Asset Pipeline and Front-end in Rails

Objectives

Incorporate JavaScript, CSS, SCSS, images, and other static files into a Rails application
Understand the Asset Pipeline
Utilize helper methods/tags for including static resources
Use gon to pass data to the front-end

We can work with JavaScript, CSS, and images in Rails just like we did in our front-end only projects. Rails uses the concept of an Asset Pipeline to handle pre-processing and loading of static assets. The "Rails way" is to put all of your assets (JavaScript, CSS/SASS, images) in the assets folder. Just do it!

JavaScript

To load JavaScript files in Rails we simply create .js files in the app/assets/javascripts directory and Rails will automatically load them in the head of the page using <script> tags.

To adjust the load order, we can add the scripts along with the //= lines at the bottom of application.js

application.js

application.js is used to load other JavaScript files and allows us to change the load order of the files. You CAN put JavaScript code directly in this file. But it's best practice to avoid this and use separate files as before.

At the bottom of application.js there are a few lines that look like comments and start with //= these are used to specify which files to load and in what order. The last line (//= require_tree .) loads all of the files in the javascripts directory that aren't loaded explicitly above.

For example, if we had a file called importantScript.js in our javascripts directory and we needed it to be loaded first we could simply do something like this...

//= require 'importantScript.js'
//= require jquery_ujs
//= require turbolinks
//= require_tree .

Blame Turbolinks

Rails includes a package called Turbolinks, which provides some optimization for our sites by minimising the amount of the page that is redrawn between requests. It's clever but can cause issues if we are expecting js/jquery page ready events to fire.

Let's remove Turbolinks

Stylesheets

Rails uses SASS, which is a preprocessor for CSS that allows us to use some additional features that standard CSS doesn't support. These files end with the .scss file extension and are processed on the server (back-end) before they are sent to the user/browser as plain CSS.

Learn SASS here: SASS

Just like with JavaScript, Rails will auto-load any CSS or SCSS file that are in app/assets/stylesheets.

Also, similar to JavaScript we can adjust the load order by editing the bottom of application.css, and again we should avoid putting CSS directly in this file.

Images

We can load images by placing them in app/assets/images and using the rails image_tag helper to load them for us.

Images in views (erb)

If we have a file app/assets/images/pic.jpg we can display it in an erb file like this:

<%= image_tag 'pic.jpg' %>

We can also get a string of the image URL/path by calling image_url or image_path.

Images in CSS (using SCSS)

We also have access to a image_url helper method in scss files. It will be used in place of the standard css url('/path/to/image.jpg') format.

.some-class {
  background-image: asset-url('pic.jpg');
}

NOTE: make sure the file is named scss and not just css. The css files are served to the browser as-is (no pre-processing), but scss are run through the sass pre-proccessor first which will run the image_url method and insert the correct image path.

Passing data to the front-end (gon)

We can pass data from a controller to front-end JavaScript using a gem called "gon" which allows us to easily pass any data from the back-end to the front-end.

gon setup

To use gon (just like any other gem) we need to add it to our GemFile. After adding it remember to run bundle install and then restart your server with rails s

in GemFile

gem 'gon'

Then we need to add a tag inside of our <head> tag that will load the backend values as javascript so we can use them in our frontend.

in layouts/application.html.erb

<%= include_gon %>

Be sure to add this line BEFORE javascript_include_tag that way the variables it defines will be available in your .js files.

Using gon

gon is very simple to use. It exposes a variable called gon the controller. gon is a hash that we can add any values to and they will be available on the front-end automatically.

In controller

def index
  gon.something = "hello"
  gon.people = ['Lenny', 'Brian', 'Daniel']
  gon.tasks = Task.all
end

In JS

console.log(gon.something);
//outputs: "hello"

console.log(gon.people);
//output: ['Lenny', 'Brian', 'Daniel']

console.log(gon.tasks);
//outputs: an array of objects for all tasks in the database

Pitfalls

There is only one major "gotcha" with gon and that is that gon won't be defined in JavaScript unless you assign something to it in the controller.

Assume we didn't assign any values to gon in the controller...

in JS

console.log(gon.taco);

This would throw an error because gon is undefined. To avoid this problem we just need to check it using typeof.

if (typeof gon !== 'undefined') {
  console.log('taco is', gon.taco);
} else {
  console.log('there is no gon (or taco)');
}

Asset Pipeline