Rails 6 Webpacker calling javascript function from Rails view

Question

I have following structure for Javascript in my Rails 6 app using Webpacker.

app/javascript
  + packs
    - application.js
  + custom
    - hello.js

Below shown is the content in the above mentioned JS files

app/javascript/custom/hello.js

  export function greet(name) {
    console.log("Hello, " + name);
  }

app/javascript/packs/application.js

  require("@rails/ujs").start()
  require("jquery")
  require("bootstrap")

  import greet from '../custom/hello'

config/webpack/environment.js

const { environment } = require('@rails/webpacker')

const webpack = require('webpack')
environment.plugins.prepend('Provide',
  new webpack.ProvidePlugin({
    $: 'jquery',
    jQuery: 'jquery',
    Popper: ['popper.js', 'default']
  })
)

module.exports = environment

Now in my Rails view I am trying to use the imported function greet like shown below

app/views/welcome/index.html.haml

  - name = 'Jignesh'

  :javascript

    var name = "#{name}"
    greet(name)

When I load the view I am seeing ReferenceError: greet is not defined error in browser's console.

I tried to search for a solution to this problem and found many resources on web but none turned out to help me. At last when I was drafting this question in the suggestions I found How to execute custom javascript functions in Rails 6 which indeed is close to my need however the solution shows a workaround but I am looking for a proper solution for the need because I have many views which needs to pass data from Rails view to JS functions to be moved custom files under app/javascript/custom folder.

Also I would highly appreciate if anybody can help me understand the cause behind the ReferenceError I am encountering.

Note:

I am not well-versed in Javascript development in Node realm and also new to Webpacker, Webpack, Javascript's modules, import, export, require syntax etc so please bear with me if you find anything silly in what I am asking. I have landed up in above situation while trying to upgrade an existing Rails app to use version 6.

Answer 1

Webpack does not make modules available to the global scope by default. That said, there are a few ways for you to pass information from Ruby to JavaScript outside of an AJAX request:

1. window.greet = function() { ... } and calling the function from the view as you have suggested is an option. I don't like have to code side effects in a lot of places so it's my least favorite.

2. You could look at using expose-loader. This would mean customizing your webpack config to "expose" selected functions from selected modules to the global scope. It could work well for a handful of cases but would get tedious for many use cases.

3. Export selected functions from your entrypoint(s) and configure webpack to package your bundle as a library. This is my favorite approach if you prefer to call global functions from the view. I've written about this approach specifically for Webpacker on my blog.

   // app/javascript/packs/application.js
   
   export * from '../myGlobalFunctions'
   
   
   // config/webpack/environment.js
   
   environment.config.merge({
     output: {
       // Makes exports from entry packs available to global scope, e.g.
       // Packs.application.myFunction
       library: ['Packs', '[name]'],
       libraryTarget: 'var'
     },
   })
   

   // app/views/welcome/index.html.haml
   
   :javascript
     Packs.application.greet("#{name}")
   

4. Take a different approach altogether and attach Ruby variables to a global object in your controller, such as with the gon gem. Assuming you setup the gem per the instructions, the gon object would be available both as Ruby object which you can mutate server-side and in your JavaScript code as a global variable to read from. You might need to come up with some other way to selectively call the greet function, such as with a DOM query for a particular selector that's only rendered on the given page or for a given url.

   # welcome_controller.rb
   
   def index
     gon.name = 'My name'
   end
   

   // app/javascript/someInitializer.js
   
   window.addEventListener('DOMContentLoaded', function() {
     if (window.location.match(/posts/)) {
       greet(window.gon.name)
     }
   })
   

Answer 2

@rossta Thanks a lot for your elaborate answer. It definitely should be hihghly helpful to the viewers of this post.

Your 1st suggestion I found while searching for solution to my problem and I did referenced it in my question. Like you I also don't like it because it is sort of a workaround.

Your 2nd and 3rd suggestions, honestly speaking went top of my head perhaps because I am novice to the concepts of Webpack.

Your 4th approach sounds more practical to me and as a matter of fact, after posting my question yesterday, along similar lines I tried out something and which did worked. I am sharing the solution below for reference

app/javascript/custom/hello.js

  function greet(name) {
    console.log("Hello, " + name)
  }

  export { greet }

app/javascript/packs/application.js

  require("@rails/ujs").start()
  require("bootstrap")

Note that in above file I removed require("jquery"). That's because it has already been made globally available in /config/webpack/environment.js through ProvidePlugin (please refer the code in my question). Thus requiring them in this file is not needed. I found this out while going through "Option 4: Adding Javascript to environment.js" in http://blog.blackninjadojo.com/ruby/rails/2019/03/01/webpack-webpacker-and-modules-oh-my-how-to-add-javascript-to-ruby-on-rails.html

app/views/welcome/index.html.haml

  - first_name = 'Jignesh'
  - last_name = 'Gohel'

  = hidden_field_tag('name', nil, "data": { firstName: first_name, lastName: last_name }.to_json)

Note: The idea for "data" attribute got from https://github.com/rails/webpacker/blob/master/docs/props.md

app/javascript/custom/welcome_page.js

  import { greet } from './hello'

  function nameField() {
    return $('#name')
  }

  function greetUser() {
    var nameData = nameField().attr('data')

    //console.log(nameData)
    //console.log(typeof(nameData))

    var nameJson = $.parseJSON(nameData)

    var name = nameJson.firstName + nameJson.lastName

    greet(name)
  }

  export { greetUser }

app/javascript/packs/welcome.js

  import { greetUser } from '../custom/welcome_page'

  greetUser()

Note: The idea for a separate pack I found while going through https://blog.capsens.eu/how-to-write-javascript-in-rails-6-webpacker-yarn-and-sprockets-cdf990387463 under section "Do not try to use Webpack as you would use Sprockets!" (quoting the paragraph for quick view)

So how would you make a button trigger a JS action? From a pack, you add a behavior to an HTML element. You can do that using vanilla JS, JQuery, StimulusJS, you name it.

Also the information in https://prathamesh.tech/2019/09/24/mastering-packs-in-webpacker/ helped in guiding me to solve my problem.

Then updated app/views/welcome/index.html.haml by adding following at the bottom

= javascript_pack_tag("welcome")

Finally reloaded the page and the webpacker compiled all the packs and I could see the greeting in console with the name in the view.

I hope this helps someone having a similar need like mine.