---
title: Don’t Repeat Your Ruby Constants in Javascript
teaser:
tags: web,javascript,rails,good code
author: Adarsh Pandit
published_on: 2012-09-27
---

Constants help us avoid [Magic
Numbers](http://stackoverflow.com/questions/47882/what-is-a-magic-number-and-why-is-it-bad)
and repeated code, which violates the
[DRY](http://en.wikipedia.org/wiki/Don't_repeat_yourself) principle.

Recently, I needed to count down the [the remaining
characters](http://stackoverflow.com/questions/2136647/character-countdown-like-on-twitter)
in a text field.

I limited the length of the attribute using a constant:

    class Event < ActiveRecord::Base
      NAME_MAX_LENGTH = 70

      validates :name, length: { maximum: NAME_MAX_LENGTH }

      # ...
    end

Then I repeated myself in the JavaScript:

    # app/assets/javascripts/countdown_event_name.js
    var max = 70

"This is a nasty [scenario](http://www.youtube.com/watch?v=JFZLq6R-ZtM) - I've
got the same value in two places!", says I. How else could I
access the constant more cleanly from my `Event` model in the JavaScript?

# Answer

I discovered you can use ERB in JavaScript if you change file extensions and
trigger preprocessing like so:

    # Rename countdown_event_name.js → countdown_event_name.js.erb
    var max = <%= Event::NAME_MAX_LENGTH %>

Appending `.js.erb` to the filename initiates [asset pipeline preprocessing](http://guides.rubyonrails.org/asset_pipeline.html#preprocessing). Processing occurs in order, right-to-left, so make sure to keep things in order.  For example, `.js.coffee.erb` is processed as ERB first, then processed as CoffeeScript, then served as a JavaScript file for the browser to interpret.