controller } from "@hotwired/stimulus"
import { Chart, registerables } from "chart.js"
Chart.register(...registerables )

Did you get it? With the newest versions of javascript supported by the browser we can import from an URL, but with importmap we make an index saying chart.js translates into[email protected]/dist/chart.esm.js so instead of doing import { Chart, registerables } from "[email protected]/dist/chart.esm.js" we just name it chart.js

I have to declare that I still get confused with modules and libraries, but I can understand that those .esm.js files are importable modules, like ChartJS, and lazysizes is just a library that gets imported right away into the document, so those are instantly available.